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BUS_DMA(9)		 BSD Kernel Developer's Manual		    BUS_DMA(9)

NAME
     bus_dma, bus_dma_tag_create, bus_dma_tag_destroy, bus_dmamap_create,
     bus_dmamap_destroy, bus_dmamap_load, bus_dmamap_load_mbuf,
     bus_dmamap_load_mbuf_segment, bus_dmamap_load_mbuf_defrag,
     bus_dmamap_load_uio, bus_dmamap_unload, bus_dmamap_sync,
     bus_dmamem_alloc, bus_dmamem_coherent, bus_dmamem_coherent_any,
     bus_dmamem_free — Bus and Machine Independent DMA Mapping Interface

SYNOPSIS
     #include <sys/bus.h>

     int
     bus_dma_tag_create(bus_dma_tag_t parent, bus_size_t alignment,
	 bus_size_t boundary, bus_addr_t lowaddr, bus_addr_t highaddr,
	 bus_dma_filter_t *filtfunc, void *filtfuncarg, bus_size_t maxsize,
	 int nsegments, bus_size_t maxsegsz, int flags, bus_dma_tag_t *dmat);

     int
     bus_dma_tag_destroy(bus_dma_tag_t dmat);

     int
     bus_dmamap_create(bus_dma_tag_t dmat, int flags, bus_dmamap_t *mapp);

     int
     bus_dmamap_destroy(bus_dma_tag_t dmat, bus_dmamap_t map);

     int
     bus_dmamap_load(bus_dma_tag_t dmat, bus_dmamap_t map, void *buf,
	 bus_size_t buflen, bus_dmamap_callback_t *callback,
	 void *callback_arg, int flags);

     int
     bus_dmamap_load_mbuf(bus_dma_tag_t dmat, bus_dmamap_t map,
	 struct mbuf *mbuf, bus_dmamap_callback2_t *callback,
	 void *callback_arg, int flags);

     int
     bus_dmamap_load_mbuf_segment(bus_dma_tag_t dmat, bus_dmamap_t map,
	 struct mbuf *mbuf, bus_dma_segment_t *segs, int maxsegs, int *nsegs,
	 int flags);

     int
     bus_dmamap_load_mbuf_defrag(bus_dma_tag_t dmat, bus_dmamap_t map,
	 struct mbuf **mbuf, bus_dma_segment_t *segs, int maxsegs, int *nsegs,
	 int flags);

     int
     bus_dmamap_load_uio(bus_dma_tag_t dmat, bus_dmamap_t map,
	 struct uio *uio, bus_dmamap_callback2_t *callback,
	 void *callback_arg, int flags);

     int
     bus_dmamem_alloc(bus_dma_tag_t dmat, void **vaddr, int flags,
	 bus_dmamap_t *mapp);

     int
     bus_dmamem_coherent(bus_dma_tag_t parent, bus_size_t alignment,
	 bus_size_t boundary, bus_addr_t lowaddr, bus_addr_t highaddr,
	 bus_size_t maxsize, int flags, bus_dmamem_t *dmem);

     void *
     bus_dmamem_coherent_any(bus_dma_tag_t parent, bus_size_t alignment,
	 bus_size_t maxsize, int flags, bus_dma_tag_t *dtag,
	 bus_dmamap_t *dmap, bus_addr_t *busaddr);

     void
     bus_dmamap_unload(bus_dma_tag_t dmat, bus_dmamap_t map);

     void
     bus_dmamap_sync(bus_dma_tag_t dmat, bus_dmamap_t map,
	 bus_dmasync_op_t op);

     void
     bus_dmamem_free(bus_dma_tag_t dmat, void *vaddr, bus_dmamap_t map);

DESCRIPTION
     Direct Memory Access (DMA) is a method of transferring data without
     involving the CPU, thus providing higher performance.  A DMA transaction
     can be achieved between device to memory, device to device, or memory to
     memory.

     The bus_dma API is a bus, device, and machine-independent (MI) interface
     to DMA mechanisms.	 It provides the client with flexibility and simplic‐
     ity by abstracting machine dependent issues like setting up DMA mappings,
     handling cache issues, bus specific features and limitations.

