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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_bio,
     bus_dmamap_load_ccb, 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_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);

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

     int
     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);

     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
     simplicity 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
             characteristics 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
             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
             callback.  The testaddr argument contains a potential starting
             address of a DMA mapping.  The filter function operates on the
             set of addresses from testaddr to `trunc_page(testaddr) +
             PAGE_SIZE - 1', 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(),
             bus_dmamap_load_bio() or bus_dmamap_load_ccb().  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
             mapping in order to guarantee memory coherency.  The
             bus_dmasync_op_t allows the type of DMA operation that will be or
             has been performed to be communicated to the system so that the
             correct coherency measures are taken.  The operations are
             represented as bitfield flags that can be combined together,
             though it only makes sense to combine PRE flags or POST flags,
             not both.  See the bus_dmamap_sync() description below for more
             details on how to use these operations.

             All operations specified below are performed from the host 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
             operations.

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

             BUS_DMA_LOCK        Acquires and/or locks the client locking
                                 primitive.

             BUS_DMA_UNLOCK      Releases and/or unlocks the client locking
                                 primitive.

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
                              created using this tag.  The alignment must be a
                              power of 2.  Hardware that can DMA starting at
                              any address would specify 1 for byte alignment.
                              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
                              window contains all addresses greater than
                              lowaddr and less than or equal to highaddr.  For
                              example, a device incapable of DMA above 4GB,
                              would specify a highaddr of BUS_SPACE_MAXADDR
                              and a lowaddr of BUS_SPACE_MAXADDR_32BIT.
                              Similarly a device that can only perform DMA to
                              addresses below 16MB would specify a highaddr of
                              BUS_SPACE_MAXADDR and a lowaddr of
                              BUS_SPACE_MAXADDR_24BIT.  Some implementations
                              requires that some region of device visible
                              address space, overlapping available host
                              memory, be outside the window.  This area of
                              `safe memory' is used to bounce requests that
                              would otherwise conflict with the exclusion
                              window.

             filtfunc         Optional filter function (may be NULL) to be
                              called for any attempt to map memory into the
                              window described 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.

             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 specified.

             maxsegsz         Maximum size, in bytes, of a segment in any DMA
                              mapped region associated with dmat.

             flags            Are as follows:

                              BUS_DMA_ALLOCNOW      Pre-allocate enough
                                                    resources to handle at
                                                    least one map load
                                                    operation on this tag.  If
                                                    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 optimization.

             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
                              specified by lockfunc.

             dmat             Pointer to a bus_dma_tag_t where the resulting
                              DMA tag will be stored.

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

     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 control data and as
                                                  opposed to streamable data
                                                  such as receive and transmit
                                                  buffers.  Use of this flag
                                                  does not remove the
                                                  requirement of using
                                                  bus_dmamap_sync(), but it
                                                  may reduce the cost of
                                                  performing these operations.
                                                  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.

                              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 required to stall subsequent
                              requests until a pending request's callback is
                              invoked.

             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 elements 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_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
                                    buffers.

             CAM_DATA_SG_PADDR      The data is a scatter/gather list of bus
                                    address 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
             codes:

                   XPT_ATA_IO
                   XPT_CONT_TARGET_IO
                   XPT_SCSI_IO

     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
             provided 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
             follows:

             dmat      DMA tag used to allocate map.

             map       The DMA mapping to be synchronized.

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

             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
             contents 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 coherent 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
             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
             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
             completing 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_bio()
           bus_dmamap_load_ccb()
           bus_dmamap_load_mbuf()
           bus_dmamap_load_mbuf_sg()
           bus_dmamap_load_uio()
           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
     following 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
     subsystem.  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 11.0-PRERELEASE          July 17, 2013         FreeBSD 11.0-PRERELEASE

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

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