BUS_SPACE(9)           NetBSD Kernel Developer's Manual           BUS_SPACE(9)

NAME
     bus_space, bus_space_barrier, bus_space_copy_region_1,
     bus_space_copy_region_2, bus_space_copy_region_4,
     bus_space_copy_region_8, bus_space_free, bus_space_handle_is_equal,
     bus_space_is_equal, bus_space_map, bus_space_mmap, bus_space_peek_1,
     bus_space_peek_2, bus_space_peek_4, bus_space_peek_8, bus_space_poke_1,
     bus_space_poke_2, bus_space_poke_4, bus_space_poke_8, bus_space_read_1,
     bus_space_read_2, bus_space_read_4, bus_space_read_8,
     bus_space_read_multi_1, bus_space_read_multi_2, bus_space_read_multi_4,
     bus_space_read_multi_8, bus_space_read_multi_stream_1,
     bus_space_read_multi_stream_2, bus_space_read_multi_stream_4,
     bus_space_read_multi_stream_8, bus_space_read_region_1,
     bus_space_read_region_2, bus_space_read_region_4,
     bus_space_read_region_8, bus_space_read_region_stream_1,
     bus_space_read_region_stream_2, bus_space_read_region_stream_4,
     bus_space_read_region_stream_8, bus_space_read_stream_1,
     bus_space_read_stream_2, bus_space_read_stream_4,
     bus_space_read_stream_8, bus_space_release, bus_space_reservation_addr,
     bus_space_reservation_init, bus_space_reservation_size,
     bus_space_reservation_map, bus_space_reservation_unmap,
     bus_space_reserve, bus_space_reserve_subregion, bus_space_set_region_1,
     bus_space_set_region_2, bus_space_set_region_4, bus_space_set_region_8,
     bus_space_subregion, bus_space_tag_create, bus_space_tag_destroy,
     bus_space_unmap, bus_space_vaddr, bus_space_write_1, bus_space_write_2,
     bus_space_write_4, bus_space_write_8, bus_space_write_multi_1,
     bus_space_write_multi_2, bus_space_write_multi_4,
     bus_space_write_multi_8, bus_space_write_multi_stream_1,
     bus_space_write_multi_stream_2, bus_space_write_multi_stream_4,
     bus_space_write_multi_stream_8, bus_space_write_region_1,
     bus_space_write_region_2, bus_space_write_region_4,
     bus_space_write_region_8, bus_space_write_region_stream_1,
     bus_space_write_region_stream_2, bus_space_write_region_stream_4,
     bus_space_write_region_stream_8, bus_space_write_stream_1,
     bus_space_write_stream_2, bus_space_write_stream_4,
     bus_space_write_stream_8 -- bus space manipulation functions

SYNOPSIS
     #include <sys/bus.h>

     bool
     bus_space_handle_is_equal(bus_space_tag_t space,
         bus_space_handle_t handle1, bus_space_handle_t handle2);

     bool
     bus_space_is_equal(bus_space_tag_t space1, bus_space_tag_t space2);

     void
     bus_space_release(bus_space_tag_t t, bus_space_reservation_t *bsr);

     int
     bus_space_reserve(bus_space_tag_t t, bus_addr_t bpa, bus_size_t size,
         int flags, bus_space_reservation_t *bsrp);

     int
     bus_space_reserve_subregion(bus_space_tag_t t, bus_addr_t reg_start,
         bus_addr_t reg_end, bus_size_t size, bus_size_t alignment,
         bus_size_t boundary, int flags, bus_space_reservation_t *bsrp);

     void
     bus_space_reservation_init(bus_space_reservation_t *bsr, bus_addr_t addr,
         bus_size_t size);

     bus_size_t
     bus_space_reservation_size(bus_space_reservation_t *bsr);

     int
     bus_space_reservation_map(bus_space_tag_t t,
         bus_space_reservation_t *bsr, int flags, bus_space_handle_t *bshp);

     void
     bus_space_reservation_unmap(bus_space_tag_t t, bus_space_handle_t bsh,
         bus_size_t size);

     int
     bus_space_map(bus_space_tag_t space, bus_addr_t address, bus_size_t size,
         int flags, bus_space_handle_t *handlep);

     void
     bus_space_unmap(bus_space_tag_t space, bus_space_handle_t handle,
         bus_size_t size);

     int
     bus_space_subregion(bus_space_tag_t space, bus_space_handle_t handle,
         bus_size_t offset, bus_size_t size, bus_space_handle_t *nhandlep);

     int
     bus_space_alloc(bus_space_tag_t space, bus_addr_t reg_start,
         bus_addr_t reg_end, bus_size_t size, bus_size_t alignment,
         bus_size_t boundary, int flags, bus_addr_t *addrp,
         bus_space_handle_t *handlep);

     void
     bus_space_free(bus_space_tag_t space, bus_space_handle_t handle,
         bus_size_t size);

     void *
     bus_space_vaddr(bus_space_tag_t space, bus_space_handle_t handle);

     paddr_t
     bus_space_mmap(bus_space_tag_t space, bus_addr_t addr, off_t off,
         int prot, int flags);

     int
     bus_space_tag_create(bus_space_tag_t obst, uint64_t present,
         uint64_t extpresent, const struct bus_space_overrides *ov, void *ctx,
         bus_space_tag_t *bstp);

     void
     bus_space_tag_destroy(bus_space_tag_t bst);

     int
     bus_space_peek_1(bus_space_tag_t space, bus_space_handle_t handle,
         bus_size_t offset, uint8_t *datap);

     int
     bus_space_peek_2(bus_space_tag_t space, bus_space_handle_t handle,
         bus_size_t offset, uint16_t *datap);

     int
     bus_space_peek_4(bus_space_tag_t space, bus_space_handle_t handle,
         bus_size_t offset, uint32_t *datap);

     int
     bus_space_peek_8(bus_space_tag_t space, bus_space_handle_t handle,
         bus_size_t offset, uint64_t *datap);

     int
     bus_space_poke_1(bus_space_tag_t space, bus_space_handle_t handle,
         bus_size_t offset, uint8_t data);

     int
     bus_space_poke_2(bus_space_tag_t space, bus_space_handle_t handle,
         bus_size_t offset, uint16_t data);

     int
     bus_space_poke_4(bus_space_tag_t space, bus_space_handle_t handle,
         bus_size_t offset, uint32_t data);

     int
     bus_space_poke_8(bus_space_tag_t space, bus_space_handle_t handle,
         bus_size_t offset, uint64_t data);

     uint8_t
     bus_space_read_1(bus_space_tag_t space, bus_space_handle_t handle,
         bus_size_t offset);

     uint16_t
     bus_space_read_2(bus_space_tag_t space, bus_space_handle_t handle,
         bus_size_t offset);

     uint32_t
     bus_space_read_4(bus_space_tag_t space, bus_space_handle_t handle,
         bus_size_t offset);

     uint64_t
     bus_space_read_8(bus_space_tag_t space, bus_space_handle_t handle,
         bus_size_t offset);

     void
     bus_space_write_1(bus_space_tag_t space, bus_space_handle_t handle,
         bus_size_t offset, uint8_t value);

     void
     bus_space_write_2(bus_space_tag_t space, bus_space_handle_t handle,
         bus_size_t offset, uint16_t value);

     void
     bus_space_write_4(bus_space_tag_t space, bus_space_handle_t handle,
         bus_size_t offset, uint32_t value);

