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

     bus_dma, bus_dma_tag_create, bus_dma_tag_destroy, bus_dmamap_create,
     bus_dmamap_destroy, bus_dmamap_load, bus_dmamap_load_mbuf,
     bus_dmamap_load_mbuf_sg, bus_dmamap_load_uio, bus_dmamap_unload,
     bus_dmamap_sync, bus_dmamem_alloc, bus_dmamem_free -- Bus and Machine
     Independent DMA Mapping Interface

     #include <machine/bus.h>

     bus_dma_tag_create(bus_dma_tag_t parent, bus_size_t alignment,
         bus_size_t boundary, bus_addr_t lowaddr, bus_addr_t highaddr,
         bus_dma_filter_t *filtfunc, void *filtfuncarg, bus_size_t maxsize,
         int nsegments, bus_size_t maxsegsz, int flags,
         bus_dma_lock_t *lockfunc, void *lockfuncarg, bus_dma_tag_t *dmat);

     bus_dma_tag_destroy(bus_dma_tag_t dmat);

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

     bus_dmamap_destroy(bus_dma_tag_t dmat, bus_dmamap_t map);

     bus_dmamap_load(bus_dma_tag_t dmat, bus_dmamap_t map, void *buf,
         bus_size_t buflen, bus_dmamap_callback_t *callback,
         void *callback_arg, int flags);

     bus_dmamap_load_mbuf(bus_dma_tag_t dmat, bus_dmamap_t map,
         struct mbuf *mbuf, bus_dmamap_callback2_t *callback,
         void *callback_arg, int flags);

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

     bus_dmamap_load_uio(bus_dma_tag_t dmat, bus_dmamap_t map,
         struct uio *uio, bus_dmamap_callback2_t *callback,
         void *callback_arg, int flags);

     bus_dmamap_unload(bus_dma_tag_t dmat, bus_dmamap_t map);

     bus_dmamap_sync(bus_dma_tag_t dmat, bus_dmamap_t map, op);

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

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

     Direct Memory Access (DMA) is a method of transferring data without
     involving the CPU, thus providing higher performance.  A DMA transaction
     can be achieved between device to memory, device to device, or memory to

     The bus_dma API is a bus, device, and machine-independent (MI) interface
     to DMA mechanisms.  It provides the client with flexibility and simplic-
     ity by abstracting machine dependent issues like setting up DMA mappings,
     handling cache issues, bus specific features and limitations.

              A machine-dependent (MD) opaque type that describes the charac-
              teristics of DMA transactions.  DMA tags are organized into a
              hierarchy, with each child tag inheriting the restrictions of
              its parent.  This allows all devices along the path of DMA
              transactions to contribute to the constraints of those transac-

              Client specified address filter having the format:

              int      client_filter(void *filtarg, bus_addr_t testaddr)

              Address filters can be specified during tag creation to allow
              for devices whose DMA address restrictions cannot be specified
              by a single window.  The filtarg is client specified during tag
              creation to be passed to all invocations of the callback.  The
              testaddr argument contains a potential starting address of a DMA
              mapping.  The filter function operates on the set of addresses
              from testaddr to `trunc_page(testaddr) + PAGE_SIZE - 1', inclu-
              sive.  The filter function should return zero for any mapping in
              this range that can be accommodated by the device and non-zero

              A machine-dependent type that describes individual DMA segments.

                      bus_addr_t      ds_addr;
                      bus_size_t      ds_len;

              The ds_addr field contains the device visible address of the DMA
              segment, and ds_len contains the length of the DMA segment.
              Although the DMA segments returned by a mapping call will adhere
              to all restrictions necessary for a successful DMA operation,
              some conversion (e.g. a conversion from host byte order to the
              device's byte order) is almost always required when presenting
              segment information to the device.

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

              Client specified callback for receiving mapping information
              resulting from the load of a bus_dmamap_t via bus_dmamap_load().
              Callbacks are of the format:

              void     client_callback(void *callback_arg, bus_dma_segment_t
                       *segs, int nseg, int error)

              The callback_arg is the callback argument passed to dmamap load
              functions.  The segs and nseg parameters describe an array of
              bus_dma_segment_t structures that represent the mapping.  This
              array is only valid within the scope of the callback function.
              The success or failure of the mapping is indicated by the error
              parameter.  More information on the use of callbacks can be
              found in the description of the individual dmamap load func-

              Client specified callback for receiving mapping information
              resulting from the load of a bus_dmamap_t via
              bus_dmamap_load_uio() or bus_dmamap_load_mbuf().

