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BUS_SPACE(9)

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NAME    [Toc]    [Back]

     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_map, 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_set_region_1, bus_space_set_region_2,
     bus_space_set_region_4, bus_space_set_region_8, bus_space_subregion,
     bus_space_unmap, bus_space_vaddr, bus_space_mmap, 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    [Toc]    [Back]

     #include <machine/bus.h>

     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_peek_1(bus_space_tag_t space, bus_space_handle_t handle,
             bus_size_t offset, u_int8_t *datap);

     int
     bus_space_peek_2(bus_space_tag_t space, bus_space_handle_t handle,
             bus_size_t offset, u_int16_t *datap);

     int
     bus_space_peek_4(bus_space_tag_t space, bus_space_handle_t handle,
             bus_size_t offset, u_int32_t *datap);

     int
     bus_space_peek_8(bus_space_tag_t space, bus_space_handle_t handle,
             bus_size_t offset, u_int64_t *datap);

     int
     bus_space_poke_1(bus_space_tag_t space, bus_space_handle_t handle,
             bus_size_t offset, u_int8_t data);

     int
     bus_space_poke_2(bus_space_tag_t space, bus_space_handle_t handle,
             bus_size_t offset, u_int16_t data);

     int
     bus_space_poke_4(bus_space_tag_t space, bus_space_handle_t handle,
             bus_size_t offset, u_int32_t data);

     int
     bus_space_poke_8(bus_space_tag_t space, bus_space_handle_t handle,
             bus_size_t offset, u_int64_t data);

     u_int8_t
     bus_space_read_1(bus_space_tag_t space, bus_space_handle_t handle,
             bus_size_t offset);

     u_int16_t
     bus_space_read_2(bus_space_tag_t space, bus_space_handle_t handle,
             bus_size_t offset);

     u_int32_t
     bus_space_read_4(bus_space_tag_t space, bus_space_handle_t handle,
             bus_size_t offset);

     u_int64_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, u_int8_t value);

     void
     bus_space_write_2(bus_space_tag_t space, bus_space_handle_t handle,
             bus_size_t offset, u_int16_t value);

     void
     bus_space_write_4(bus_space_tag_t space, bus_space_handle_t handle,
             bus_size_t offset, u_int32_t value);

     void
     bus_space_write_8(bus_space_tag_t space, bus_space_handle_t handle,
             bus_size_t offset, u_int64_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, u_int8_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, u_int16_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, u_int32_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, u_int64_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, u_int8_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, u_int16_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, u_int32_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, u_int64_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 u_int8_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 u_int16_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 u_int32_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 u_int64_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 u_int8_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 u_int16_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 u_int32_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 u_int64_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, u_int8_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, u_int16_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, u_int32_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, u_int64_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, u_int8_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, u_int16_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, u_int32_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, u_int64_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, u_int8_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, u_int16_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, u_int32_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, u_int64_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 u_int8_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 u_int16_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 u_int32_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 u_int64_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 u_int8_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 u_int16_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 u_int32_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 u_int64_t *datap, bus_size_t count);

DESCRIPTION    [Toc]    [Back]

     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 ~ <machine/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 memory
 space on a second system.  On a third system, architectural limitations
 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 document,
 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 functions.
  Implementations are encouraged to implement prototyped (typechecked)
 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.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    [Toc]    [Back]

     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.  Architectures
 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 handles,
 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 memory
 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 barriers.


     Ordering barriers prevent some operations from bypassing other operations.
  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
     preceeding 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 exceptions
 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 behaviour
     results.  In that case, they may cause the system to halt, either intentionally
 (via panic) or unintentionally (by causing a fatal trap of by
     some other means) or may cause improper operation which is not immediately
 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    [Toc]    [Back]

     Several types are defined in ~ <machine/bus.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 unmapping
 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 mapping
 regions and offsets into regions when performing space access operations.


     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.

MAPPING AND UNMAPPING BUS SPACE    [Toc]    [Back]

     Bus space must be mapped before it can be used, and should be unmapped
     when it is no longer needed.  The bus_space_map() 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 maps the region of bus space named by the
     space, address, and size 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 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 compatibility.


           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 dereferencing
                                    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 software
 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 specified, 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
     spaces.  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 prefetchable
     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 no there is no device on the
     bus.  If space in those areas is accessed, the results are bus-dependent.

     bus_space_unmap(space, handle, size)

     The bus_space_unmap() function unmaps a region of bus space 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_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 nonzero
 and leaves the bus space handle pointed to by nhandlep in an undefined
 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 subregions
 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 indicated
 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.

ALLOCATING AND FREEING BUS SPACE    [Toc]    [Back]

     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() and bus_space_free() functions provide these capabilities.


     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 constraint
 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 constraint,
 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 undefined
 behaviour.  The bus_space_subregion() function can be used on handles
 created by bus_space_alloc().

     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    [Toc]    [Back]

     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 functions
 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 written.
  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 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.

PROBING BUS SPACE FOR HARDWARE WHICH MAY NOT RESPOND    [Toc]    [Back]

     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 gracefully
 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 complete
 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, therefore,
 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 specified
 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    [Toc]    [Back]

     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 barrier
 may be issued.

     Those flags can be combined (or-ed together) to enforce ordering on different
 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 offset
 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 second
 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    [Toc]    [Back]

     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 memory,
 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, 

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