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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_dma_template_init,
     bus_dma_template_tag, bus_dma_template_clone, bus_dma_template_fill,
     BUS_DMA_TEMPLATE_FILL, bus_dmamap_create, bus_dmamap_destroy,
     bus_dmamap_load, bus_dmamap_load_bio, bus_dmamap_load_ccb,
     bus_dmamap_load_crp, bus_dmamap_load_crp_buffer, 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 In-
     dependent DMA Mapping Interface

     #include <machine/bus.h>

     bus_dma_tag_create(bus_dma_tag_t parent, bus_size_t alignment,
	 bus_addr_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_dma_template_init(bus_dma_template_t *template,
	 bus_dma_tag_t parent);

     bus_dma_template_tag(bus_dma_template_t *template,	bus_dma_tag_t *dmat);

     bus_dma_template_clone(bus_dma_template_t *template, bus_dma_tag_t	dmat);

     bus_dma_template_fill(bus_dma_template_t *template,
	 bus_dma_param_t params[], u_int count);

     BUS_DMA_TEMPLATE_FILL(bus_dma_template_t *template,
	 bus_dma_param_t param ...);

     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_bio(bus_dma_tag_t dmat, bus_dmamap_t map,
	 struct	bio *bio, bus_dmamap_callback_t	*callback, void	*callback_arg,
	 int flags);

     bus_dmamap_load_ccb(bus_dma_tag_t dmat, bus_dmamap_t map, union ccb *ccb,
	 bus_dmamap_callback_t *callback, void *callback_arg, int flags);

     bus_dmamap_load_crp(bus_dma_tag_t dmat, bus_dmamap_t map,
	 struct	crypto *crp, bus_dmamap_callback_t *callback,
	 void *callback_arg, int flags);

     bus_dmamap_load_crp_buffer(bus_dma_tag_t dmat, bus_dmamap_t map,
	 struct	crypto_buffer *cb, 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 in-
     volving 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 tag structure (bus_dma_tag_t) is	used to	describe the properties	of a
     group of related DMA transactions.	 One way to view this is that a	tag
     describes the limitations of a DMA	engine.	 For example, if a DMA engine
     in	a device is limited to 32-bit addresses, that limitation is specified
     by	a parameter when creating the tag for that device.  Similarly, a tag
     can be marked as requiring	buffers	whose addresses	are aligned to a spe-
     cific boundary.

     Some devices may require multiple tags to describe	DMA transactions with
     differing properties.  For	example, a device might	require	16-byte	align-
     ment of its descriptor ring while permitting arbitrary alignment of I/O
     buffers.  In this case, the driver	must create one	tag for	the descriptor
     ring and a	separate tag for I/O buffers.  If a device has restrictions
     that are common to	all DMA	transactions in	addition to restrictions that
     differ between unrelated groups of	transactions, the driver can first
     create a "parent" tag that	decribes the common restrictions.  The per-
     group tags	can then inherit these restrictions from this "parent" tag
     rather than having	to list	them explicitly	when creating the per-group

     A mapping structure (bus_dmamap_t)	represents a mapping of	a memory re-
     gion for DMA.  On systems with I/O	MMUs, the mapping structure tracks any
     I/O MMU entries used by a request.	 For DMA requests that require bounce
     pages, the	mapping	tracks the bounce pages	used.

     To	prepare	for one	or more	DMA transactions, a mapping must be bound to a
     memory region by calling one of the bus_dmamap_load() functions.  These
     functions configure the mapping which can include programming entries in
     an	I/O MMU	and/or allocating bounce pages.	 An output of these functions
     (either directly or indirectly by invoking	a callback routine) is the
     list of scatter/gather address ranges a consumer can pass to a DMA	engine
     to	access the memory region.  When	a mapping is no	longer needed, the
     mapping must be unloaded via bus_dmamap_unload().

     Before and	after each DMA transaction, bus_dmamap_sync() must be used to
     ensure that the correct data is used by the DMA engine and	the CPU.  If a
     mapping uses bounce pages,	the sync operations copy data between the
     bounce pages and the memory region	bound to the mapping.  Sync operations
     also handle architecture-specific details such as CPU cache flushing and
     CPU memory	operation ordering.

     bus_dma handles two types of DMA transactions: static and dynamic.
     Static transactions are used with a long-lived memory region that is
     reused for	many transactions such as a descriptor ring.  Dynamic transac-
     tions are used for	transfers to or	from transient buffers such as I/O
     buffers holding a network packet or disk block.  Each transaction type
     uses a different subset of	the bus_dma API.

   Static Transactions
     Static transactions use memory regions allocated by bus_dma.  Each	static
     memory region is allocated	by calling bus_dmamem_alloc().	This function
     requires a	valid tag describing the properties of the DMA transactions to
     this region such as alignment or address restrictions.  Multiple regions
     can share a single	tag if they share the same restrictions.

     bus_dmamem_alloc()	allocates a memory region along	with a mapping object.
     The associated tag, memory	region,	and mapping object must	then be	passed
     to	bus_dmamap_load() to bind the mapping to the allocated region and ob-
     tain the scatter/gather list.