              Callback2s are of the format:

              void     client_callback2(void *callback_arg, bus_dma_segment_t
                       *segs, int nseg, bus_size_t mapsize, int error)

              Callback2's behavior is the same as bus_dmamap_callback_t with
              the addition that the length of the data mapped is provided via

              Memory synchronization operation specifier.  Bus DMA requires
              explicit synchronization of memory with its device visible map-
              ping in order to guarantee memory coherency.  The
              bus_dmasync_op_t allows the type of DMA operation that will be
              or has been performed to be communicated to the system so that
              the correct coherency measures are taken.  The operations are
              represented as bitfield flags that can be combined together,
              though it only makes sense to combine PRE flags or POST flags,
              not both.  See the bus_dmamap_sync() description below for more
              details on how to use these operations.

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

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

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

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

              BUS_DMASYNC_POSTWRITE  Perform any synchronization required
                                     after DMA write operations.

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

              void     lockfunc(void *lockfunc_arg, bus_dma_lock_op_t op)

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

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

              BUS_DMA_LOCK    Acquires and/or locks the client locking primi-

              BUS_DMA_UNLOCK  Releases and/or unlocks the client locking prim-

     bus_dma_tag_create(parent, alignment, boundary, lowaddr, highaddr,
              *filtfunc, *filtfuncarg, maxsize, nsegments, maxsegsz, flags,
              lockfunc, lockfuncarg, *dmat)
              Allocates a device specific DMA tag, and initializes it accord-
              ing to the arguments provided:
              parent        Indicates restrictions between the parent bridge,
                            CPU memory, and the device.  May be NULL, if no
                            DMA restrictions are to be inherited.
              alignment     Alignment constraint, in bytes, of any mappings
                            created using this tag.  The alignment must be a
                            power of 2.  Hardware that can DMA starting at any
                            address would specify 1 for byte alignment.  Hard-
                            ware requiring DMA transfers to start on a multi-
                            ple of 4K would specify 4096.
              boundary      Boundary constraint, in bytes, of the target DMA
                            memory region.  The boundary indicates the set of
                            addresses, all multiples of the boundary argument,
                            that cannot be crossed by a single
                            bus_dma_segment_t.  The boundary must be a power
                            of 2 and must be no smaller than the maximum seg-
                            ment size.  `0' indicates that there are no bound-
                            ary restrictions.
              highaddr      Bounds of the window of bus address space that
                            cannot be directly accessed by the device.  The
                            window contains all addresses greater than lowaddr
                            and less than or equal to highaddr.  For example,
                            a device incapable of DMA above 4GB, would specify
                            a highaddr of BUS_SPACE_MAXADDR and a lowaddr of
                            BUS_SPACE_MAXADDR_32BIT.  Similarly a device that
                            can only dma to addresses bellow 16MB would spec-
                            ify a highaddr of BUS_SPACE_MAXADDR and a lowaddr
                            of BUS_SPACE_MAXADDR_24BIT.  Some implementations
                            requires that some region of device visible
                            address space, overlapping available host memory,
                            be outside the window.  This area of `safe memory'
                            is used to bounce requests that would otherwise
                            conflict with the exclusion window.
              filtfunc      Optional filter function (may be NULL) to be
                            called for any attempt to map memory into the win-
                            dow described by lowaddr and highaddr. A filter
                            function is only required when the single window
                            described by lowaddr and highaddr cannot ade-
                            quately describe the constraints of the device.
                            The filter function will be called for every
                            machine page that overlaps the exclusion window.
              filtfuncarg   Argument passed to all calls to the filter func-
                            tion for this tag.  May be NULL.
              maxsize       Maximum size, in bytes, of the sum of all segment
                            lengths in a given DMA mapping associated with
                            this tag.
              nsegments     Number of discontinuities (scatter/gather seg-
                            ments) allowed in a DMA mapped region.  If there
                            is no restriction, BUS_SPACE_UNRESTRICTED may be
              maxsegsz      Maximum size, in bytes, of a segment in any DMA
                            mapped region associated with dmat.
              flags         Are as follows:
                            BUS_DMA_ALLOCNOW  Pre-allocate enough resources to
                                              handle at least one map load
                                              operation on this tag.  If suf-
                                              ficient resources are not avail-
                                              able, ENOMEM is returned.  This
                                              should not be used for tags that
                                              only describe buffers that will
                                              be allocated with
                                              bus_dmamem_alloc().  Also, due
                                              to resource sharing with other
                                              tags, this flag does not guaran-
                                              tee that resources will be allo-
                                              cated or reserved exclusively
                                              for this tag.  It should be
                                              treated only as a minor opti-
              lockfunc      Optional lock manipulation function (may be NULL)
                            to be called when busdma needs to manipulate a
                            lock on behalf of the client.  If NULL is speci-
                            fied, dflt_lock() is used.
              lockfuncarg   Optional argument to be passed to the function
                            specified by lockfunc.
              dmat          Pointer to a bus_dma_tag_t where the resulting DMA
                            tag will be stored.

