Implementation Notes

All data types and constants will be defined by the port-specific header <machine/bus_defs.h>. All functions will be defined by the port-specific header <machine/bus_funcs.h>. Note that this document assumes the existence of types already defined by the current bus_space(9) interface.

Unless otherwise noted, all function calls in this interface may be defined as cpp(1) macros.

Data Types

Individual implementations may name these structures whatever they wish, providing that the external representations are:

bus_dma_tag_t

A machine-dependent opaque type describing the implementation of DMA for a given bus.

bus_dma_segment_t

A structure with at least the following members:

bus_addr_t      ds_addr;
bus_size_t      ds_len;

The structure may have machine-dependent members and arbitrary layout. The values in ds_addr and ds_len are suitable for programming into DMA controller address and length registers.

bus_dmamap_t

A pointer to a structure with at least the following members:

bus_size_t      dm_maxsegsz;
bus_size_t      dm_mapsize;
int             dm_nsegs;
bus_dma_segment_t *dm_segs;

The structure may have machine-dependent members and arbitrary layout. The dm_maxsegsz member indicates the maximum number of bytes that may be transferred by any given DMA segment. The dm_mapsize member indicates the size of the mapping. A value of 0 indicates the mapping is invalid. The dm_segs member may be an array of segments or a pointer to an array of segments. The dm_nsegs member indicates the number of segments in dm_segs.

Functions

bus_dmamap_create(tag, size, nsegments, maxsegsz, boundary, flags, dmamp)

Allocates a DMA handle and initializes it according to the parameters provided.

tag

This is the bus_dma_tag_t passed down from the parent driver via <bus>_attach_args.

size

This is the maximum DMA transfer that can be mapped by the handle.

nsegments

Number of segments the device can support in a single DMA transaction. This may be the number of scatter-gather descriptors supported by the device.

maxsegsz

The maximum number of bytes that may be transferred by any given DMA segment and will be assigned to the dm_maxsegsz member.

boundary

Some DMA controllers are not able to transfer data that crosses a particular boundary. This argument allows this boundary to be specified. The boundary lines begin at 0, and occur every boundary bytes. Mappings may begin on a boundary line but may not end on or cross a boundary line. If no boundary condition needs to be observed, a boundary argument of 0 should be used.

flags

BUS_DMA_WAITOK

It is safe to wait (sleep) for resources during this call.

BUS_DMA_NOWAIT

It is not safe to wait (sleep) for resources during this call.

BUS_DMA_ALLOCNOW

Perform any resource allocation this handle may need now. If this is not specified, the allocation may be deferred to bus_dmamap_load(). If this flag is specified, bus_dmamap_load() will not block on resource allocation.

BUS_DMA_BUS[1-4]

These flags are placeholders, and may be used by busses to provide bus-dependent functionality.

dmamp

This is a pointer to a bus_dmamap_t. A DMA map will be allocated and pointed to by dmamp upon successful completion of this routine. dmamp is undefined if this routine fails.

Behavior is not defined if invalid arguments are passed to bus_dmamap_create().

Returns 0 on success, or an error code to indicate mode of failure.

bus_dmamap_destroy(tag, dmam)

Frees all resources associated with a given DMA handle.

tag

This is the bus_dma_tag_t passed down from the parent driver via <bus>_attach_args.

dmam

The DMA handle to destroy.

In the event that the DMA handle contains a valid mapping, the mapping will be unloaded via the same mechanism used by bus_dmamap_unload().

Behavior is not defined if invalid arguments are passed to bus_dmamap_destroy().

If given valid arguments, bus_dmamap_destroy() always succeeds.

bus_dmamap_load(tag, dmam, buf, buflen, p, flags)

Loads a DMA handle with mappings for a DMA transfer. It assumes that all pages involved in a DMA transfer are wired.

tag

This is the bus_dma_tag_t passed down from the parent driver via <bus>_attach_args.

dmam

The DMA handle with which to map the transfer.

buf

The buffer to be used for the DMA transfer.

buflen

The size of the buffer.

p

Used to indicate the address space in which the buffer is located. If NULL, the buffer is assumed to be in kernel space. Otherwise, the buffer is assumed to be in proc p's address space.

flags

BUS_DMA_WAITOK

It is safe to wait (sleep) for resources during this call.

BUS_DMA_NOWAIT

It is not safe to wait (sleep) for resources during this call.

BUS_DMA_STREAMING

By default, the bus_dma API assumes that there is coherency between memory and the device performing the DMA transaction. Some platforms, however, have special hardware, such as an “I/O cache”, which may improve performance of some types of DMA transactions, but which break the assumption that there is coherency between memory and the device performing the DMA transaction. This flag allows the use of this special hardware, provided that the device is doing sequential, unidirectional transfers which conform to certain alignment and size constraints defined by the platform. If the platform does not support the feature, or if the buffer being loaded into the DMA map does not conform to the constraints required for use of the feature, then this flag will be silently ignored. Also refer to the use of this flag with the bus_dmamem_alloc() function.

BUS_DMA_READ

This is a hint to the machine-dependent back-end that indicates the mapping will be used only for a device → memory transaction. The back-end may perform optimizations based on this information.

BUS_DMA_WRITE

This is a hint to the machine-dependent back-end that indicates the mapping will be used only for a memory → device transaction. The back-end may perform optimizations based on this information.

BUS_DMA_BUS[1-4]

These flags are placeholders, and may be used by busses to provide bus-dependent functionality.

As noted above, if a DMA handle is created with BUS_DMA_ALLOCNOW, bus_dmamap_load() will never block.

If a call to bus_dmamap_load() fails, the mapping in the DMA handle will be invalid. It is the responsibility of the caller to clean up any inconsistent device state resulting from incomplete iteration through the uio.