     It	is expected that bus_dmamem_alloc() will attempt to allocate memory
     requiring less expensive sync operations (for example, implementations
     should not	allocate regions requiring bounce pages), but sync operations
     should still be used.  For	example, a driver should use bus_dmamap_sync()
     in	an interrupt handler before reading descriptor ring entries written by
     the device	prior to the interrupt.

     When a consumer is	finished with a	memory region, it should unload	the
     mapping via bus_dmamap_unload() and then release the memory region	and
     mapping object via	bus_dmamem_free().

   Dynamic Transactions
     Dynamic transactions map memory regions provided by other parts of	the
     system.  A	tag must be created via	bus_dma_tag_create() to	describe the
     DMA transactions to and from these	memory regions,	and a pool of mapping
     objects must be allocated via bus_dmamap_create() to track	the mappings
     of	any in-flight transactions.

     When a consumer wishes to schedule	a transaction for a memory region, the
     consumer must first obtain	an unused mapping object from its pool of map-
     ping objects.  The	memory region must be bound to the mapping object via
     one of the	bus_dmamap_load() functions.  Before scheduling	the transac-
     tion, the consumer	should sync the	memory region via bus_dmamap_sync()
     with one or more of the "PRE" flags.  After the transaction has com-
     pleted, the consumer should sync the memory region	via bus_dmamap_sync()
     with one or more of the "POST" flags.  The	mapping	can then be unloaded
     via bus_dmamap_unload(), and the mapping object can be returned to	the
     pool of unused mapping objects.

     When a consumer is	no longer scheduling DMA transactions, the mapping ob-
     jects should be freed via bus_dmamap_destroy(), and the tag should	be
     freed via bus_dma_tag_destroy().

	     A machine-dependent (MD) opaque type that describes the charac-
	     teristics of a group 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-

	     A template	is a structure for creating a bus_dma_tag_t from a set
	     of	defaults.  Once	initialized with bus_dma_template_init(), a
	     driver can	over-ride individual fields to suit its	needs.	The
	     following fields start with the indicated default values:

		     alignment	     1
		     boundary	     0
		     lowaddr	     BUS_SPACE_MAXADDR
		     highaddr	     BUS_SPACE_MAXADDR
		     maxsize	     BUS_SPACE_MAXSIZE
		     nsegments	     BUS_SPACE_UNRESTRICTED
		     maxsegsize	     BUS_SPACE_MAXSIZE
		     flags	     0
		     lockfunc	     NULL
		     lockfuncarg     NULL

	     Descriptions of each field	are documented with
	     bus_dma_tag_create().  Note that the filtfunc and filtfuncarg at-
	     tributes of the DMA tag are not supported with templates.

	     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	ad-
	     dress 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.

	     Note: The use of filters is deprecated. Proper operation is not

	     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.  Al-
	     though 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 de-
	     vice's byte order)	is almost always required when presenting seg-
	     ment information to the device.

	     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.

	     Client specified callback for receiving mapping information re-
	     sulting from the load of a	bus_dmamap_t via bus_dmamap_load(),
	     bus_dmamap_load_bio(), bus_dmamap_load_ccb(),
	     bus_dmamap_load_crp(), or bus_dmamap_load_crp_buffer().  Call-
	     backs 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.

	     Client specified callback for receiving mapping information re-
	     sulting 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 ex-
	     plicit synchronization of memory with its device visible mapping
	     in	order to guarantee memory coherency.  The bus_dmasync_op_t al-
	     lows 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	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_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.

	     Client specified lock/mutex manipulation method.  This will be
	     called from within	busdma whenever	a client lock needs to be ma-
	     nipulated.	 In its	current	form, the function will	be called im-
	     mediately 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	de-
	     ferred, 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-

	     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	primi-

     bus_dma_tag_create(parent,	alignment, boundary, lowaddr, highaddr,
	     *filtfunc,	*filtfuncarg, maxsize, nsegments, maxsegsz, flags,
	     lockfunc, lockfuncarg, *dmat)
	     Allocates a DMA tag, and initializes it according to the argu-
	     ments provided:

	     parent	  A parent tag from which to inherit restrictions.
			  The restrictions passed in other arguments can only
			  further tighten the restrictions inherited from the
			  parent tag.

			  All tags created by a	device driver must inherit
			  from the tag returned	by bus_get_dma_tag() to	honor
			  restrictions between the parent bridge, CPU memory,
			  and the device.

	     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	re-
			  quiring 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 re-

	     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 de-
			  vice 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 require 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	de-
			  scribed by lowaddr and highaddr.  A filter function
			  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.

			  Note:	The use	of filters is deprecated. Proper
			  operation is not guaranteed.

	     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

	     nsegments	  Number of discontinuities (scatter/gather segments)
			  allowed in a DMA mapped region.

	     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 de-
					    scribe buffers that	will be	allo-
					    cated 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-

			  BUS_DMA_COHERENT  Indicate that the DMA engine and
					    CPU	are cache-coherent.  Cached
					    memory may be used to back alloca-
					    tions created by
					    bus_dmamem_alloc().	 For
					    bus_dma_tag_create(), the
					    BUS_DMA_COHERENT flag is currently
					    implemented	on arm64.