STRUCTURES AND TYPES
     bus_dma_tag_t
	      A machine-dependent (MD) opaque type that describes the charac‐
	      teristics of DMA transactions.  DMA tags are organized into a
	      hierarchy, with each child tag inheriting the restrictions of
	      its parent.  This allows all devices along the path of DMA
	      transactions to contribute to the constraints of those transac‐
	      tions.

     bus_dma_filter_t
	      Client specified address filter having the format:

	      int      client_filter(void *filtarg, bus_addr_t testaddr)

	      Address filters can be specified during tag creation to allow
	      for devices who's DMA address restrictions cannot be specified
	      by a single window.  The filtarg is client specified during tag
	      creation to be passed to all invocations of the callback.	 The
	      testaddr argument contains a potential starting address of a DMA
	      mapping.	The filter function operates on the set of addresses
	      from testaddr to ‘trunc_page(testaddr) + PAGE_SIZE - 1’, inclu‐
	      sive.  The filter function should return zero for any mapping in
	      this range that can be accommodated by the device and non-zero
	      otherwise.

     bus_dma_segment_t
	      A machine-dependent type that describes individual DMA segments.

		      bus_addr_t      ds_addr;
		      bus_size_t      ds_len;

	      The ds_addr field contains the device visible address of the DMA
	      segment, and ds_len contains the length of the DMA segment.
	      Although the DMA segments returned by a mapping call will adhere
	      to all restrictions necessary for a successful DMA operation,
	      some conversion (e.g. a conversion from host byte order to the
	      device's byte order) is almost always required when presenting
	      segment information to the device.

     bus_dmamap_t
	      A machine-dependent opaque type describing an individual map‐
	      ping.  Multiple DMA maps can be associated with one DMA tag.

     bus_dmamem_t
	      A machine-dependent type that describes DMA memory created by
	      bus_dmamem_coherent().

		      bus_dma_tag_t   dmem_tag;
		      bus_dmamap_t    dmem_map;
		      void	      *dmem_addr;
		      bus_addr_t      dmem_busaddr;

	      The dmem_tag field contains the DMA tag of the DMA memory and
	      dmem_map field contains the DMA map of the DMA memory.  The
	      dmem_addr field points to the allocated DMA memory in kernel
	      virtual address space.  The dmem_busaddr field contains the
	      device visible address of the DMA memory.

     bus_dmamap_callback_t
	      Client specified callback for receiving mapping information
	      resulting from the load of a bus_dmamap_t via bus_dmamap_load().
	      Callbacks are of the format:

	      void     client_callback(void *callback_arg, bus_dma_segment_t
		       *segs, int nseg, int error)

	      The callback_arg is the callback argument passed to dmamap load
	      functions.  The segs and nseg parameters describe an array of
	      bus_dma_segment_t structures that represent the mapping.	This
	      array is only valid within the scope of the callback function.
	      The success or failure of the mapping is indicated by the error
	      parameter.  More information on the use of callbacks can be
	      found in the description of the individual dmamap load func‐
	      tions.

     bus_dmamap_callback2_t
	      Client specified callback for receiving mapping information
	      resulting from the load of a bus_dmamap_t via
	      bus_dmamap_load_uio() or bus_dmamap_load_mbuf().

	      Callback2s are of the format:

	      void     client_callback2(void *callback_arg, bus_dma_segment_t
		       *segs, int nseg, bus_size_t mapsize, int error)

	      Callback2's behavior is the same as bus_dmamap_callback_t with
	      the addition that the length of the data mapped is provided via
	      mapsize.

     bus_dmasync_op_t
	      Memory synchronization operation specifier.  Bus DMA requires
	      explicit synchronization of memory with it's device visible map‐
	      ping in order to guarantee memory coherency.  The
	      bus_dmasync_op_t allows the type of DMA operation that will be
	      or has been performed to be communicated to the system so that
	      the correct coherency measures are taken.	 All operations speci‐
	      fied below are performed from the DMA engine's point of view:

	      BUS_DMASYNC_PREREAD    Perform any synchronization required
				     after an update of memory by the CPU but
				     prior to DMA read operations.

	      BUS_DMASYNC_PREWRITE   Perform any synchronization required
				     after an update of memory by the CPU but
				     prior to DMA write operations.

	      BUS_DMASYNC_POSTREAD   Perform any synchronization required
				     after DMA read operations, but prior to
				     CPU access of the memory.

	      BUS_DMASYNC_POSTWRITE  Perform any synchronization required
				     after DMA write operations, but prior to
				     CPU access of the memory.

