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Version: 328768 (ubuntu - 08/07/09)

Section: 9 (Appels noyau Linux)

BSD mandoc


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


In machine/bus.h Ft int Fn 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 Ft int Fn bus_dma_tag_destroy bus_dma_tag_t dmat Ft int Fn bus_dmamap_create bus_dma_tag_t dmat int flags bus_dmamap_t *mapp Ft int Fn bus_dmamap_destroy bus_dma_tag_t dmat bus_dmamap_t map Ft int Fn 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 Ft int Fn 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 Ft int Fn 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 Ft int Fn 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 Ft void Fn bus_dmamap_unload bus_dma_tag_t dmat bus_dmamap_t map Ft void Fn bus_dmamap_sync bus_dma_tag_t dmat bus_dmamap_t map op Ft int Fn bus_dmamem_alloc bus_dma_tag_t dmat void **vaddr int flags bus_dmamap_t *mapp Ft void Fn 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 memory.

The 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.


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:
Ft int
Fn 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 Fa filtarg argument is specified by the client during tag creation to be passed to all invocations of the callback. The Fa testaddr argument contains a potential starting address of a DMA mapping. The filter function operates on the set of addresses from Fa 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.

Vt 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 Fa ds_addr field contains the device visible address of the DMA segment, and Fa 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. 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.
Vt bus_dmamap_callback_t
Client specified callback for receiving mapping information resulting from the load of a Vt bus_dmamap_t via Fn bus_dmamap_load . Callbacks are of the format:
Ft void
Fn client_callback void *callback_arg bus_dma_segment_t *segs int nseg int error

The Fa callback_arg is the callback argument passed to dmamap load functions. The Fa segs and Fa nseg arguments 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 Fa error argument. 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 Fn bus_dmamap_load_uio or Fn bus_dmamap_load_mbuf .

Callback2s are of the format:

Ft void
Fn 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 Fa 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. 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 Fn 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 .

Perform any synchronization required prior to an update of host memory by the device.
Perform any synchronization required after an update of host memory by the CPU and prior to device access to host memory.
Perform any synchronization required after an update of host memory by the device and prior to CPU access to host memory.
Perform any synchronization required after device access to host memory.
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:
Ft void
Fn lockfunc void *lockfunc_arg bus_dma_lock_op_t op

The Fa lockfuncarg argument is specified by the client during tag creation to be passed to all invocations of the callback. The Fa op argument specifies the lock operation to perform.

Two Vt lockfunc implementations are provided for convenience. Fn busdma_lock_mutex performs standard mutex operations on the sleep mutex provided via Fa lockfuncarg . Fn dflt_lock will generate a system panic if it is called. It is substituted into the tag when Fa lockfunc is passed as NULL to Fn bus_dma_tag_create and is useful for tags that should not be used with deferred load operations.

Vt bus_dma_lock_op_t
Operations to be performed by the client-specified Fn lockfunc .
Acquires and/or locks the client locking primitive.
Releases and/or unlocks the client locking primitive.


Fn 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:
Fa parent
Indicates restrictions between the parent bridge, CPU memory, and the device. Each device must use a master parent tag by calling Fn bus_get_dma_tag .
Fa 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
Fa 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.
Fa 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.
Fa filtfunc
Optional filter function (may be NULL to be called for any attempt to map memory into the window described by Fa lowaddr and Fa highaddr . A filter function is only required when the single window described by Fa lowaddr and Fa highaddr cannot adequately describe the constraints of the device. The filter function will be called for every machine page that overlaps the exclusion window.
Fa filtfuncarg
Argument passed to all calls to the filter function for this tag. May be NULL
Fa maxsize
Maximum size, in bytes, of the sum of all segment lengths in a given DMA mapping associated with this tag.
Fa nsegments
Number of discontinuities (scatter/gather segments) allowed in a DMA mapped region. If there is no restriction, BUS_SPACE_UNRESTRICTED may be specified.
Fa maxsegsz
Maximum size, in bytes, of a segment in any DMA mapped region associated with Fa dmat .
Fa flags
Are as follows:
Pre-allocate enough resources to handle at least one map load operation on this tag. If sufficient resources are not available, Er ENOMEM is returned. This should not be used for tags that only describe buffers that will be allocated with Fn 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.
Fa 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, Fn dflt_lock is used.
Fa lockfuncarg
Optional argument to be passed to the function specified by Fa lockfunc .
Fa dmat
Pointer to a bus_dma_tag_t where the resulting DMA tag will be stored.

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

Fn bus_dma_tag_destroy dmat
Deallocate the DMA tag Fa dmat that was created by Fn bus_dma_tag_create .

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

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

Returns Er ENOMEM if sufficient memory is not available for creating the map or allocating mapping resources.

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

Returns Er EBUSY if a mapping is still active for Fa map .

