table of contents
BUS_DMA(9) | Kernel Developer's Manual | BUS_DMA(9) |
NAME¶
bus_dma, bus_dma_tag_create, bus_dma_tag_destroy, bus_dmamap_create, bus_dmamap_destroy, bus_dmamap_load, bus_dmamap_load_mbuf, bus_dmamap_load_mbuf_sg, bus_dmamap_load_uio, bus_dmamap_unload, bus_dmamap_sync, bus_dmamem_alloc, bus_dmamem_free — Bus and Machine Independent DMA Mapping InterfaceSYNOPSIS¶
#include <machine/bus.h> intbus_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)
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;
- 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 beNULL
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)
- 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)
- 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 withBUS_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)
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 ofBUS_SPACE_MAXADDR_32BIT
. Similarly a device that can only perform DMA to addresses below 16MB would specify a highaddr ofBUS_SPACE_MAXADDR
and a lowaddr ofBUS_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. IfNULL
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.
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.
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.
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.
- 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.
- 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 isUIO_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_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.
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, anotherBUS_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 ofBUS_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 aBUS_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.
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()
- bus_dma_tag_create()
- bus_dmamap_create()
- bus_dmamem_alloc()
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.May 12, 2009 | Debian |