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

              Deallocate the DMA tag dmat that was created by

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

     bus_dmamap_create(dmat, flags, *mapp)
              Allocates and initializes a DMA map.  Arguments are as follows:
              dmat       DMA tag.
              flags      The value of this argument is currently undefined and
                         should be specified as `0'.
              mapp       Pointer to a bus_dmamap_t where the resulting DMA map
                         will be stored.

              Returns ENOMEM if sufficient memory is not available for creat-
              ing the map or allocating mapping resources.

     bus_dmamap_destroy(dmat, map)
              Frees all resources associated with a given DMA map.  Arguments
              are as follows:
              dmat  DMA tag used to allocate map.
              map   The DMA map to destroy.

              Returns EBUSY if a mapping is still active for map.

     bus_dmamap_load(dmat, map, buf, buflen, *callback, callback_arg, flags)
              Creates a mapping in device visible address space of buflen
              bytes of buf, associated with the DMA map map.  This call will
              always return immediately and will not block for any reason.
              Arguments are as follows:
              dmat    DMA tag used to allocate map.
              map     A DMA map without a currently active mapping.
              buf     A kernel virtual address pointer to a contiguous (in
                      KVA) buffer, to be mapped into device visible address
              buflen  The size of the buffer.
              callback callback_arg
                      The callback function, and its argument.  This function
                      is called once sufficient mapping resources are avail-
                      able for the DMA operation.  If resources are temporar-
                      ily unavailable, this function will be deferred until
                      later, but the load operation will still return immedi-
                      ately to the caller.  Thus, callers should not assume
                      that the callback will be called before the load
                      returns, and code should be structured appropriately to
                      handle this.  See below for specific flags and error
                      codes that control this behavior.
              flags   Are as follows:
                      BUS_DMA_NOWAIT  The load should not be deferred in case
                                      of insufficient mapping resources, and
                                      instead should return immediately with
                                      an appropriate error.

              Return values to the caller are as follows:
              0            The callback has been called and completed.  The
                           status of the mapping has been delivered to the
              EINPROGRESS  The mapping has been deferred for lack of
                           resources.  The callback will be called as soon as
                           resources are available.  Callbacks are serviced in
                           FIFO order.  To ensure that ordering is guaranteed,
                           all subsequent load requests will also be deferred
                           until all callbacks have been processed.
              ENOMEM       The load request has failed due to insufficient
                           resources, and the caller specifically used the
                           BUS_DMA_NOWAIT flag.
              EINVAL       The load request was invalid.  The callback has
                           been called and has been provided the same error.
                           This error value may indicate that dmat, map, buf,
                           or callback were invalid, or buflen was larger than
                           the maxsize argument used to create the dma tag

              When the callback is called, it is presented with an error value
              indicating the disposition of the mapping.  Error may be one of
              the following:
              0            The mapping was successful and the dm_segs callback
                           argument contains an array of bus_dma_segment_t
                           elements describing the mapping.  This array is
                           only valid during the scope of the callback func-
              EFBIG        A mapping could not be achieved within the segment
                           constraints provided in the tag even though the
                           requested allocation size was less than maxsize.

     bus_dmamap_load_mbuf(dmat, map, mbuf, callback2, callback_arg, flags)
              This is a variation of bus_dmamap_load() which maps mbuf chains
              for DMA transfers.  A bus_size_t argument is also passed to the
              callback routine, which contains the mbuf chain's packet header
              length.  The BUS_DMA_NOWAIT flag is implied, thus no callback
              deferral will happen.

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

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

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

     bus_dmamap_load_uio(dmat, map, uio, callback2, callback_arg, flags)
              This is a variation of bus_dmamap_load() which maps buffers
              pointed to by uio for DMA transfers.  A bus_size_t argument is
              also passed to the callback routine, which contains the size of
              uio, i.e.  uio-_uio_resid.  The BUS_DMA_NOWAIT flag is implied,
              thus no callback deferral will happen.  Returns the same errors
              as bus_dmamap_load().