FUNCTIONS
     bus_dma_tag_create(parent, alignment, boundary, lowaddr, highaddr,
	      *filtfunc, *filtfuncarg, maxsize, nsegments, maxsegsz, flags,
	      *dmat)
	      Allocates a device specific DMA tag, and initializes it accord‐
	      ing to the arguments provided:
	      parent	    Indicates restrictions between the parent bridge,
			    CPU memory, and the device.	 May be NULL, if no
			    DMA restrictions are to be inherited.
	      alignment	    Alignment constraint, in bytes, of any mappings
			    created using this tag.  The alignment must be a
			    power of 2.	 Hardware that can DMA starting at any
			    address would specify 1 for byte alignment.	 Hard‐
			    ware requiring DMA transfers to start on a multi‐
			    ple of 4K would specify 4096.
	      boundary	    Boundary constraint, in bytes, of the target DMA
			    memory region.  The boundary indicates the set of
			    addresses, all multiples of the boundary argument,
			    that cannot be crossed by a single
			    bus_dma_segment_t.	The boundary must be either a
			    power of 2 or 0.  ‘0’ indicates that there are no
			    boundary restrictions.
	      lowaddr
	      highaddr	    Bounds of the window of bus address space that
			    cannot be directly accessed by the device.	The
			    window contains all address greater than lowaddr
			    and less than or equal to highaddr.	 For example,
			    a device incapable of DMA above 4GB, would specify
			    a highaddr of BUS_SPACE_MAXADDR and a lowaddr of
			    BUS_SPACE_MAXADDR_32BIT.  Similarly a device that
			    can only dma to addresses bellow 16MB would spec‐
			    ify a highaddr of BUS_SPACE_MAXADDR and a lowaddr
			    of BUS_SPACE_MAXADDR_24BIT.	 Some implementations
			    requires that some region of device visible
			    address space, overlapping available host memory,
			    be outside the window.  This area of ‘safe memory’
			    is used to bounce requests that would otherwise
			    conflict with the exclusion window.
	      filtfunc	    Optional filter function (may be NULL) to be
			    called for any attempt to map memory into the win‐
			    dow described by lowaddr and highaddr.  A filter
			    function is only required when the single window
			    described by lowaddr and highaddr cannot ade‐
			    quately describe the constraints of the device.
			    The filter function will be called for every
			    machine page that overlaps the exclusion window.
	      filtfuncarg   Argument passed to all calls to the filter func‐
			    tion for this tag.	May be NULL.
	      maxsize	    Maximum size, in bytes, of the sum of all segment
			    lengths in a given DMA mapping associated with
			    this tag.
	      nsegments	    Number of discontinuities (scatter/gather seg‐
			    ments) allowed in a DMA mapped region.  If there
			    is no restriction, BUS_SPACE_UNRESTRICTED may be
			    specified.
	      maxsegsz	    Maximum size, in bytes, of a segment in any DMA
			    mapped region associated with dmat.
	      flags	    Are as follows:
			    BUS_DMA_ALLOCNOW   Allocate the minimum resources
					       necessary to guarantee that all
					       map load operations associated
					       with this tag may not block.
					       If sufficient resources are not
					       available, ENOMEM is returned.
			    BUS_DMA_WAITOK     Indicates that it is OK to wait
					       for resources.  However, unlike
					       kmalloc(9), it is not guaran‐
					       teed that the resource alloca‐
					       tion will succeed.  This flag
					       is the default one, if
					       BUS_DMA_NOWAIT is not supplied.
			    BUS_DMA_NOWAIT     If the resource allocation
					       request cannot be immediately
					       fulfilled, ENOMEM is returned.
			    BUS_DMA_ONEBPAGE   Allocte one bounce page at
					       most, even if the maxsize indi‐
					       cates that multiple bounce
					       pages are needed.
			    BUS_DMA_ALIGNED    Indicates that all memory to be
					       loaded into the DMA maps asso‐
					       ciated with this DMA tag is
					       properly aligned according to
					       alignment constraint.  No
					       resources, e.g. bounce pages,
					       will be allocated due to the
					       alignment constraint.  If
					       unaligned memory was loaded
					       into the DMA maps associated
					       with this DMA tag, system will
					       panic.
			    BUS_DMA_PRIVBZONE  Uses a private bounce zone
					       instead of a shared one.	 A
					       private bounce zone will vanish
					       if the DMA tag is destroyed.
			    BUS_DMA_ALLOCALL   Allocate all required resources
					       (mainly the bounce buffer).  If
					       any allocation fails,
					       bus_dma_tag_create() fails.
			    BUS_DMA_PROTECTED  All of the functions called
					       with the DMA tag are already
					       protected by the caller, so the
					       bus_dma code need not protect
					       the internal data structures.
	      dmat	    Pointer to a bus_dma_tag_t where the resulting DMA
			    tag will be stored.

