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BUS_DMA (9) | Kernel routines | Unix Manual Pages | :man

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

CONTENTS

Synopsis
Description
Structures And Types
Functions
Return Values
See Also
History
Authors

SYNOPSIS


.In 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

Vt 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.
Vt 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 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’, inclusive. The filter function should return zero for any mapping in this range that can be accommodated by the device and non-zero otherwise.

Vt bus_dma_segment_t
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.

Vt bus_dmamap_t
A machine-dependent opaque type describing an individual mapping. Multiple DMA maps can be associated with one DMA tag.
Vt bus_dmamap_callback_t
Client specified callback for receiving mapping information resulting from the load of a
.Vt 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
.Vt 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 functions.

Vt bus_dmamap_callback2_t
Client specified callback for receiving mapping information resulting from the load of a
.Vt 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
.Vt bus_dmamap_callback_t with the addition that the length of the data mapped is provided via mapsize.

Vt 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
.Vt 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. All operations specified below are performed from the CPU’s point of view (for a complete description, see the bus_dmamap_sync description below):
BUS_DMASYNC_PREREAD Perform any synchronization required after an update of memory by the CPU but prior to DMA read operations.
BUS_DMASYNC_PREWRITE Perform any synchronization required after an update of memory by the CPU but prior to DMA write operations.
BUS_DMASYNC_PREREAD|BUS_DMASYNC_PREWRITE
Perform any synchronization required prior to a combination of DMA read and write operations.
BUS_DMASYNC_POSTREAD Perform any synchronization required after DMA read operations, but prior to CPU access of the memory.
BUS_DMASYNC_POSTWRITE
Perform any synchronization required after DMA write operations, but prior to CPU access of the memory.
BUS_DMASYNC_POSTREAD|BUS_DMASYNC_POSTWRITE
Perform any synchronization required after a combination of DMA read and write operations.
Vt 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"

Two
.Vt 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.

Vt 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. 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. 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
.Vt 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 MAXADDR and a lowaddr of BUS MAXADDR_32BIT. Similarly a device that can only dma to addresses bellow 16MB would specify a highaddr of BUS MAXADDR and a lowaddr of BUS 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 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 without blocking. If sufficient resources are not available, ENOMEM is returned. This should not be used for tags that will not be directly associated with a map.
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
.Vt 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. 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.
flags The value of this argument is currently undefined, and should be specified as ‘0’.

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.
EINVAL The load request was invalid. The callback has not, and will not be called. This error value may indicate that dmat, map, buf, or callback were invalid, or buslen 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
.Vt 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
.Vt bus_size_t argument is also passed to the callback routine, which contains the mbuf chain’s packet header length.

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

Returns EINVAL 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
.Vt bus_size_t argument is also passed to the callback routine, which contains the size of uio, i.e. uio->uio_resid.

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
.Vt 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 undefined.

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
.Vt 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.
BUS_DMA_ZERO Causes the allocated memory to be set to all zeros.
mapp Pointer to storage for the returned DMA map.

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

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

Although no explicit loading is required to access the memory referenced by the returned map, the synchronization requirements as described in the bus_dmamap_sync section still apply.

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,
.Vt void , will return 0 on success or an error code, as discussed above.

All
.Vt void routines will succeed if provided with valid arguments.

SEE ALSO

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


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