     void
     bus_space_write_8(bus_space_tag_t space, bus_space_handle_t handle,
         bus_size_t offset, uint64_t value);

     void
     bus_space_barrier(bus_space_tag_t space, bus_space_handle_t handle,
         bus_size_t offset, bus_size_t length, int flags);

     void
     bus_space_read_region_1(bus_space_tag_t space, bus_space_handle_t handle,
         bus_size_t offset, uint8_t *datap, bus_size_t count);

     void
     bus_space_read_region_2(bus_space_tag_t space, bus_space_handle_t handle,
         bus_size_t offset, uint16_t *datap, bus_size_t count);

     void
     bus_space_read_region_4(bus_space_tag_t space, bus_space_handle_t handle,
         bus_size_t offset, uint32_t *datap, bus_size_t count);

     void
     bus_space_read_region_8(bus_space_tag_t space, bus_space_handle_t handle,
         bus_size_t offset, uint64_t *datap, bus_size_t count);

     void
     bus_space_read_region_stream_1(bus_space_tag_t space,
         bus_space_handle_t handle, bus_size_t offset, uint8_t *datap,
         bus_size_t count);

     void
     bus_space_read_region_stream_2(bus_space_tag_t space,
         bus_space_handle_t handle, bus_size_t offset, uint16_t *datap,
         bus_size_t count);

     void
     bus_space_read_region_stream_4(bus_space_tag_t space,
         bus_space_handle_t handle, bus_size_t offset, uint32_t *datap,
         bus_size_t count);

     void
     bus_space_read_region_stream_8(bus_space_tag_t space,
         bus_space_handle_t handle, bus_size_t offset, uint64_t *datap,
         bus_size_t count);

     void
     bus_space_write_region_1(bus_space_tag_t space,
         bus_space_handle_t handle, bus_size_t offset, const uint8_t *datap,
         bus_size_t count);

     void
     bus_space_write_region_2(bus_space_tag_t space,
         bus_space_handle_t handle, bus_size_t offset, const uint16_t *datap,
         bus_size_t count);

     void
     bus_space_write_region_4(bus_space_tag_t space,
         bus_space_handle_t handle, bus_size_t offset, const uint32_t *datap,
         bus_size_t count);

     void
     bus_space_write_region_8(bus_space_tag_t space,
         bus_space_handle_t handle, bus_size_t offset, const uint64_t *datap,
         bus_size_t count);

     void
     bus_space_write_region_stream_1(bus_space_tag_t space,
         bus_space_handle_t handle, bus_size_t offset, const uint8_t *datap,
         bus_size_t count);

     void
     bus_space_write_region_stream_2(bus_space_tag_t space,
         bus_space_handle_t handle, bus_size_t offset, const uint16_t *datap,
         bus_size_t count);

     void
     bus_space_write_region_stream_4(bus_space_tag_t space,
         bus_space_handle_t handle, bus_size_t offset, const uint32_t *datap,
         bus_size_t count);

     void
     bus_space_write_region_stream_8(bus_space_tag_t space,
         bus_space_handle_t handle, bus_size_t offset, const uint64_t *datap,
         bus_size_t count);

     void
     bus_space_copy_region_1(bus_space_tag_t space,
         bus_space_handle_t srchandle, bus_size_t srcoffset,
         bus_space_handle_t dsthandle, bus_size_t dstoffset,
         bus_size_t count);

     void
     bus_space_copy_region_2(bus_space_tag_t space,
         bus_space_handle_t srchandle, bus_size_t srcoffset,
         bus_space_handle_t dsthandle, bus_size_t dstoffset,
         bus_size_t count);

     void
     bus_space_copy_region_4(bus_space_tag_t space,
         bus_space_handle_t srchandle, bus_size_t srcoffset,
         bus_space_handle_t dsthandle, bus_size_t dstoffset,
         bus_size_t count);

     void
     bus_space_copy_region_8(bus_space_tag_t space,
         bus_space_handle_t srchandle, bus_size_t srcoffset,
         bus_space_handle_t dsthandle, bus_size_t dstoffset,
         bus_size_t count);

     void
     bus_space_set_region_1(bus_space_tag_t space, bus_space_handle_t handle,
         bus_size_t offset, uint8_t value, bus_size_t count);

     void
     bus_space_set_region_2(bus_space_tag_t space, bus_space_handle_t handle,
         bus_size_t offset, uint16_t value, bus_size_t count);

     void
     bus_space_set_region_4(bus_space_tag_t space, bus_space_handle_t handle,
         bus_size_t offset, uint32_t value, bus_size_t count);

     void
     bus_space_set_region_8(bus_space_tag_t space, bus_space_handle_t handle,
         bus_size_t offset, uint64_t value, bus_size_t count);

     void
     bus_space_read_multi_1(bus_space_tag_t space, bus_space_handle_t handle,
         bus_size_t offset, uint8_t *datap, bus_size_t count);

     void
     bus_space_read_multi_2(bus_space_tag_t space, bus_space_handle_t handle,
         bus_size_t offset, uint16_t *datap, bus_size_t count);

     void
     bus_space_read_multi_4(bus_space_tag_t space, bus_space_handle_t handle,
         bus_size_t offset, uint32_t *datap, bus_size_t count);

     void
     bus_space_read_multi_8(bus_space_tag_t space, bus_space_handle_t handle,
         bus_size_t offset, uint64_t *datap, bus_size_t count);

     void
     bus_space_read_multi_stream_1(bus_space_tag_t space,
         bus_space_handle_t handle, bus_size_t offset, uint8_t *datap,
         bus_size_t count);

     void
     bus_space_read_multi_stream_2(bus_space_tag_t space,
         bus_space_handle_t handle, bus_size_t offset, uint16_t *datap,
         bus_size_t count);

     void
     bus_space_read_multi_stream_4(bus_space_tag_t space,
         bus_space_handle_t handle, bus_size_t offset, uint32_t *datap,
         bus_size_t count);

     void
     bus_space_read_multi_stream_8(bus_space_tag_t space,
         bus_space_handle_t handle, bus_size_t offset, uint64_t *datap,
         bus_size_t count);

     void
     bus_space_write_multi_1(bus_space_tag_t space, bus_space_handle_t handle,
         bus_size_t offset, const uint8_t *datap, bus_size_t count);

     void
     bus_space_write_multi_2(bus_space_tag_t space, bus_space_handle_t handle,
         bus_size_t offset, const uint16_t *datap, bus_size_t count);

     void
     bus_space_write_multi_4(bus_space_tag_t space, bus_space_handle_t handle,
         bus_size_t offset, const uint32_t *datap, bus_size_t count);

     void
     bus_space_write_multi_8(bus_space_tag_t space, bus_space_handle_t handle,
         bus_size_t offset, const uint64_t *datap, bus_size_t count);

     void
     bus_space_write_multi_stream_1(bus_space_tag_t space,
         bus_space_handle_t handle, bus_size_t offset, const uint8_t *datap,
         bus_size_t count);

     void
     bus_space_write_multi_stream_2(bus_space_tag_t space,
         bus_space_handle_t handle, bus_size_t offset, const uint16_t *datap,
         bus_size_t count);

     void
     bus_space_write_multi_stream_4(bus_space_tag_t space,
         bus_space_handle_t handle, bus_size_t offset, const uint32_t *datap,
         bus_size_t count);

     void
     bus_space_write_multi_stream_8(bus_space_tag_t space,
         bus_space_handle_t handle, bus_size_t offset, const uint64_t *datap,
         bus_size_t count);

DESCRIPTION
     The bus_space functions exist to allow device drivers machine-independent
     access to bus memory and register areas.  All of the functions and types
     described in this document can be used by including the <sys/bus.h>
     header file.