	      Returns ENOMEM if sufficient memory is not available for tag
	      creation or allocating mapping resources.

     bus_dma_tag_destroy(dmat)
	      Deallocate the DMA tag dmat that was created by
	      bus_dma_tag_create().

	      Returns EBUSY if any DMA maps remain associated with dmat or ‘0’
	      on success.

     bus_dmamap_create(dmat, flags, *mapp)
	      Allocates and initializes a DMA map.  Arguments are as follows:
	      dmat	 DMA tag.
	      flags	 Are as follows:
			 BUS_DMA_WAITOK	   Indicates that it is OK to wait for
					   resources.  However, unlike
					   kmalloc(9), it is not guaranteed
					   that the resource allocation will
					   succeed.  This flag is the default
					   one, if BUS_DMA_NOWAIT is not sup‐
					   plied.
			 BUS_DMA_NOWAIT	   If the resource allocation request
					   cannot be immediately fulfilled,
					   ENOMEM is returned.
			 BUS_DMA_ONEBPAGE  Allocte one bounce page at most,
					   even if the maxsize used to create
					   the dmat indicates that multiple
					   bounce pages are needed.
	      mapp	 Pointer to a bus_dmamap_t where the resulting DMA map
			 will be stored.

	      Returns ENOMEM if sufficient memory is not available for creat‐
	      ing the map or allocating mapping resources.

     bus_dmamap_destroy(dmat, map)
	      Frees all resources associated with a given DMA map.  Arguments
	      are as follows:
	      dmat  DMA tag used to allocate map.
	      map   The DMA map to destroy.

	      Returns EBUSY if a mapping is still active for map.

     bus_dmamap_load(dmat, map, buf, buflen, *callback, ...)
	      Creates a mapping in device visible address space of buflen
	      bytes of buf, associated with the DMA map map.  Arguments are as
	      follows:
	      dmat    DMA tag used to allocate map.
	      map     A DMA map without a currently active mapping.
	      buf     A kernel virtual address pointer to a contiguous (in
		      KVA) buffer, to be mapped into device visible address
		      space.
	      buflen  The size of the buffer.
	      callback callback_arg
		      The callback function, and its argument.
	      flags   The value of this argument is currently undefined, and
		      should be specified as ‘0’.

	      Return values to the caller are as follows:
	      0		   The callback has been called and completed.	The
			   status of the mapping has been delivered to the
			   callback.
	      EINPROGRESS  The mapping has been deferred for lack of
			   resources.  The callback will be called as soon as
			   resources are available.  Callbacks are serviced in
			   FIFO order.	DMA maps created from DMA tags that
			   are allocated with the BUS_DMA_ALLOCNOW flag will
			   never return this status for a load operation.
	      EINVAL	   The load request was invalid.  The callback has
			   not, and will not be called.	 This error value may
			   indicate that dmat, map, buf, or callback were
			   invalid, or buslen was larger than the maxsize
			   argument used to create the dma tag dmat.

	      When the callback is called, it is presented with an error value
	      indicating the disposition of the mapping.  Error may be one of
	      the following:
	      0		   The mapping was successful and the dm_segs callback
			   argument contains an array of bus_dma_segment_t
			   elements describing the mapping.  This array is
			   only valid during the scope of the callback func‐
			   tion.
	      EFBIG	   A mapping could not be achieved within the segment
			   constraints provided in the tag even though the
			   requested allocation size was less than maxsize.

     bus_dmamap_load_mbuf(dmat, map, mbuf, callback2, callback_arg, flags)
	      This is a variation of bus_dmamap_load() which maps mbuf chains
	      for DMA transfers.  A bus_size_t argument is also passed to the
	      callback routine, which contains the mbuf chain's packet header
	      length.

	      Mbuf chains are assumed to be in kernel virtual address space.

