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BUS_DMA(9)	       FreeBSD 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_sg, bus_dmamap_load_uio, bus_dmamap_unload,
     bus_dmamap_sync, bus_dmamem_alloc,	bus_dmamem_free	-- Bus and Machine
     Independent DMA Mapping Interface

SYNOPSIS
     #include <machine/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_lock_t	*lockfunc, void	*lockfuncarg, 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_sg(bus_dma_tag_t dmat, bus_dmamap_t map,
	 struct	mbuf *mbuf, bus_dma_segment_t *segs, 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);

     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, op);

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

     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 transac-
	     tions to contribute to the	constraints of those transactions.

     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 whose DMA address restrictions cannot be specified	by a
	     single window.  The filtarg argument is specified by the client
	     during tag	creation to be passed to all invocations of the	call-
	     back.  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', inclusive.	 The filter function should return
	     zero if any mapping in this range can be accommodated by the
	     device and	non-zero otherwise.

     bus_dma_segment_t
	     A machine-dependent type that describes individual	DMA segments.
	     It	contains the following fields:

		     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 mapping.
	     One map is	used for each memory allocation	that will be loaded.
	     Maps can be reused	once they have been unloaded.  Multiple	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.

     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 arguments 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
	     argument.	More information on the	use of callbacks can be	found
	     in	the description	of the individual dmamap load functions.

     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 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 repre-
	     sented 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	memory
	     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.  Alternatively, 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 device.

	     BUS_DMASYNC_PREWRITE   Perform any	synchronization	required after
				    an update of host memory by	the CPU	and
				    prior to device access to host memory.

	     BUS_DMASYNC_POSTREAD   Perform any	synchronization	required after
				    an update of host memory by	the device and
				    prior to CPU access	to host	memory.

	     BUS_DMASYNC_POSTWRITE  Perform any	synchronization	required after
				    device access to host memory.

     bus_dma_lock_t
	     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)

	     The lockfuncarg argument is specified by the client during	tag
	     creation to be passed to all invocations of the callback.	The op
	     argument specifies	the lock operation to perform.

	     Two lockfunc implementations are provided for convenience.
	     busdma_lock_mutex() performs standard mutex operations on the
	     sleep mutex provided via lockfuncarg.  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() and is
	     useful for	tags that should not be	used with deferred load	opera-
	     tions.

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

	     BUS_DMA_LOCK    Acquires and/or locks the client locking primi-
			     tive.

	     BUS_DMA_UNLOCK  Releases and/or unlocks the client	locking	primi-
			     tive.

FUNCTIONS
     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 according
	     to	the arguments provided:

	     parent	  Indicates restrictions between the parent bridge,
			  CPU memory, and the device.  Each device must	use a
			  master parent	tag by calling bus_get_dma_tag().

	     alignment	  Alignment constraint,	in bytes, of any mappings cre-
			  ated 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.  Hardware
			  requiring DMA	transfers to start on a	multiple 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 segment
			  size.	 `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 win-
			  dow 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 perform DMA to addresses below 16MB would
			  specify a highaddr of	BUS_SPACE_MAXADDR and a
			  lowaddr of BUS_SPACE_MAXADDR_24BIT.  Some implemen-
			  tations 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 con-
			  flict	with the exclusion window.

	     filtfunc	  Optional filter function (may	be NULL) to be called
			  for any attempt to map memory	into the window
			  described by lowaddr and highaddr.  A	filter func-
			  tion is only required	when the single	window
			  described by lowaddr and highaddr cannot adequately
			  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 function
			  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 segments)
			  allowed in a DMA mapped region.  If there is no
			  restriction, BUS_SPACE_UNRESTRICTED may be speci-
			  fied.

	     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 oper-
					    ation on this tag.	If sufficient
					    resources are not available,
					    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
					    guarantee that resources will be
					    allocated or reserved exclusively
					    for	this tag.  It should be
					    treated only as a minor optimiza-
					    tion.

	     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 specified,
			  dflt_lock() is used.

	     lockfuncarg  Optional argument to be passed to the	function spec-
			  ified	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 cre-
	     ation 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_COHERENT  Attempt to map the memory loaded
					  with this map	such that cache	sync
					  operations are as cheap as possible.
					  This flag is typically set on	maps
					  when the memory loaded with these
					  will be accessed by both a CPU and a
					  DMA engine, frequently such as con-
					  trol data and	as opposed to stream-
					  able data such as receive and	trans-
					  mit buffers.	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.  For bus_dmamap_create(), the
					  BUS_DMA_COHERENT flag	is currently
					  implemented on sparc64.

	     mapp	Pointer	to a bus_dmamap_t where	the resulting DMA map
			will be	stored.

	     Returns ENOMEM if sufficient memory is not	available for creating
	     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 space.

	     buflen  The size of the buffer.

	     callback callback_arg
		     The callback function, and	its argument.  This function
		     is	called once sufficient mapping resources are available
		     for the DMA operation.  If	resources are temporarily
		     unavailable, this function	will be	deferred until later,
		     but the load operation will still return immediately 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.