     Many common devices are used on multiple architectures, but are accessed
     differently on each because of architectural constraints.  For instance,
     a device which is mapped in one system's I/O space may be mapped in mem-
     ory space on a second system.  On a third system, architectural limita-
     tions might change the way registers need to be accessed (e.g., creating
     a non-linear register space).  In some cases, a single driver may need to
     access the same type of device in multiple ways in a single system or
     architecture.  The goal of the bus_space functions is to allow a single
     driver source file to manipulate a set of devices on different system
     architectures, and to allow a single driver object file to manipulate a
     set of devices on multiple bus types on a single architecture.

     Not all busses have to implement all functions described in this docu-
     ment, though that is encouraged if the operations are logically supported
     by the bus.  Unimplemented functions should cause compile-time errors if
     possible.

     All of the interface definitions described in this document are shown as
     function prototypes and discussed as if they were required to be func-
     tions.  Implementations are encouraged to implement prototyped (type-
     checked) versions of these interfaces, but may implement them as macros
     if appropriate.  Machine-dependent types, variables, and functions should
     be marked clearly in <machine/bus_defs.h> and in <machine/bus_funcs.h> to
     avoid confusion with the machine-independent types and functions, and, if
     possible, should be given names which make the machine-dependence clear.

CONCEPTS AND GUIDELINES
     Bus spaces are described by bus space tags, which can be created only by
     machine-dependent code.  A given machine may have several different types
     of bus space (e.g., memory space and I/O space), and thus may provide
     multiple different bus space tags.  Individual busses or devices on a
     machine may use more than one bus space tag.  For instance, ISA devices
     are given an ISA memory space tag and an ISA I/O space tag.  Architec-
     tures may have several different tags which represent the same type of
     space, for instance because of multiple different host bus interface
     chipsets.

     A range in bus space is described by a bus address and a bus size.  The
     bus address describes the start of the range in bus space.  The bus size
     describes the size of the range in bytes.  Busses which are not byte
     addressable may require use of bus space ranges with appropriately
     aligned addresses and properly rounded sizes.

     Access to regions of bus space is facilitated by use of bus space han-
     dles, which are usually created by mapping a specific range of a bus
     space.  Handles may also be created by allocating and mapping a range of
     bus space, the actual location of which is picked by the implementation
     within bounds specified by the caller of the allocation function.

     All of the bus space access functions require one bus space tag argument,
     at least one handle argument, and at least one offset argument (a bus
     size).  The bus space tag specifies the space, each handle specifies a
     region in the space, and each offset specifies the offset into the region
     of the actual location(s) to be accessed.  Offsets are given in bytes,
     though busses may impose alignment constraints.  The offset used to
     access data relative to a given handle must be such that all of the data
     being accessed is in the mapped region that the handle describes.  Trying
     to access data outside that region is an error.

     Because some architectures' memory systems use buffering to improve mem-
     ory and device access performance, there is a mechanism which can be used
     to create ``barriers'' in the bus space read and write stream.

     There are two types of barriers: ordering barriers and completion barri-
     ers.

     Ordering barriers prevent some operations from bypassing other opera-
     tions.  They are relatively light weight and described in terms of the
     operations they are intended to order.  The important thing to note is
     that they create specific ordering constraint surrounding bus accesses
     but do not necessarily force any synchronization themselves.  So, if
     there is enough distance between the memory operations being ordered, the
     preceding ones could complete by themselves resulting in no performance
     penalty.

     For instance, a write before read barrier will force any writes issued
     before the barrier instruction to complete before any reads after the
     barrier are issued.  This forces processors with write buffers to read
     data from memory rather than from the pending write in the write buffer.

     Ordering barriers are usually sufficient for most circumstances, and can
     be combined together.  For instance a read before write barrier can be
     combined with a write before write barrier to force all memory operations
     to complete before the next write is started.

     Completion barriers force all memory operations and any pending excep-
     tions to be completed before any instructions after the barrier may be
     issued.  Completion barriers are extremely expensive and almost never
     required in device driver code.  A single completion barrier can force
     the processor to stall on memory for hundreds of cycles on some machines.

     Correctly-written drivers will include all appropriate barriers, and
     assume only the read/write ordering imposed by the barrier operations.

     People trying to write portable drivers with the bus_space functions
     should try to make minimal assumptions about what the system allows.  In
     particular, they should expect that the system requires bus space
     addresses being accessed to be naturally aligned (i.e., base address of
     handle added to offset is a multiple of the access size), and that the
     system does alignment checking on pointers (i.e., pointer to objects
     being read and written must point to properly-aligned data).

     The descriptions of the bus_space functions given below all assume that
     they are called with proper arguments.  If called with invalid arguments
     or arguments that are out of range (e.g., trying to access data outside
     of the region mapped when a given handle was created), undefined behav-
     iour results.  In that case, they may cause the system to halt, either
     intentionally (via panic) or unintentionally (by causing a fatal trap or
     by some other means) or may cause improper operation which is not immedi-
     ately fatal.  Functions which return void or which return data read from
     bus space (i.e., functions which don't obviously return an error code) do
     not fail.  They could only fail if given invalid arguments, and in that
     case their behaviour is undefined.  Functions which take a count of bytes
     have undefined results if the specified count is zero.

TYPES
     Several types are defined in <machine/bus_defs.h> to facilitate use of
     the bus_space functions by drivers.

     bus_addr_t

     The bus_addr_t type is used to describe bus addresses.  It must be an
     unsigned integral type capable of holding the largest bus address usable
     by the architecture.  This type is primarily used when mapping and unmap-
     ping bus space.

     bus_size_t

     The bus_size_t type is used to describe sizes of ranges in bus space.  It
     must be an unsigned integral type capable of holding the size of the
     largest bus address range usable on the architecture.  This type is used
     by virtually all of the bus_space functions, describing sizes when map-
     ping regions and offsets into regions when performing space access opera-
     tions.

     bus_space_tag_t

     The bus_space_tag_t type is used to describe a particular bus space on a
     machine.  Its contents are machine-dependent and should be considered
     opaque by machine-independent code.  This type is used by all bus_space
     functions to name the space on which they're operating.

     bus_space_handle_t

     The bus_space_handle_t type is used to describe a mapping of a range of
     bus space.  Its contents are machine-dependent and should be considered
     opaque by machine-independent code.  This type is used when performing
     bus space access operations.
     bus_space_reservation_t

     The bus_space_reservation_t type is used to describe a range of bus
     space.  It logically consists of a bus_addr_t, the first address in the
     range, and a bus_size_t, the length in bytes of the range.  Machine-inde-
     pendent code creates and interrogates a bus_space_reservation_t using a
     constructor, bus_space_reservation_init(), and accessor functions,
     bus_space_reservation_addr() and bus_space_reservation_size().

COMPARING BUS SPACE TAGS
     To check whether or not one bus_space_tag_t refers to the same space as
     another in machine-independent code, do not use either memcmp(9) or the C
     equals (==) operator.  Use bus_space_is_equal(), instead.