	      Returns EINVAL if the size of the mbuf chain exceeds the maximum
	      limit of the DMA tag.

     bus_dmamap_load_mbuf_segment(dmat, map, mbuf, *segs, maxsegs, *nsegs,
	      flags)
	      It is like bus_dmamap_load_mbuf() without callback.  Segmenta‐
	      tion information are saved in the segs and nsegs if the loading
	      is successful.  The maxsegs, which indicates the number of ele‐
	      ments in the segs, must be set by the caller and must be at
	      least 1 but less than the nsegments used to create the dmat.
	      The flags must have BUS_DMA_NOWAIT turned on.

	      This function will not block.  When system is short of DMA
	      resources, this function will return ENOMEM, instead of
	      EINPROGRESS.

     bus_dmamap_load_mbuf_defrag(dmat, map, *mbuf, *segs, maxsegs, *nsegs,
	      flags)
	      This function is like bus_dmamap_load_mbuf_segment(), but it
	      will call m_defrag() on the *mbuf and try reloading, if low
	      level code indicates too many fragments in the *mbuf; the mbuf
	      will be updated under this situation.  However, *mbuf would not
	      be freed by this function, even if m_defrag() failed.

	      Return ENOBUFS, if the calling of m_defrag() failed.

     bus_dmamap_load_uio(dmat, map, uio, callback2, callback_arg, flags)
	      This is a variation of bus_dmamap_load() which maps buffers
	      pointed to by uio for DMA transfers.  A bus_size_t argument is
	      also passed to the callback routine, which contains the size of
	      uio, i.e.	 uio->uio_resid.

	      If uio->uio_segflg is UIO_USERSPACE, then it is assumed that the
	      buffer, uio is in uio->uio_td->td_proc's address space.  User
	      space memory must be in-core and wired prior to attempting a map
	      load operation.

     bus_dmamap_unload(dmat, map)
	      Unloads a DMA map.  Arguments are as follows:
	      dmat  DMA tag used to allocate map.
	      map   The DMA map that is to be unloaded.

	      bus_dmamap_unload() will not perform any implicit synchroniza‐
	      tion of DMA buffers.  This must be done explicitly by a call to
	      bus_dmamap_sync() prior to unloading the map.

     bus_dmamap_sync(dmat, map, op)
	      Performs synchronization of a device visible mapping with the
	      CPU visible memory referenced by that mapping.  Arguments are as
	      follows:
	      dmat  DMA tag used to allocate map.
	      map   The DMA mapping to be synchronized.
	      op    Type of synchronization operation to perform.  See the
		    definition of bus_dmasync_op_t for a description of the
		    acceptable values for op.

	      bus_dmamap_sync() is the method used to ensure that CPU and
	      device DMA access to shared memory is coherent.  For example,
	      the CPU might be used to setup the contents of a buffer that is
	      to be DMA'ed into a device.  To ensure that the data are visible
	      via the device's mapping of that memory, the buffer must be
	      loaded and a dma sync operation of BUS_DMASYNC_PREREAD must be
	      performed.  Additional sync operations must be performed after
	      every CPU write to this memory if additional DMA reads are to be
	      performed.  Conversely, for the DMA write case, the buffer must
	      be loaded, and a dma sync operation of BUS_DMASYNC_PREWRITE must
	      be performed.  The CPU will only be able to see the results of
	      this DMA write once the DMA has completed and a
	      BUS_DMASYNC_POSTWRITE operation has been performed.

	      If DMA read and write operations are not preceded and followed
	      by the appropriate synchronization operations, behavior is unde‐
	      fined.

     bus_dmamem_alloc(dmat, **vaddr, flags, mapp)
	      Allocates memory that is mapped into KVA at the address returned
	      in vaddr that is permanently loaded into the newly created
	      bus_dmamap_t returned via mapp.  Arguments are as follows:
	      dmat	 DMA tag describing the constraints of the DMA map‐
			 ping.
	      vaddr	 Pointer to a pointer that will hold the returned KVA
			 mapping of the allocated region.
	      flags	 Flags are defined as follows:
			 BUS_DMA_WAITOK	 The routine can safely wait (sleep)
					 for resources.
			 BUS_DMA_NOWAIT	 The routine is not allowed to wait
					 for resources.	 If resources are not
					 available, ENOMEM is returned.
			 BUS_DMA_COHERENT
					 Attempt to map this memory such that
					 cache sync operations are as cheap as
					 possible.  This flag is typically set
					 on memory that will be accessed by
					 both a CPU and a DMA engine, fre‐
					 quently.  Use of this flag does not
					 remove the requirement of using
					 bus_dmamap_sync, but it may reduce
					 the cost of performing these opera‐
					 tions.
			 BUS_DMA_ZERO	 Causes the allocated memory to be set
					 to all zeros.
	      mapp	 Pointer to storage for the returned DMA map.

