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

     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_sg, bus_dmamap_load_uio, bus_dmamap_unload,
     bus_dmamap_sync, bus_dmamem_alloc,	bus_dmamem_free	-- Bus and Machine
     Independent DMA Mapping Interface

     #include <machine/bus.h>

     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_lock_t	*lockfunc, void	*lockfuncarg, bus_dma_tag_t *dmat);

     bus_dma_tag_destroy(bus_dma_tag_t dmat);

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

     bus_dmamap_destroy(bus_dma_tag_t dmat, bus_dmamap_t map);

     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);

     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);

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

     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);

     bus_dmamap_unload(bus_dma_tag_t dmat, bus_dmamap_t	map);

     bus_dmamap_sync(bus_dma_tag_t dmat, bus_dmamap_t map, op);

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

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

     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

     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.

	      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-

	      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 whose	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

	      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.

	      A	machine-dependent opaque type describing an individual map-
	      ping.  One map is	used for each memory allocation	that will be
	      loaded.  Maps can	be reused once they have been unloaded.	 Mul-
	      tiple maps can be	associated with	one DMA	tag.  While the	value
	      of the map may evaluate to NULL on some platforms	under certain
	      conditions, it should never be assumed that it will be NULL in
	      all cases.

	      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-

	      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

	      Memory synchronization operation specifier.  Bus DMA requires
	      explicit synchronization of memory with its 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.	 The operations	are
	      represented as bitfield flags that can be	combined together,
	      though it	only makes sense to combine PRE	flags or POST flags,
	      not both.	 See the bus_dmamap_sync() description below for more
	      details on how to	use these operations.

	      All operations specified below are performed from	the host mem-
	      ory point	of view, where a read implies data coming from the
	      device to	the host memory, and a write implies data going	from
	      the host memory to the device.  Alternately, the operations can
	      be thought of in terms of	driver operations, where reading a
	      network packet or	storage	sector corresponds to a	read operation
	      in bus_dma.

	      BUS_DMASYNC_PREREAD    Perform any synchronization required
				     prior to an update	of host	memory by the
				     DMA read operation.

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

	      BUS_DMASYNC_POSTREAD   Perform any synchronization required
				     after DMA read operations and prior to
				     CPU access	to host	memory.

	      BUS_DMASYNC_POSTWRITE  Perform any synchronization required
				     after DMA write operations.

	      Client specified lock/mutex manipulation method.	This will be
	      called from within busdma	whenever a client lock needs to	be
	      manipulated.  In its current form, the function will be called
	      immediately before the callback for a dma	load operation that
	      has been deferred	with BUS_DMA_LOCK and immediately after	with
	      BUS_DMA_UNLOCK.  If the load operation does not need to be
	      deferred,	then it	will not be called since the function loading
	      the map should be	holding	the appropriate	locks.	This method is
	      of the format:

	      void     lockfunc(void *lockfunc_arg, bus_dma_lock_op_t op)

	      Two lockfunc implementations are provided	for convenience.
	      busdma_lock_mutex() performs standard mutex operations on	the
	      sleep mutex provided via the lockfuncarg.	 passed	into
	      bus_dma_tag_create().  dflt_lock() will generate a system	panic
	      if it is called.	It is substituted into the tag when lockfunc
	      is passed	as NULL	to bus_dma_tag_create().

	      Operations to be performed by the	client-specified lockfunc().

	      BUS_DMA_LOCK    Acquires and/or locks the	client locking primi-

	      BUS_DMA_UNLOCK  Releases and/or unlocks the client locking prim-

     bus_dma_tag_create(parent,	alignment, boundary, lowaddr, highaddr,
	      *filtfunc, *filtfuncarg, maxsize,	nsegments, maxsegsz, flags,
	      lockfunc,	lockfuncarg, *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 a power
			    of 2 and must be no	smaller	than the maximum seg-
			    ment size.	`0' indicates that there are no	bound-
			    ary	restrictions.
	      highaddr	    Bounds of the window of bus	address	space that
			    cannot be directly accessed	by the device.	The
			    window contains all	addresses 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
	      maxsegsz	    Maximum size, in bytes, of a segment in any	DMA
			    mapped region associated with dmat.
	      flags	    Are	as follows:
			    BUS_DMA_ALLOCNOW  Pre-allocate enough resources to
					      handle at	least one map load
					      operation	on this	tag.  If suf-
					      ficient resources	are not	avail-
					      able, ENOMEM is returned.	 This
					      should not be used for tags that
					      only describe buffers that will
					      be allocated with
					      bus_dmamem_alloc().  Also, due
					      to resource sharing with other
					      tags, this flag does not guaran-
					      tee that resources will be allo-
					      cated or reserved	exclusively
					      for this tag.  It	should be
					      treated only as a	minor opti-
	      lockfunc	    Optional lock manipulation function	(may be	NULL)
			    to be called when busdma needs to manipulate a
			    lock on behalf of the client.  If NULL is speci-
			    fied, dflt_lock() is used.
	      lockfuncarg   Optional argument to be passed to the function
			    specified by lockfunc.
	      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.