MAPPING AND UNMAPPING BUS SPACE
     Bus space must be mapped before it can be used, and should be unmapped
     when it is no longer needed.  The bus_space_map(),
     bus_space_reservation_map(), bus_space_reservation_unmap(), and
     bus_space_unmap() functions provide these capabilities.

     Some drivers need to be able to pass a subregion of already-mapped bus
     space to another driver or module within a driver.  The
     bus_space_subregion() function allows such subregions to be created.

     bus_space_map(space, address, size, flags, handlep)

     The bus_space_map() function exclusively reserves and maps the region of
     bus space named by the space, address, and size arguments.  If success-
     ful, it returns zero and fills in the bus space handle pointed to by
     handlep with the handle that can be used to access the mapped region.  If
     unsuccessful, it will return non-zero and leave the bus space handle
     pointed to by handlep in an undefined state.

     The flags argument controls how the space is to be mapped.  Supported
     flags include:

           BUS_SPACE_MAP_CACHEABLE  Try to map the space so that accesses can
                                    be cached by the system cache.  If this
                                    flag is not specified, the implementation
                                    should map the space so that it will not
                                    be cached.  This mapping method will only
                                    be useful in very rare occasions.

                                    This flag must have a value of 1 on all
                                    implementations for backward compatibil-
                                    ity.

           BUS_SPACE_MAP_PREFETCHABLE
                                    Try to map the space so that accesses can
                                    be prefetched by the system, and writes
                                    can be buffered.  This means, accesses
                                    should be side effect free (idempotent).
                                    The bus_space_barrier() methods will flush
                                    the write buffer or force actual read
                                    accesses.  If this flag is not specified,
                                    the implementation should map the space so
                                    that it will not be prefetched or delayed.

           BUS_SPACE_MAP_LINEAR     Try to map the space so that its contents
                                    can be accessed linearly via normal memory
                                    access methods (e.g., pointer dereferenc-
                                    ing and structure accesses).  The
                                    bus_space_vaddr() method can be used to
                                    obtain the kernel virtual address of the
                                    mapped range.  This is useful when soft-
                                    ware wants to do direct access to a memory
                                    device, e.g., a frame buffer.  If this
                                    flag is specified and linear mapping is
                                    not possible, the bus_space_map() call
                                    should fail.  If this flag is not speci-
                                    fied, the system may map the space in
                                    whatever way is most convenient.  Use of
                                    this mapping method is not encouraged for
                                    normal device access; where linear access
                                    is not essential, use of the
                                    bus_space_read/write() methods is strongly
                                    recommended.

     Not all combinations of flags make sense or are supported with all spa-
     ces.  For instance, BUS_SPACE_MAP_CACHEABLE may be meaningless when used
     on many systems' I/O port spaces, and on some systems
     BUS_SPACE_MAP_LINEAR without BUS_SPACE_MAP_PREFETCHABLE may never work.
     When the system hardware or firmware provides hints as to how spaces
     should be mapped (e.g., the PCI memory mapping registers' "prefetchable"
     bit), those hints should be followed for maximum compatibility.  On some
     systems, requesting a mapping that cannot be satisfied (e.g., requesting
     a non-prefetchable mapping when the system can only provide a prefetch-
     able one) will cause the request to fail.

     Some implementations may keep track of use of bus space for some or all
     bus spaces and refuse to allow duplicate allocations.  This is encouraged
     for bus spaces which have no notion of slot-specific space addressing,
     such as ISA and VME, and for spaces which coexist with those spaces
     (e.g., EISA and PCI memory and I/O spaces co-existing with ISA memory and
     I/O spaces).

     Mapped regions may contain areas for which there is no device on the bus.
     If space in those areas is accessed, the results are bus-dependent.

     bus_space_reservation_map(space, bsr, flags, handlep)

     The bus_space_reservation_map() function is similar to bus_space_map()
     but it maps a region of bus space that was previously reserved by a call
     to bus_space_reserve() or bus_space_reserve_subregion().  The region is
     given by the space and bsr arguments.  If successful, it returns zero and
     fills in the bus space handle pointed to by handlep with the handle that
     can be used to access the mapped region.  If unsuccessful, it will return
     non-zero and leave the bus space handle pointed to by handlep in an unde-
     fined state.

     A region mapped by bus_space_reservation_map() may only be unmapped by a
     call to bus_space_reservation_unmap().

     For more details, see the description of bus_space_map().

     bus_space_unmap(space, handle, size)

     The bus_space_unmap() function unmaps and relinquishes a region of bus
     space reserved and mapped with bus_space_map().  When unmapping a region,
     the size specified should be the same as the size given to
     bus_space_map() when mapping that region.

     After bus_space_unmap() is called on a handle, that handle is no longer
     valid.  (If copies were made of the handle they are no longer valid,
     either.)

     This function will never fail.  If it would fail (e.g., because of an
     argument error), that indicates a software bug which should cause a
     panic.  In that case, bus_space_unmap() will never return.

     bus_space_reservation_unmap(space, handle, size)

     The bus_space_reservation_unmap() function is similar to
     bus_space_unmap() but it should be called on handles mapped by
     bus_space_reservation_map() and only on such handles.  Unlike
     bus_space_unmap(), bus_space_reservation_unmap() does not relinquish
     exclusive use of the bus space named by handle and size; that is the job
     of bus_space_release().

     bus_space_subregion(space, handle, offset, size, nhandlep)

     The bus_space_subregion() function is a convenience function which makes
     a new handle to some subregion of an already-mapped region of bus space.
     The subregion described by the new handle starts at byte offset offset
     into the region described by handle, with the size given by size, and
     must be wholly contained within the original region.

     If successful, bus_space_subregion() returns zero and fills in the bus
     space handle pointed to by nhandlep.  If unsuccessful, it returns non-
     zero and leaves the bus space handle pointed to by nhandlep in an unde-
     fined state.  In either case, the handle described by handle remains
     valid and is unmodified.

     When done with a handle created by bus_space_subregion(), the handle
     should be thrown away.  Under no circumstances should bus_space_unmap()
     be used on the handle.  Doing so may confuse any resource management
     being done on the space, and will result in undefined behaviour.  When
     bus_space_unmap() or bus_space_free() is called on a handle, all subre-
     gions of that handle become invalid.

     bus_space_vaddr(tag, handle)

     This method returns the kernel virtual address of a mapped bus space if
     and only if it was mapped with the BUS_SPACE_MAP_LINEAR flag.  The range
     can be accessed by normal (volatile) pointer dereferences.  If mapped
     with the BUS_SPACE_MAP_PREFETCHABLE flag, the bus_space_barrier() method
     must be used to force a particular access order.

     bus_space_mmap(tag, addr, off, prot, flags)

     This method is used to provide support for memory mapping bus space into
     user applications.  If an address space is addressable via volatile
     pointer dereferences, bus_space_mmap() will return the physical address
     (possibly encoded as a machine-dependent cookie) of the bus space indi-
     cated by addr and off.  addr is the base address of the device or device
     region, and off is the offset into that region that is being requested.
     If the request is made with BUS_SPACE_MAP_LINEAR as a flag, then a linear
     region must be returned to the caller.  If the region cannot be mapped
     (either the address does not exist, or the constraints can not be met),
     bus_space_mmap() returns -1 to indicate failure.