	      The size of memory to be allocated is maxsize as specified in
	      dmat.

	      The current implementation of bus_dmamem_alloc() will allocate
	      all requests as a single segment.

	      Although no explicit loading is required to access the memory
	      referenced by the returned map, the synchronization requirements
	      as described in the bus_dmamap_sync() section still apply.

	      Returns ENOMEM if sufficient memory is not available for com‐
	      pleting the operation.

     bus_dmamem_coherent(parent, alignment, boundary, lowaddr, highaddr,
	      maxsize, flags, *dmem)
	      This is a convenient function to create one segment of DMA mem‐
	      ory.  It combines following bus_dma(9) function calls:

		      bus_dma_tag_create(..., dtag);
		      bus_dmamem_alloc(*dtag, vaddr, ..., dmap);
		      bus_dmamap_load(*dtag, *dmap, *vaddr, ..., \
				      callback, busaddr, ...);

	      The final results of the above function calls are: DMA tag, DMA
	      map, DMA memory's kernel virtual address and its device visible
	      address.	bus_dmamem_coherent() saves the results in *dmem.

	      The parent, alignment, boundary, lowaddr and highaddr will be
	      passed to bus_dma_tag_create() as they are.  The maxsize will be
	      passed to bus_dma_tag_create() as its maxsize and maxsegsz and
	      ‘1’ will be passed to bus_dma_tag_create() as its nsegments.
	      When bus_dmamem_alloc() is called, flags will be first or'ed
	      with BUS_DMA_COHERENT then passed to it.	The final results of
	      the above three functions, i.e. DMA tag, DMA map, DMA memory's
	      kernel virtual address and its device visible address, are saved
	      in *dmem.	 If any of the three functions failed, this function
	      will return the error code and the *dmem should not be used.

     bus_dmamem_coherent_any(parent, alignment, maxsize, flags, *dtag, *dmap,
	      *busaddr)
	      This function is a simplified version of bus_dmamem_coherent()
	      with its boundary set to ‘0’, lowaddr set to BUS_SPACE_MAXADDR
	      and highaddr set to BUS_SPACE_MAXADDR.  The parent usually
	      should not be NULL.

	      Return the DMA memory's kernel virtual address.  The DMA tag,
	      DMA map and device visible address are returned in *dtag, *dmap,
	      and *busaddr.  If this function failed, NULL will be returned;
	      *dtag, *dmap, and *busaddr are left unchanged.

     bus_dmamem_free(dmat, *vaddr, map)
	      Frees memory previously allocated by bus_dmamem_alloc().	Any
	      mappings will be invalidated.  Arguments are as follows:
	      dmat   DMA tag.
	      vaddr  Kernel virtual address of the memory.
	      map    DMA map to be invalidated.

RETURN VALUES
     Behavior is undefined if invalid arguments are passed to any of the above
     functions.	 If sufficient resources cannot be allocated for a given
     transaction, ENOMEM is returned.  All routines that are not of type,
     void, will return 0 on success or an error code, as discussed above.

     All void routines will succeed if provided with valid arguments.

SEE ALSO
     devclass(9), device(9), driver(9), rman(9)

     Jason R. Thorpe, "A Machine-Independent DMA Framework for NetBSD",
     Proceedings of the Summer 1998 USENIX Technical Conference, USENIX
     Association, June 1998.

HISTORY
     The bus_dma interface first appeared in NetBSD 1.3.

     The bus_dma API was adopted from NetBSD for use in the CAM SCSI subsys‐
     tem.  The alterations to the original API were aimed to remove the need
     for a bus_dma_segment_t array stored in each bus_dmamap_t while allowing
     callers to queue up on scarce resources.

AUTHORS
     The bus_dma interface was designed and implemented by Jason R. Thorpe of
     the Numerical Aerospace Simulation Facility, NASA Ames Research Center.
     Additional input on the bus_dma design was provided by Chris Demetriou,
     Charles Hannum, Ross Harvey, Matthew Jacob, Jonathan Stone, and Matt
     Thomas.

     This manual page was written by Hiten Pandya and Justin T. Gibbs.

BSD			       January 11, 2010				   BSD
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