Fn bus_dmamap_load dmat map buf buflen *callback
"callback_arg" "flags" Creates a mapping in device visible address space of Fa buflen bytes of Fa buf , associated with the DMA map Fa map . This call will always return immediately and will not block for any reason. Arguments are as follows:
Fa dmat
DMA tag used to allocate Fa map .
Fa map
A DMA map without a currently active mapping.
Fa buf
A kernel virtual address pointer to a contiguous (in KVA) buffer, to be mapped into device visible address space.
Fa buflen
The size of the buffer.
Fa callback Fa 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.
Fa flags
Are as follows:
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:

The callback has been called and completed. The status of the mapping has been delivered to the callback.
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.
The load request has failed due to insufficient resources, and the caller specifically used the BUS_DMA_NOWAIT flag.
The load request was invalid. The callback has been called and has been provided the same error. This error value may indicate that Fa dmat , Fa map , Fa buf , or Fa callback were invalid, or Fa buflen was larger than the Fa maxsize argument used to create the dma tag Fa 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:

The mapping was successful and the Fa 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.
A mapping could not be achieved within the segment constraints provided in the tag even though the requested allocation size was less than maxsize.
Fn bus_dmamap_load_mbuf dmat map mbuf callback2 callback_arg
"flags" This is a variation of Fn 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. 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 Fn bus_dmamap_load , Er EINVAL will be returned if the size of the mbuf chain exceeds the maximum limit of the DMA tag.

Fn bus_dmamap_load_mbuf_sg dmat map mbuf segs nsegs flags
This is just like Fn 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 Fn bus_dmamap_load_mbuf .
Fn bus_dmamap_load_uio dmat map uio callback2 callback_arg flags
This is a variation of Fn bus_dmamap_load which maps buffers pointed to by Fa uio for DMA transfers. A Vt bus_size_t argument is also passed to the callback routine, which contains the size of Fa uio , i.e. Fa uio->uio_resid . The BUS_DMA_NOWAIT flag is implied, thus no callback deferral will happen. Returns the same errors as Fn bus_dmamap_load .

If Fa uio->uio_segflg is UIO_USERSPACE then it is assumed that the buffer, Fa uio is in Fa uio->uio_td->td_proc Ns '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).

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

Fn bus_dmamap_unload will not perform any implicit synchronization of DMA buffers. This must be done explicitly by a call to Fn bus_dmamap_sync prior to unloading the map.

Fn 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:
Fa dmat
DMA tag used to allocate Fa map .
Fa map
The DMA mapping to be synchronized.
Fa op
Type of synchronization operation to perform. See the definition of Vt bus_dmasync_op_t for a description of the acceptable values for Fa op .

The Fn 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.

Fn bus_dmamem_alloc dmat **vaddr flags *mapp
Allocates memory that is mapped into KVA at the address returned in Fa vaddr and that is permanently loaded into the newly created Vt bus_dmamap_t returned via Fa mapp . Arguments are as follows:
Fa dmat
DMA tag describing the constraints of the DMA mapping.
Fa vaddr
Pointer to a pointer that will hold the returned KVA mapping of the allocated region.
Fa flags
Flags are defined as follows:
The routine can safely wait (sleep) for resources.
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, 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.
Causes the allocated memory to be set to all zeros.
Fa mapp
Pointer to a Vt bus_dmamap_t where the resulting DMA map will be stored.

The size of memory to be allocated is Fa maxsize as specified in the call to Fn bus_dma_tag_create for Fa dmat .

The current implementation of Fn 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 Fn 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 Fn bus_dmamap_sync section still apply and should be used to achieve portability on architectures without coherent buses.

Returns Er ENOMEM if sufficient memory is not available for completing the operation.

Fn bus_dmamem_free dmat *vaddr map
Frees memory previously allocated by Fn bus_dmamem_alloc . Any mappings will be invalidated. Arguments are as follows:
Fa dmat
DMA tag.
Fa vaddr
Kernel virtual address of the memory.
Fa 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, Er ENOMEM is returned. All routines that are not of type Vt void will return 0 on success or an error code on failure as discussed above.

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


Two locking protocols are used by . 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 and is not exposed to clients of the API.

The second protocol involves protecting various resources stored in the tag. Since almost all 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 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 Fn bus_dmamap_load callback function is called from a deferred context instead of the driver context. This means that certain functions must always be called with the same lock held that is specified in the tag. These functions include:

Fn bus_dmamap_load
Fn bus_dmamap_load_uio
Fn bus_dmamap_load_mbuf
Fn bus_dmamap_load_mbuf_sg
Fn bus_dmamap_unload
Fn 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 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:

Fn bus_dma_tag_create
Fn bus_dmamap_create
Fn 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.


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 interface first appeared in Nx 1.3 .

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


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

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

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