     Note that it is not necessary that the region being requested by a
     bus_space_mmap() call be mapped into a bus_space_handle_t.

     bus_space_mmap() is called once per PAGE_SIZE page in the range.  The
     prot argument indicates the memory protection requested by the user
     application for the range.

     bus_space_handle_is_equal(space, handle1, handle2)
     Use bus_space_handle_is_equal() to check whether or not handle1 and
     handle2 refer to regions starting at the same address in the bus space
     space.

ALLOCATING AND FREEING BUS SPACE
     Some devices require or allow bus space to be allocated by the operating
     system for device use.  When the devices no longer need the space, the
     operating system should free it for use by other devices.  The
     bus_space_alloc(), bus_space_free(), bus_space_reserve(),
     bus_space_reserve_subregion(), and bus_space_release() functions provide
     these capabilities.  The functions bus_space_reserve(),
     bus_space_reserve_subregion(), and bus_space_release() are not yet avail-
     able on all architectures.

     bus_space_alloc(space, reg_start, reg_end, size, alignment, boundary,
     flags, addrp, handlep)

     The bus_space_alloc() function allocates and maps a region of bus space
     with the size given by size, corresponding to the given constraints.  If
     successful, it returns zero, fills in the bus address pointed to by addrp
     with the bus space address of the allocated region, and fills in the bus
     space handle pointed to by handlep with the handle that can be used to
     access that region.  If unsuccessful, it returns non-zero and leaves the
     bus address pointed to by addrp and the bus space handle pointed to by
     handlep in an undefined state.

     Constraints on the allocation are given by the reg_start, reg_end,
     alignment, and boundary parameters.  The allocated region will start at
     or after reg_start and end before or at reg_end.  The alignment con-
     straint must be a power of two, and the allocated region will start at an
     address that is an even multiple of that power of two.  The boundary con-
     straint, if non-zero, ensures that the region is allocated so that first
     address in region / boundary has the same value as last address in region
     / boundary.  If the constraints cannot be met, bus_space_alloc() will
     fail.  It is an error to specify a set of constraints that can never be
     met (for example, size greater than boundary).

     The flags parameter is the same as the like-named parameter to
     bus_space_map, the same flag values should be used, and they have the
     same meanings.

     Handles created by bus_space_alloc() should only be freed with
     bus_space_free().  Trying to use bus_space_unmap() on them causes unde-
     fined behaviour.  The bus_space_subregion() function can be used on han-
     dles created by bus_space_alloc().

     bus_space_reserve(t, bpa, size, flags, bsrp)

     The bus_space_reserve() function reserves, for the caller's exclusive
     use, size bytes starting at the address bpa in the space referenced by t.

     bus_space_reserve() does not map the space.  The caller should use
     bus_space_reservation_map() to map the reservation.  flags contains a
     hint how the caller may map the reservation, later.  Whenever possible,
     callers should pass the same flags to bus_space_reserve() as they will
     pass to bus_space_reservation_map() to map the reservation.

     On success, bus_space_reserve() records the reservation at bsrp and
     returns 0.  On failure, bsrp is undefined, and bus_space_reserve()
     returns a non-zero error code.  Possible error codes include

           EOPNOTSUPP  bus_space_reserve() is not supported on this architec-
                       ture, or flags was incompatible with the bus space rep-
                       resented by t.

           ENOMEM      There was not sufficient bus space at bpa to satisfy
                       the request.

     bus_space_reserve_subregion(t, reg_start, reg_end, size, alignment,
     boundary, flags, bsrp)

     The bus_space_reserve_subregion() function reserves, for the caller's
     exclusive use, size bytes in the space referenced by t.  The parameters
     reg_start, reg_end, alignment, boundary, and flags each work alike to the
     bus_space_alloc() parameters of the same names.

     On success, bus_space_reserve_subregion() records the reservation at bsrp
     and returns 0.  On failure, bsrp is undefined, and
     bus_space_reserve_subregion() returns a non-zero error code.  Possible
     error codes include

           EOPNOTSUPP  bus_space_reserve() is not supported on this architec-
                       ture, or flags was incompatible with the bus space rep-
                       resented by t.

           ENOMEM      There was not sufficient bus space at bpa to satisfy
                       the request.

     bus_space_release(t, bsr)

     The bus_space_release() function releases the bus space bsr in t that was
     previously reserved by bus_space_reserve() or
     bus_space_reserve_subregion().

     If bus_space_release() is called on a reservation that has been mapped by
     bus_space_reservation_map() without subsequently being unmapped, the
     behavior of the system is undefined.

     bus_space_free(space, handle, size)

     The bus_space_free() function unmaps and frees a region of bus space
     mapped and allocated with bus_space_alloc().  When unmapping a region,
     the size specified should be the same as the size given to
     bus_space_alloc() when allocating the region.

     After bus_space_free() is called on a handle, that handle is no longer
     valid.  (If copies were made of the handle, they are no longer valid,
     either.)

     This function will never fail.  If it would fail (e.g., because of an
     argument error), that indicates a software bug which should cause a
     panic.  In that case, bus_space_free() will never return.

READING AND WRITING SINGLE DATA ITEMS
     The simplest way to access bus space is to read or write a single data
     item.  The bus_space_read_N() and bus_space_write_N() families of func-
     tions provide the ability to read and write 1, 2, 4, and 8 byte data
     items on busses which support those access sizes.

     bus_space_read_1(space, handle, offset)
     bus_space_read_2(space, handle, offset)
     bus_space_read_4(space, handle, offset)
     bus_space_read_8(space, handle, offset)

     The bus_space_read_N() family of functions reads a 1, 2, 4, or 8 byte
     data item from the offset specified by offset into the region specified
     by handle of the bus space specified by space.  The location being read
     must lie within the bus space region specified by handle.

     For portability, the starting address of the region specified by handle
     plus the offset should be a multiple of the size of data item being read.
     On some systems, not obeying this requirement may cause incorrect data to
     be read, on others it may cause a system crash.

     Read operations done by the bus_space_read_N() functions may be executed
     out of order with respect to other pending read and write operations
     unless order is enforced by use of the bus_space_barrier() function.

     These functions will never fail.  If they would fail (e.g., because of an
     argument error), that indicates a software bug which should cause a
     panic.  In that case, they will never return.

     bus_space_write_1(space, handle, offset, value)
     bus_space_write_2(space, handle, offset, value)
     bus_space_write_4(space, handle, offset, value)
     bus_space_write_8(space, handle, offset, value)

     The bus_space_write_N() family of functions writes a 1, 2, 4, or 8 byte
     data item to the offset specified by offset into the region specified by
     handle of the bus space specified by space.  The location being written
     must lie within the bus space region specified by handle.

     For portability, the starting address of the region specified by handle
     plus the offset should be a multiple of the size of data item being writ-
     ten.  On some systems, not obeying this requirement may cause incorrect
     data to be written, on others it may cause a system crash.

     Write operations done by the bus_space_write_N() functions may be exe-
     cuted out of order with respect to other pending read and write opera-
     tions unless order is enforced by use of the bus_space_barrier() func-
     tion.

     These functions will never fail.  If they would fail (e.g., because of an
     argument error), that indicates a software bug which should cause a
     panic.  In that case, they will never return.

PROBING BUS SPACE FOR HARDWARE WHICH MAY NOT RESPOND
     One problem with the bus_space_read_N() and bus_space_write_N() family of
     functions is that they provide no protection against exceptions which can
     occur when no physical hardware or device responds to the read or write
     cycles.  In such a situation, the system typically would panic due to a
     kernel-mode bus error.  The bus_space_peek_N() and bus_space_poke_N()
     family of functions provide a mechanism to handle these exceptions grace-
     fully without the risk of crashing the system.