              If uio-_uio_segflg is UIO_USERSPACE, then it is assumed that the
              buffer, uio is in uio-_uio_td-_td_proc's address space.  User
              space memory must be in-core and wired prior to attempting a map
              load operation.  Pages may be locked using vslock(9).

     bus_dmamap_unload(dmat, map)
              Unloads a DMA map.  Arguments are as follows:
              dmat  DMA tag used to allocate map.
              map   The DMA map that is to be unloaded.

              bus_dmamap_unload() will not perform any implicit synchroniza-
              tion of DMA buffers.  This must be done explicitly by a call to
              bus_dmamap_sync() prior to unloading the map.

     bus_dmamap_sync(dmat, map, op)
              Performs synchronization of a device visible mapping with the
              CPU visible memory referenced by that mapping.  Arguments are as
              dmat  DMA tag used to allocate map.
              map   The DMA mapping to be synchronized.
              op    Type of synchronization operation to perform.  See the
                    definition of bus_dmasync_op_t for a description of the
                    acceptable values for op.

              bus_dmamap_sync() is the method used to ensure that CPU and
              device DMA access to shared memory is coherent.  For example,
              the CPU might be used to setup the contents of a buffer that is
              to be DMA'ed into a device.  To ensure that the data are visible
              via the device's mapping of that memory, the buffer must be
              loaded and a dma sync operation of BUS_DMASYNC_PREREAD must be
              performed.  Additional sync operations must be performed after
              every CPU write to this memory if additional DMA reads are to be
              performed.  Conversely, for the DMA write case, the buffer must
              be loaded, and a dma sync operation of BUS_DMASYNC_PREWRITE must
              be performed.  The CPU will only be able to see the results of
              this DMA write once the DMA has completed and a
              BUS_DMASYNC_POSTWRITE operation has been performed.

              If DMA read and write operations are not preceded and followed
              by the appropriate synchronization operations, behavior is unde-

     bus_dmamem_alloc(dmat, **vaddr, flags, *mapp)
              Allocates memory that is mapped into KVA at the address returned
              in vaddr that is permanently loaded into the newly created
              bus_dmamap_t returned via mapp.  Arguments are as follows:
              dmat       DMA tag describing the constraints of the DMA map-
              vaddr      Pointer to a pointer that will hold the returned KVA
                         mapping of the allocated region.
              flags      Flags are defined as follows:
                         BUS_DMA_WAITOK  The routine can safely wait (sleep)
                                         for resources.
                         BUS_DMA_NOWAIT  The routine is not allowed to wait
                                         for resources.  If resources are not
                                         available, ENOMEM is returned.
                                         Attempt to map this memory such that
                                         cache sync operations are as cheap as
                                         possible.  This flag is typically set
                                         on memory that will be accessed by
                                         both a CPU and a DMA engine, fre-
                                         quently.  Use of this flag does not
                                         remove the requirement of using
                                         bus_dmamap_sync, but it may reduce
                                         the cost of performing these opera-
                         BUS_DMA_ZERO    Causes the allocated memory to be set
                                         to all zeros.
              mapp       Pointer to a bus_dmamap_t where the resulting DMA map
                         will be stored.

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

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

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

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

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

     bus_dmamem_free(dmat, *vaddr, map)
              Frees memory previously allocated by bus_dmamem_alloc().  Any
              mappings will be invalidated.  Arguments are as follows:
              dmat   DMA tag.
              vaddr  Kernel virtual address of the memory.
              map    DMA map to be invalidated.

     Behavior is undefined if invalid arguments are passed to any of the above
     functions.  If sufficient resources cannot be allocated for a given
     transaction, ENOMEM is returned.  All routines that are not of type,
     void, will return 0 on success or an error code, as discussed above.

     All void routines will succeed if provided with valid arguments.

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

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

     The bus_dma interface first appeared in NetBSD 1.3.

     The bus_dma API was adopted from NetBSD for use in the CAM SCSI subsys-
     tem.  The alterations to the original API were aimed to remove the need
     for a bus_dma_segment_t array stored in each bus_dmamap_t while allowing
     callers to queue up on scarce resources.

     The bus_dma interface was designed and implemented by Jason R. Thorpe of
     the Numerical Aerospace Simulation Facility, NASA Ames Research Center.
     Additional input on the bus_dma design was provided by Chris Demetriou,
     Charles Hannum, Ross Harvey, Matthew Jacob, Jonathan Stone, and Matt

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

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

FreeBSD 6.2                    December 5, 2005                    FreeBSD 6.2


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