	     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.

	     Deallocate	the DMA	tag dmat that was created by

	     Returns EBUSY if any DMA maps remain associated with dmat or `0'
	     on	success.

     bus_dma_template_init(*template, parent)
	     Initializes a bus_dma_template_t structure.  If the parent	argu-
	     ment is non-NULL, this parent tag is associated with the template
	     and will be compiled into the dma tag that	is later created.  The
	     values of the parent are not copied into the template.  During
	     tag creation in bus_dma_tag_template(), any parameters from the
	     parent tag	that are more restrictive than what is in the provided
	     template will overwrite what goes into the	new tag.

     bus_dma_template_tag(*template, *dmat)
	     Unpacks a template	into a tag, and	returns	the tag	via the	dmat.
	     All return	values are identical to	bus_dma_tag_create().  The
	     template is not modified by this function,	and can	be reused
	     and/or freed upon return.

     bus_dma_template_clone(*template, dmat)
	     Copies the	fields from an existing	tag to a template.  The	tem-
	     plate does	not need to be initialized first.  All of its fields
	     will be overwritten by the	values contained in the	tag.  When
	     paired with bus_dma_template_tag(), this function is useful for
	     creating copies of	tags.

     bus_dma_template_fill(*template, params[],	count)
	     Fills in the selected fields of the template with the keyed val-
	     ues from the params array.	 This is not meant to be called	di-
	     rectly, use BUS_DMA_TEMPLATE_FILL() instead.

     BUS_DMA_TEMPLATE_FILL(*template, param ...)
	     Fills in the selected fields of the template with a variable num-
	     ber of key-value parameters.  The macros listed below take	an ar-
	     gument of the specified type and encapsulate it into a key-value
	     structure that is directly	usable as a parameter argument.	 Muli-
	     ple parameters may	be provided at once.

		     BD_PARENT()     void *
		     BD_ALIGNMENT()  uintmax_t
		     BD_BOUNDARY()   uintmax_t
		     BD_LOWADDR()    vm_paddr_t
		     BD_HIGHADDR()   vm_paddr_t
		     BD_MAXSIZE()    uintmax_t
		     BD_NSEGMENTS()  uintmax_t
		     BD_MAXSEGSIZE() uintmax_t
		     BD_FLAGS()	     uintmax_t
		     BD_LOCKFUNC()   void *
		     BD_LOCKFUNCARG() void *

     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-

	     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 un-
		     available,	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.

				     The generated transactions	to and from
				     the virtual page are non-cacheable.

	     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.

			  Note that subsequent load operations for the same
			  tag that do not require extra	resources will still
			  succeed.  This may result in out-of-order processing
			  of requests.	If the caller requires the order of
			  requests to be preserved, then the caller is re-
			  quired to stall subsequent requests until a pending
			  request's callback is	invoked.

	     ENOMEM	  The load request has failed due to insufficient re-
			  sources, 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 re-
			  quested allocation size was less than	maxsize.

     bus_dmamap_load_bio(dmat, map, bio, callback, callback_arg, flags)
	     This is a variation of bus_dmamap_load() which maps buffers
	     pointed to	by bio for DMA transfers.  bio may point to either a
	     mapped or unmapped	buffer.

     bus_dmamap_load_ccb(dmat, map, ccb, callback, callback_arg, flags)
	     This is a variation of bus_dmamap_load() which maps data pointed
	     to	by ccb for DMA transfers.  The data for	ccb may	be any of the
	     following types:

	     CAM_DATA_VADDR	The data is a single KVA buffer.

	     CAM_DATA_PADDR	The data is a single bus address range.

	     CAM_DATA_SG	The data is a scatter/gather list of KVA buf-

	     CAM_DATA_SG_PADDR	The data is a scatter/gather list of bus ad-
				dress ranges.

	     CAM_DATA_BIO	The data is contained in a struct bio attached
				to the CCB.

	     bus_dmamap_load_ccb() supports the	following CCB XPT function


     bus_dmamap_load_crp(dmat, map, crp, callback, callback_arg, flags)
	     This is a variation of bus_dmamap_load() which maps the input
	     buffer pointed to by crp for DMA transfers.  The BUS_DMA_NOWAIT
	     flag is implied, thus no callback deferral	will happen.

     bus_dmamap_load_crp_buffer(dmat, map, cb, callback, callback_arg, flags)
	     This is a variation of bus_dmamap_load() which maps the crypto
	     data buffer pointed to by cb for DMA transfers.  The
	     BUS_DMA_NOWAIT flag is implied, thus no callback deferral will

     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 de-
	     ferral 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-

	     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	ac-
		   ceptable 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 ac-
	     cess.  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 be-
	     fore 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

	     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.

					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 im-
					plemented on arm and arm64.

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

					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 im-
					plemented 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 be-
	     fore freeing the memory, as described below in bus_dmamem_free().
	     Maps are automatically handled by this function 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	re-
	     quirements	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.

     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.

     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:


     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:


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

     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	13.0			 May 25, 2020			  FreeBSD 13.0


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