	      Deallocate the DMA tag dmat that was created by

	      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	 The value of this argument is currently undefined and
			 should	be specified as	`0'.
	      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,	callback_arg, flags)
	      Creates a	mapping	in device visible address space	of buflen
	      bytes of buf, associated with the	DMA map	map.  This call	will
	      always return immediately	and will not block for any reason.
	      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
	      buflen  The size of the buffer.
	      callback callback_arg
		      The callback function, and its argument.	This function
		      is called	once sufficient	mapping	resources are avail-
		      able for the DMA operation.  If resources	are temporar-
		      ily unavailable, this function will be deferred until
		      later, but the load operation will still return immedi-
		      ately to the caller.  Thus, callers should not assume
		      that the callback	will be	called before the load
		      returns, and code	should be structured appropriately to
		      handle this.  See	below for specific flags and error
		      codes that control this behavior.
	      flags   Are as follows:
		      BUS_DMA_NOWAIT  The load should not be deferred in case
				      of insufficient mapping resources, and
				      instead should return immediately	with
				      an appropriate error.

	      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
	      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.	To ensure that ordering	is guaranteed,
			   all subsequent load requests	will also be deferred
			   until all callbacks have been processed.
	      ENOMEM	   The load request has	failed due to insufficient
			   resources, and the caller specifically used the
			   BUS_DMA_NOWAIT flag.
	      EINVAL	   The load request was	invalid.  The callback has
			   been	called and has been provided the same error.
			   This	error value may	indicate that dmat, map, buf,
			   or callback were invalid, or	buflen was larger than
			   the maxsize argument	used to	create the dma tag

	      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-
	      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.  The BUS_DMA_NOWAIT flag is implied, thus	no callback
	      deferral will happen.

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

	      Beside the error values listed for bus_dmamap_load(), EINVAL
	      will be returned if the size of the mbuf chain exceeds the maxi-
	      mum limit	of the DMA tag.

     bus_dmamap_load_mbuf_sg(dmat, map,	mbuf, segs, nsegs, flags)
	      This is just like	bus_dmamap_load_mbuf() except that it returns
	      immediately without calling a callback function.	It is provided
	      for efficiency.  The scatter/gather segment array	segs is	pro-
	      vided by the caller and filled in	directly by the	function.  The
	      nsegs argument is	returned with the number of segments filled
	      in.  Returns the same errors as bus_dmamap_load_mbuf().

     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.  The BUS_DMA_NOWAIT flag is implied,
	      thus no callback deferral	will happen.  Returns the same errors
	      as bus_dmamap_load().

	      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.  Pages may be locked using vslock(9).

     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
	      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-

     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-
	      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.
					 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-
			 BUS_DMA_ZERO	 Causes	the allocated memory to	be set
					 to all	zeros.
	      mapp	 Pointer to a bus_dmamap_t where the resulting DMA map
			 will be stored.

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

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

	      An initial load operation	is required to obtain the bus address
	      of the allocated memory, and an unload operation is required
	      before freeing the memory, as described below in
	      bus_dmamem_free().  Maps are automatically handled by this func-
	      tion and should not be explicitly	allocated or destroyed.

	      Although an explicit load	is not required	for each access	to the
	      memory referenced	by the returned	map, the synchronization
	      requirements as described	in the bus_dmamap_sync() section still
	      apply and	should be used to achieve portability on architecu-
	      tures without coherent buses.

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

     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.

     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.

     devclass(9), device(9), driver(9),	rman(9), vslock(9)

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

     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.

     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

     The bus_dma interface in FreeBSD benefits from the	contributions of
     Justin T. Gibbs, Peter Wemm, Doug Rabson, Matthew N. Dodd,	Sam Leffler,
     Maxime Henrion, Jake Burkholder, Takahashi	Yoshihiro, Scott Long and many

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

FreeBSD	11.1		       December	5, 2005			  FreeBSD 11.1


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