|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.|
|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.
A machine-dependent type that describes individual
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 devices byte order) is almost always required when presenting segment information to the device.
|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.|
Client specified callback for receiving mapping information resulting from
the load of a
.Vt bus_dmamap_t via bus_dmamap_load, bus_dmamap_load_bio or bus_dmamap_load_ccb. Callbacks are of the format:
|void||client_callback void *callback_arg bus_dma_segment_t *segs "int nseg" "int error"|
is the callback argument passed to dmamap load functions.
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 error argument. More information on the use of callbacks can be found in the description of the individual dmamap load functions.
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"|
Callback2s behavior is the same as
.Vt bus_dmamap_callback_t with the addition that the length of the data mapped is provided via mapsize.
Memory synchronization operation specifier.
Bus DMA requires explicit synchronization of memory with its device
visible mapping in order to guarantee memory coherency.
.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 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.|
|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:|
|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.
.Vt 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.
|Operations to be performed by the client-specified lockfunc.|
|BUS_DMA_LOCK||Acquires and/or locks the client locking primitive.|
|Releases and/or unlocks the client locking primitive.|
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
.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 perform DMA to addresses below 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. 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 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
.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. 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.
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.
Note that subsequent load operations for the same tag that do not require extra resources will still succeed. This may result in out-of-order processing of requests. If the caller requires the order of requests to be preserved, then the caller is required to stall subsequent requests until a pending requests callback is invoked.
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
.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_bio dmat map bio callback callback_arg flags This is a variation of bus_dmamap_load which maps buffers pointed to by bio for DMA transfers. bio may point to either a mapped or unmapped buffer. bus_dmamap_load_ccb dmat map ccb callback callback_arg flags This is a variation of bus_dmamap_load which maps data pointed to by ccb for DMA transfers. The data for ccb may be any of the following types:
CAM_DATA_VADDR The data is a single KVA buffer. CAM_DATA_PADDR The data is a single bus address range. CAM_DATA_SG The data is a scatter/gather list of KVA buffers. CAM_DATA_SG_PADDR The data is a scatter/gather list of bus address ranges. CAM_DATA_BIO The data is contained in a
.Vt struct bio attached to the CCB.
bus_dmamap_load_ccb supports the following CCB XPT function codes:
XPT_ATA_IO XPT_CONT_TARGET_IO XPT_SCSI_IO
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 chains 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
.Vt 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
.Vt 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 CPUs and devices 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 devices 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
.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 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
.Vt 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.
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 on failure as discussed above.
.Vt void routines will succeed if provided with valid arguments.
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_bio bus_dmamap_load_ccb bus_dmamap_load_mbuf bus_dmamap_load_mbuf_sg bus_dmamap_load_uio 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.
devclass(9), device(9), driver(9), rman(9), vslock(9)
.Rs A Machine-Independent DMA Framework for NetBSD
The bus_dma interface first appeared in
.Nx 1.3 .
The bus_dma 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 bus_dma interface was designed and implemented by
.An Jason R. Thorpe of the Numerical Aerospace Simulation Facility, NASA Ames Research Center. Additional input on the bus_dma design was provided by
.An Chris Demetriou ,
.An Charles Hannum ,
.An Ross Harvey ,
.An Matthew Jacob ,
.An Jonathan Stone , and
.An Matt Thomas .
The bus_dma 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 .