     As with bus_space_read_N() and bus_space_write_N(), the peek and poke
     functions provide the ability to read and write 1, 2, 4, and 8 byte data
     items on busses which support those access sizes.  All of the constraints
     specified in the descriptions of the bus_space_read_N() and
     bus_space_write_N() functions also apply to bus_space_peek_N() and
     bus_space_poke_N().

     In addition, explicit calls to the bus_space_barrier() function are not
     required as the implementation will ensure all pending operations com-
     plete before the peek or poke operation starts.  The implementation will
     also ensure that the peek or poke operations complete before returning.

     The return value indicates the outcome of the peek or poke operation.  A
     return value of zero implies that a hardware device is responding to the
     operation at the specified offset in the bus space.  A non-zero return
     value indicates that the kernel intercepted a hardware exception (e.g.,
     bus error) when the peek or poke operation was attempted.  Note that some
     busses are incapable of generating exceptions when non-existent hardware
     is accessed.  In such cases, these functions will always return zero and
     the value of the data read by bus_space_peek_N() will be unspecified.

     Finally, it should be noted that at this time the bus_space_peek_N() and
     bus_space_poke_N() functions are not re-entrant and should not, there-
     fore, be used from within an interrupt service routine.  This constraint
     may be removed at some point in the future.

     bus_space_peek_1(space, handle, offset, datap)
     bus_space_peek_2(space, handle, offset, datap)
     bus_space_peek_4(space, handle, offset, datap)
     bus_space_peek_8(space, handle, offset, datap)

     The bus_space_peek_N() family of functions cautiously read a 1, 2, 4, or
     8 byte data item from the offset specified by offset in the region speci-
     fied by handle of the bus space specified by space.  The data item read
     is stored in the location pointed to by datap.  It is permissible for
     datap to be NULL, in which case the data item will be discarded after
     being read.

     bus_space_poke_1(space, handle, offset, value)
     bus_space_poke_2(space, handle, offset, value)
     bus_space_poke_4(space, handle, offset, value)
     bus_space_poke_8(space, handle, offset, value)

     The bus_space_poke_N() family of functions cautiously write a 1, 2, 4, or
     8 byte data item specified by value to the offset specified by offset in
     the region specified by handle of the bus space specified by space.

BARRIERS
     In order to allow high-performance buffering implementations to avoid bus
     activity on every operation, read and write ordering should be specified
     explicitly by drivers when necessary.  The bus_space_barrier() function
     provides that ability.

     bus_space_barrier(space, handle, offset, length, flags)

     The bus_space_barrier() function enforces ordering of bus space read and
     write operations for the specified subregion (described by the offset and
     length parameters) of the region named by handle in the space named by
     space.

     The flags argument controls what types of operations are to be ordered.
     Supported flags are:

           BUS_SPACE_BARRIER_READ_BEFORE_READ    Force all reads before the
                                                 barrier to complete before
                                                 any reads after the barrier
                                                 may be issued.

           BUS_SPACE_BARRIER_READ_BEFORE_WRITE   Force all reads before the
                                                 barrier to complete before
                                                 any writes after the barrier
                                                 may be issued.

           BUS_SPACE_BARRIER_WRITE_BEFORE_READ   Force all writes before the
                                                 barrier to complete before
                                                 any reads after the barrier
                                                 may be issued.

           BUS_SPACE_BARRIER_WRITE_BEFORE_WRITE  Force all writes before the
                                                 barrier to complete before
                                                 any writes after the barrier
                                                 may be issued.

           BUS_SPACE_BARRIER_SYNC                Force all memory operations
                                                 and any pending exceptions to
                                                 be completed before any
                                                 instructions after the bar-
                                                 rier may be issued.

     Those flags can be combined (or-ed together) to enforce ordering on dif-
     ferent combinations of read and write operations.

     All of the specified type(s) of operation which are done to the region
     before the barrier operation are guaranteed to complete before any of the
     specified type(s) of operation done after the barrier.

     Example: Consider a hypothetical device with two single-byte ports, one
     write-only input port (at offset 0) and a read-only output port (at off-
     set 1).  Operation of the device is as follows: data bytes are written to
     the input port, and are placed by the device on a stack, the top of which
     is read by reading from the output port.  The sequence to correctly write
     two data bytes to the device then read those two data bytes back would
     be:

     /*
      * t and h are the tag and handle for the mapped device's
      * space.
      */
     bus_space_write_1(t, h, 0, data0);
     bus_space_barrier(t, h, 0, 1, BUS_SPACE_BARRIER_WRITE_BEFORE_WRITE); /* 1 */
     bus_space_write_1(t, h, 0, data1);
     bus_space_barrier(t, h, 0, 2, BUS_SPACE_BARRIER_WRITE_BEFORE_READ);  /* 2 */
     ndata1 = bus_space_read_1(t, h, 1);
     bus_space_barrier(t, h, 1, 1, BUS_SPACE_BARRIER_READ_BEFORE_READ);   /* 3 */
     ndata0 = bus_space_read_1(t, h, 1);
     /* data0 == ndata0, data1 == ndata1 */

     The first barrier makes sure that the first write finishes before the
     second write is issued, so that two writes to the input port are done in
     order and are not collapsed into a single write.  This ensures that the
     data bytes are written to the device correctly and in order.

     The second barrier forces the writes to the output port finish before any
     of the reads to the input port are issued, thereby making sure that all
     of the writes are finished before data is read.  This ensures that the
     first byte read from the device really is the last one that was written.

     The third barrier makes sure that the first read finishes before the sec-
     ond read is issued, ensuring that data is read correctly and in order.

     The barriers in the example above are specified to cover the absolute
     minimum number of bus space locations.  It is correct (and often easier)
     to make barrier operations cover the device's whole range of bus space,
     that is, to specify an offset of zero and the size of the whole region.

     The following barrier operations are obsolete and should be removed from
     existing code:

           BUS_SPACE_BARRIER_READ   Synchronize read operations.

           BUS_SPACE_BARRIER_WRITE  Synchronize write operations.

REGION OPERATIONS
     Some devices use buffers which are mapped as regions in bus space.
     Often, drivers want to copy the contents of those buffers to or from mem-
     ory, e.g., into mbufs which can be passed to higher levels of the system
     or from mbufs to be output to a network.  In order to allow drivers to do
     this as efficiently as possible, the bus_space_read_region_N() and
     bus_space_write_region_N() families of functions are provided.

     Drivers occasionally need to copy one region of a bus space to another,
     or to set all locations in a region of bus space to contain a single
     value.  The bus_space_copy_region_N() family of functions and the
     bus_space_set_region_N() family of functions allow drivers to perform
     these operations.

     bus_space_read_region_1(space, handle, offset, datap, count)
     bus_space_read_region_2(space, handle, offset, datap, count)
     bus_space_read_region_4(space, handle, offset, datap, count)
     bus_space_read_region_8(space, handle, offset, datap, count)

     The bus_space_read_region_N() family of functions reads count 1, 2, 4, or
     8 byte data items from bus space starting at byte offset offset in the
     region specified by handle of the bus space specified by space and writes
     them into the array specified by datap.  Each successive data item is
     read from an offset 1, 2, 4, or 8 bytes after the previous data item
     (depending on which function is used).  All locations being read must lie
     within the bus space region specified by handle.

