busdma man page on FreeBSD

Man page or keyword search:  
man Server   9747 pages
apropos Keyword Search (all sections)
Output format
FreeBSD logo
[printable version]

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

NAME
     bus_dma, bus_dma_tag_create, bus_dma_tag_destroy, bus_dmamap_create,
     bus_dmamap_destroy, bus_dmamap_load, bus_dmamap_load_mbuf,
     bus_dmamap_load_mbuf_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.

BSD				 May 12, 2009				   BSD
[top]

List of man pages available for FreeBSD

Copyright (c) for man pages and the logo by the respective OS vendor.

For those who want to learn more, the polarhome community provides shell access and support.

[legal] [privacy] [GNU] [policy] [cookies] [netiquette] [sponsors] [FAQ]
Tweet
Polarhome, production since 1999.
Member of Polarhome portal.
Based on Fawad Halim's script.
...................................................................
Vote for polarhome
Free Shell Accounts :: the biggest list on the net