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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
     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().
             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 dma to addresses bellow 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      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 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.

             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
                          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 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_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 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,
                                        frequently.  Use of this flag does not
                                        remove the requirement of using
                                        bus_dmamap_sync, but it may reduce the
                                        cost of performing these operations.
                                        The BUS_DMA_COHERENT flag is currently
                                        implemented on sparc64 and arm.

                        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
             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_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
     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.