     For portability, the starting address of the region specified by handle
     plus the offset should be a multiple of the size of data items being read
     and the data array pointer should be properly aligned.  On some systems,
     not obeying these requirements may cause incorrect data to be read, on
     others it may cause a system crash.

     Read operations done by the bus_space_read_region_N() functions may be
     executed in any order.  They may also be executed out of order with
     respect to other pending read and write operations unless order is
     enforced by use of the bus_space_barrier() function.  There is no way to
     insert barriers between reads of individual bus space locations executed
     by the bus_space_read_region_N() functions.

     These functions will never fail.  If they would fail (e.g., because of an
     argument error), that indicates a software bug which should cause a
     panic.  In that case, they will never return.

     bus_space_write_region_1(space, handle, offset, datap, count)
     bus_space_write_region_2(space, handle, offset, datap, count)
     bus_space_write_region_4(space, handle, offset, datap, count)
     bus_space_write_region_8(space, handle, offset, datap, count)

     The bus_space_write_region_N() family of functions reads count 1, 2, 4,
     or 8 byte data items from the array specified by datap and writes them to
     bus space starting at byte offset offset in the region specified by
     handle of the bus space specified by space.  Each successive data item is
     written to an offset 1, 2, 4, or 8 bytes after the previous data item
     (depending on which function is used).  All locations being written must
     lie within the bus space region specified by handle.

     For portability, the starting address of the region specified by handle
     plus the offset should be a multiple of the size of data items being
     written and the data array pointer should be properly aligned.  On some
     systems, not obeying these requirements may cause incorrect data to be
     written, on others it may cause a system crash.

     Write operations done by the bus_space_write_region_N() functions may be
     executed in any order.  They may also be executed out of order with
     respect to other pending read and write operations unless order is
     enforced by use of the bus_space_barrier() function.  There is no way to
     insert barriers between writes of individual bus space locations executed
     by the bus_space_write_region_N() functions.

     These functions will never fail.  If they would fail (e.g., because of an
     argument error), that indicates a software bug which should cause a
     panic.  In that case, they will never return.

     bus_space_copy_region_1(space, srchandle, srcoffset, dsthandle,
     dstoffset, count)
     bus_space_copy_region_2(space, srchandle, srcoffset, dsthandle,
     dstoffset, count)
     bus_space_copy_region_4(space, srchandle, srcoffset, dsthandle,
     dstoffset, count)
     bus_space_copy_region_8(space, srchandle, srcoffset, dsthandle,
     dstoffset, count)

     The bus_space_copy_region_N() family of functions copies count 1, 2, 4,
     or 8 byte data items in bus space from the area starting at byte offset
     srcoffset in the region specified by srchandle of the bus space specified
     by space to the area starting at byte offset dstoffset in the region
     specified by dsthandle in the same bus space.  Each successive data item
     read or written has an offset 1, 2, 4, or 8 bytes after the previous data
     item (depending on which function is used).  All locations being read and
     written must lie within the bus space region specified by their respec-
     tive handles.

     For portability, the starting addresses of the regions specified by each
     handle plus its respective offset should be a multiple of the size of
     data items being copied.  On some systems, not obeying this requirement
     may cause incorrect data to be copied, on others it may cause a system
     crash.

     Read and write operations done by the bus_space_copy_region_N() functions
     may be executed in any order.  They may also be executed out of order
     with respect to other pending read and write operations unless order is
     enforced by use of the bus_space_barrier(function).  There is no way to
     insert barriers between reads or writes of individual bus space locations
     executed by the bus_space_copy_region_N() functions.

     Overlapping copies between different subregions of a single region of bus
     space are handled correctly by the bus_space_copy_region_N() functions.

     These functions will never fail.  If they would fail (e.g., because of an
     argument error), that indicates a software bug which should cause a
     panic.  In that case, they will never return.

     bus_space_set_region_1(space, handle, offset, value, count)
     bus_space_set_region_2(space, handle, offset, value, count)
     bus_space_set_region_4(space, handle, offset, value, count)
     bus_space_set_region_8(space, handle, offset, value, count)

     The bus_space_set_region_N() family of functions writes the given value
     to count 1, 2, 4, or 8 byte data items in bus space starting at byte off-
     set offset in the region specified by handle of the bus space specified
     by space.  Each successive data item has an offset 1, 2, 4, or 8 bytes
     after the previous data item (depending on which function is used).  All
     locations being written must lie within the bus space region specified by
     handle.

     For portability, the starting address of the region specified by handle
     plus the offset should be a multiple of the size of data items being
     written.  On some systems, not obeying this requirement may cause incor-
     rect data to be written, on others it may cause a system crash.

     Write operations done by the bus_space_set_region_N() functions may be
     executed in any order.  They may also be executed out of order with
     respect to other pending read and write operations unless order is
     enforced by use of the bus_space_barrier() function.  There is no way to
     insert barriers between writes of individual bus space locations executed
     by the bus_space_set_region_N() functions.

     These functions will never fail.  If they would fail (e.g., because of an
     argument error), that indicates a software bug which should cause a
     panic.  In that case, they will never return.

READING AND WRITING A SINGLE LOCATION MULTIPLE TIMES
     Some devices implement single locations in bus space which are to be read
     or written multiple times to communicate data, e.g., some ethernet
     devices' packet buffer FIFOs.  In order to allow drivers to manipulate
     these types of devices as efficiently as possible, the
     bus_space_read_multi_N() and bus_space_write_multi_N() families of func-
     tions are provided.

     bus_space_read_multi_1(space, handle, offset, datap, count)
     bus_space_read_multi_2(space, handle, offset, datap, count)
     bus_space_read_multi_4(space, handle, offset, datap, count)
     bus_space_read_multi_8(space, handle, offset, datap, count)

     The bus_space_read_multi_N() family of functions reads count 1, 2, 4, or
     8 byte data items from bus space at byte offset offset in the region
     specified by handle of the bus space specified by space and writes them
     into the array specified by datap.  Each successive data item is read
     from the same location in bus space.  The location being read must lie
     within the bus space region specified by handle.

     For portability, the starting address of the region specified by handle
     plus the offset should be a multiple of the size of data items being read
     and the data array pointer should be properly aligned.  On some systems,
     not obeying these requirements may cause incorrect data to be read, on
     others it may cause a system crash.

     Read operations done by the bus_space_read_multi_N() functions may be
     executed out of order with respect to other pending read and write opera-
     tions unless order is enforced by use of the bus_space_barrier() func-
     tion.  Because the bus_space_read_multi_N() functions read the same bus
     space location multiple times, they place an implicit read barrier
     between each successive read of that bus space location.

     These functions will never fail.  If they would fail (e.g., because of an
     argument error), that indicates a software bug which should cause a
     panic.  In that case, they will never return.

     bus_space_write_multi_1(space, handle, offset, datap, count)
     bus_space_write_multi_2(space, handle, offset, datap, count)
     bus_space_write_multi_4(space, handle, offset, datap, count)
     bus_space_write_multi_8(space, handle, offset, datap, count)

     The bus_space_write_multi_N() family of functions reads count 1, 2, 4, or
     8 byte data items from the array specified by datap and writes them into
     bus space at byte offset offset in the region specified by handle of the
     bus space specified by space.  Each successive data item is written to
     the same location in bus space.  The location being written must lie
     within the bus space region specified by handle.