		     BUS_DMA_NOCACHE
				     The generated transactions	to and from
				     the virtual page are non-cacheable.  For
				     bus_dmamap_load(),	the BUS_DMA_NOCACHE
				     flag is currently implemented on sparc64.

	     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.
			  To ensure that ordering is guaranteed, all subse-
			  quent	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 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 ele-
			  ments	describing the mapping.	 This array is only
			  valid	during the scope of the	callback function.

	     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 maximum
	     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 synchronization
	     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 fol-
	     lows:

	     dmat  DMA tag used	to allocate map.

	     map   The DMA mapping to be synchronized.

	     op	   Type	of synchronization operation to	perform.  See the def-
		   inition of bus_dmasync_op_t for a description of the
		   acceptable values for op.

	     The bus_dmamap_sync() function is the method used to ensure that
	     CPU's and device's	direct memory access (DMA) to shared memory is
	     coherent.	For example, the CPU might be used to set up the con-
	     tents of a	buffer that is to be made available to 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_PREWRITE must be performed after the CPU has updated
	     the buffer	and before the device access is	initiated.  If the CPU
	     modifies this buffer again	later, another BUS_DMASYNC_PREWRITE
	     sync operation must be performed before an	additional device
	     access.  Conversely, suppose a device updates memory that is to
	     be	read by	a CPU.	In this	case, the buffer must be loaded, and a
	     DMA sync operation	of BUS_DMASYNC_PREREAD must be performed
	     before the	device access is initiated.  The CPU will only be able
	     to	see the	results	of this	memory update once the DMA operation
	     has completed and a BUS_DMASYNC_POSTREAD sync operation has been
	     performed.

	     If	read and write operations are not preceded and followed	by the
	     appropriate synchronization operations, behavior is undefined.

     bus_dmamem_alloc(dmat, **vaddr, flags, *mapp)
	     Allocates memory that is mapped into KVA at the address returned
	     in	vaddr and 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 mapping.

	     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 in a	coher-
					ent fashion.  See bus_dmamap_create()
					above for a description	of this	flag.
					For bus_dmamem_alloc(),	the
					BUS_DMA_COHERENT flag is currently
					implemented on arm and sparc64.

			BUS_DMA_ZERO	Causes the allocated memory to be set
					to all zeros.

			BUS_DMA_NOCACHE
					The allocated memory will not be
					cached in the processor	caches.	 All
					memory accesses	appear on the bus and
					are executed without reordering.  For
					bus_dmamem_alloc(), the
					BUS_DMA_NOCACHE	flag is	currently
					implemented on amd64 and i386 where it
					results	in the Strong Uncacheable PAT
					to be set for the allocated virtual
					address	range.

	     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
	     call to bus_dma_tag_create() for dmat.

	     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	architectures
	     without coherent buses.

	     Returns ENOMEM if sufficient memory is not	available for complet-
	     ing 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.

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 on failure as discussed above.

     All void routines will succeed if provided	with valid arguments.

LOCKING
     Two locking protocols are used by bus_dma.	 The first is a	private	global
     lock that is used to synchronize access to	the bounce buffer pool on the
     architectures that	make use of them.  This	lock is	strictly a leaf	lock
     that is only used internally to bus_dma and is not	exposed	to clients of
     the API.

     The second	protocol involves protecting various resources stored in the
     tag.  Since almost	all bus_dma operations are done	through	requests from
     the driver	that created the tag, the most efficient way to	protect	the
     tag resources is through the lock that the	driver uses.  In cases where
     bus_dma acts on its own without being called by the driver, the lock
     primitive specified in the	tag is acquired	and released automatically.
     An	example	of this	is when	the bus_dmamap_load() callback function	is
     called from a deferred context instead of the driver context.  This means
     that certain bus_dma functions must always	be called with the same	lock
     held that is specified in the tag.	 These functions include:

	   bus_dmamap_load()
	   bus_dmamap_load_uio()
	   bus_dmamap_load_mbuf()
	   bus_dmamap_load_mbuf_sg()
	   bus_dmamap_unload()
	   bus_dmamap_sync()

     There is one exception to this rule.  It is common	practice to call some
     of	these functions	during driver start-up without any locks held.	So
     long as there is a	guarantee of no	possible concurrent use	of the tag by
     different threads during this operation, it is safe to not	hold a lock
     for these functions.

     Certain bus_dma operations	should not be called with the driver lock
     held, either because they are already protected by	an internal lock, or
     because they might	sleep due to memory or resource	allocation.  The fol-
     lowing functions must not be called with any non-sleepable	locks held:

	   bus_dma_tag_create()
	   bus_dmamap_create()
	   bus_dmamem_alloc()

     All other functions do not	have a locking protocol	and can	thus be	called
     with or without any system	or driver locks	held.

SEE ALSO
     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.

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.

     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
     others.

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

FreeBSD	10.1			 May 12, 2009			  FreeBSD 10.1

NAME | SYNOPSIS | DESCRIPTION | STRUCTURES AND TYPES | FUNCTIONS | RETURN VALUES | LOCKING | SEE ALSO | HISTORY | AUTHORS

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