     For portability, the starting address of the region specified by handle
     plus the offset should be a multiple of the size of data items being
     written and the data array pointer should be properly aligned.  On some
     systems, not obeying these requirements may cause incorrect data to be
     written, on others it may cause a system crash.

     Write operations done by the bus_space_write_multi_N() functions may be
     executed out of order with respect to other pending read and write opera-
     tions unless order is enforced by use of the bus_space_barrier() func-
     tion.  Because the bus_space_write_multi_N() functions write the same bus
     space location multiple times, they place an implicit write barrier
     between each successive write of that bus space location.

     These functions will never fail.  If they would fail (e.g., because of an
     argument error), that indicates a software bug which should cause a
     panic.  In that case, they will never return.

STREAM FUNCTIONS
     Most of the bus_space functions imply a host byte-order and a bus byte-
     order and take care of any translation for the caller.  In some cases,
     however, hardware may map a FIFO or some other memory region for which
     the caller may want to use multi-word, yet untranslated access.  Access
     to these types of memory regions should be with the
     bus_space_*_stream_N() functions.

     bus_space_read_stream_1(space, handle, offset)
     bus_space_read_stream_2(space, handle, offset)
     bus_space_read_stream_4(space, handle, offset)
     bus_space_read_stream_8(space, handle, offset)
     bus_space_read_multi_stream_1(space, handle, offset, datap, count)
     bus_space_read_multi_stream_2(space, handle, offset, datap, count)
     bus_space_read_multi_stream_4(space, handle, offset, datap, count)
     bus_space_read_multi_stream_8(space, handle, offset, datap, count)
     bus_space_read_region_stream_1(space, handle, offset, datap, count)
     bus_space_read_region_stream_2(space, handle, offset, datap, count)
     bus_space_read_region_stream_4(space, handle, offset, datap, count)
     bus_space_read_region_stream_8(space, handle, offset, datap, count)
     bus_space_write_stream_1(space, handle, offset, value)
     bus_space_write_stream_2(space, handle, offset, value)
     bus_space_write_stream_4(space, handle, offset, value)
     bus_space_write_stream_8(space, handle, offset, value)
     bus_space_write_multi_stream_1(space, handle, offset, datap, count)
     bus_space_write_multi_stream_2(space, handle, offset, datap, count)
     bus_space_write_multi_stream_4(space, handle, offset, datap, count)
     bus_space_write_multi_stream_8(space, handle, offset, datap, count)
     bus_space_write_region_stream_1(space, handle, offset, datap, count)
     bus_space_write_region_stream_2(space, handle, offset, datap, count)
     bus_space_write_region_stream_4(space, handle, offset, datap, count)
     bus_space_write_region_stream_8(space, handle, offset, datap, count)

     These functions are defined just as their non-stream counterparts, except
     that they provide no byte-order translation.

IMPLEMENTING BUS SPACES IN MACHINE-INDEPENDENT CODE
     bus_space_tag_create(obst, present, extpresent, ov, ctx, bstp)
     Create a copy of the tag obst at *bstp.  Except for the behavior overrid-
     den by ov, *bstp inherits the behavior of obst under bus_space calls.

     ov contains function pointers corresponding to bus_space routines.  Each
     function pointer has a corresponding bit in present or extpresent, and if
     that bit is 1, the function pointer overrides the corresponding bus_space
     call for the new tag.  Any combination of these bits may be set in
     present:

     BUS_SPACE_OVERRIDE_MAP
     BUS_SPACE_OVERRIDE_UNMAP
     BUS_SPACE_OVERRIDE_ALLOC
     BUS_SPACE_OVERRIDE_FREE
     BUS_SPACE_OVERRIDE_RESERVE
     BUS_SPACE_OVERRIDE_RELEASE
     BUS_SPACE_OVERRIDE_RESERVATION_MAP
     BUS_SPACE_OVERRIDE_RESERVATION_UNMAP
     BUS_SPACE_OVERRIDE_RESERVE_SUBREGION

     bus_space_tag_create() does not copy ov.  After a new tag is created by
     bus_space_tag_create(), ov must not be destroyed until after the tag is
     destroyed by bus_space_tag_destroy().

     The first argument of every override-function is a void *, and ctx is
     passed in that argument.

     Return 0 if the call succeeds.  Return EOPNOTSUPP if the architecture
     does not support overrides.  Return EINVAL if present is 0, if ov is
     NULL, or if present indicates that an override is present, but the corre-
     sponding override in ov is NULL.

     If the call does not succeed, *bstp is undefined.
     bus_space_tag_destroy(bst)
     Destroy a tag, bst, created by a prior call to bus_space_tag_create().
     If bst was not created by bus_space_tag_create(), results are undefined.
     If bst was already destroyed, results are undefined.

EXPECTED CHANGES TO THE BUS_SPACE FUNCTIONS
     The definition of the bus_space functions should not yet be considered
     finalized.  There are several changes and improvements which should be
     explored, including:

        Providing a mechanism by which incorrectly-written drivers will be
         automatically given barriers and properly-written drivers won't be
         forced to use more barriers than they need.  This should probably be
         done via a #define in the incorrectly-written drivers.  Unfortu-
         nately, at this time, few drivers actually use barriers correctly (or
         at all).  Because of that, bus_space implementations on architectures
         which do buffering must always do the barriers inside the bus_space
         calls, to be safe.  That has a potentially significant performance
         impact.

        Exporting the bus_space functions to user-land so that applications
         (such as X servers) have easier, more portable access to device
         space.

        Redefining bus space tags and handles so that machine-independent bus
         interface drivers (for example PCI to VME bridges) could define and
         implement bus spaces without requiring machine-dependent code.  If
         this is done, it should be done in such a way that machine-dependent
         optimizations should remain possible.

        Converting bus spaces (such as PCI configuration space) which cur-
         rently use space-specific access methods to use the bus_space func-
         tions where that is appropriate.

        Redefining the way bus space is mapped and allocated, so that mapping
         and allocation are done with bus specific functions which return bus
         space tags.  This would allow further optimization than is currently
         possible, and would also ease translation of the bus_space functions
         into user space (since mapping in user space would look like it just
         used a different bus-specific mapping function).

COMPATIBILITY
     The current version of the bus_space interface specification differs
     slightly from the original specification that came into wide use.  A few
     of the function names and arguments have changed for consistency and
     increased functionality.  Drivers that were written to the old, depre-
     cated specification can be compiled by defining the
     __BUS_SPACE_COMPAT_OLDDEFS preprocessor symbol before including
     <sys/bus.h>.

SEE ALSO
     bus_dma(9), mb(9)

HISTORY
     The bus_space functions were introduced in a different form (memory and
     I/O spaces were accessed via different sets of functions) in NetBSD 1.2.
     The functions were merged to work on generic ``spaces'' early in the
     NetBSD 1.3 development cycle, and many drivers were converted to use
     them.  This document was written later during the NetBSD 1.3 development
     cycle and the specification was updated to fix some consistency problems
     and to add some missing functionality.

AUTHORS
     The bus_space interfaces were designed and implemented by the NetBSD
     developer community.  Primary contributors and implementors were Chris
     Demetriou, Jason Thorpe, and Charles Hannum, but the rest of the NetBSD
     developers and the user community played a significant role in develop-
     ment.

     Chris Demetriou wrote this manual page.

NetBSD 6.0.1                     July 6, 2011                     NetBSD 6.0.1

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