Behavior is not defined if invalid arguments are passed to bus_dmamap_load().

Returns 0 on success, or an error code to indicate mode of failure. Possible error codes include the following:

EFBIG

Too many segments.

EINVAL

buflen is too large for the DMA map.

ENOMEM

Could not allocate memory for, e.g., a bounce buffer.

bus_dmamap_load_mbuf(tag, dmam, chain, flags)

This is a variation of bus_dmamap_load() which maps mbuf chains for DMA transfers. Mbuf chains are assumed to be in kernel virtual address space.

bus_dmamap_load_uio(tag, dmam, uio, flags)

This is a variation of bus_dmamap_load() which maps buffers pointed to by uio for DMA transfers. Determination if the buffers are in user or kernel virtual address space is done internally, according to uio->uio_vmspace. See uiomove(9) for details of the uio structure.

bus_dmamap_load_raw(tag, dmam, segs, nsegs, size, flags)

This is a variation of bus_dmamap_load() which maps buffers allocated by bus_dmamem_alloc() (see below). The segs argument is an array of bus_dma_segment_t's filled in by bus_dmamem_alloc(). The nsegs argument is the number of segments in the array. The size argument is the size of the DMA transfer.

bus_dmamap_unload(tag, dmam)

Deletes the mappings for a given DMA handle.

tag

This is the bus_dma_tag_t passed down from the parent driver via <bus>_attach_args.

dmam

The DMA handle containing the mappings which are to be deleted.

If the DMA handle was created with BUS_DMA_ALLOCNOW, bus_dmamap_unload() will not free the corresponding resources which were allocated by bus_dmamap_create(). This is to ensure that bus_dmamap_load() will never block on resources if the handle was created with BUS_DMA_ALLOCNOW.

bus_dmamap_unload() will not perform any implicit synchronization of DMA buffers. This must be done explicitly by bus_dmamap_sync().

bus_dmamap_unload() will restore the dm_maxsegsz member to its initial value assigned by bus_dmamap_create().

Behavior is not defined if invalid arguments are passed to bus_dmamap_unload().

If given valid arguments, bus_dmamap_unload() always succeeds.

bus_dmamap_sync(tag, dmam, offset, len, ops)

Performs pre- and post-DMA operation cache and/or buffer synchronization.

tag

This is the bus_dma_tag_t passed down from the parent driver via <bus>_attach_args.

dmam

The DMA mapping to be synchronized.

offset

The offset into the DMA mapping to synchronize.

len

The length of the mapping from offset to synchronize.

ops

One or more synchronization operations to perform. The following DMA synchronization operations are defined:

BUS_DMASYNC_PREREAD

Perform any synchronization required prior to an update of host memory by the device.

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_PREWRITE

Perform any synchronization required after an update of host memory by the CPU and prior to device access to host memory.

BUS_DMASYNC_POSTWRITE

Perform any synchronization required after device access to host memory.

where each operation may involve cache flush/invalidation, bounce buffer copying, and/or memory barriers.

More than one operation may be performed in a given synchronization call. Mixing of PRE and POST operations is not allowed, and behavior is undefined if this is attempted.

Synchronization operations are expressed from the perspective of the host RAM, i.e., a device → memory operation is a READ, and a memory → device operation is a WRITE.

bus_dmamap_sync() may consult state kept within the DMA map to determine if the memory is mapped in a DMA coherent fashion. If so, bus_dmamap_sync() may elect to skip certain expensive operations, such as flushing of the data cache. See bus_dmamem_map() for more information on this subject.

On platforms which implement a weak memory access ordering model, bus_dmamap_sync() will always cause the appropriate memory barriers to be issued.

This function exists to ensure that the host and the device have a consistent view of a range of DMA memory, before and after a DMA operation.

An example of using bus_dmamap_sync(), involving multiple read-write use of a single mapping might look like this:

bus_dmamap_load(...);

while (not done) {
        /* invalidate soon-to-be-stale cache blocks */
        bus_dmamap_sync(..., BUS_DMASYNC_PREREAD);

        [ do read DMA ]

        /* copy from bounce */
        bus_dmamap_sync(..., BUS_DMASYNC_POSTREAD);

        /* read data now in driver-provided buffer */

        [ computation ]

        /* data to be written now in driver-provided buffer */

        /* flush write buffers and writeback, copy to bounce */
        bus_dmamap_sync(..., BUS_DMASYNC_PREWRITE);

        [ do write DMA ]

        /* probably a no-op, but provided for consistency */
        bus_dmamap_sync(..., BUS_DMASYNC_POSTWRITE);
}

bus_dmamap_unload(...);

This function must be called to synchronize DMA buffers before and after a DMA operation. Other bus_dma functions can not be relied on to do this synchronization implicitly. If DMA read and write operations are not preceded and followed by the appropriate synchronization operations, behavior is undefined.

Behavior is not defined if invalid arguments are passed to bus_dmamap_sync().

If given valid arguments, bus_dmamap_sync() always succeeds.

bus_dmamem_alloc(tag, size, alignment, boundary, segs, nsegs, rsegs, flags)

Allocates memory that is “DMA safe” for the bus corresponding to the given tag.

The mapping of this memory is machine-dependent (or “opaque”); machine-independent code must not assume that the addresses returned are valid in kernel virtual address space, or that the addresses returned are system physical addresses. The address value returned as part of segs can thus not be used to program DMA controller address registers. Only the values in the dm_segs array of a successfully loaded DMA map (using bus_dmamap_load()) can be used for this purpose.

Allocations will always be rounded to the hardware page size. Callers may wish to take advantage of this, and cluster allocation of small data structures.

tag

This is the bus_dma_tag_t passed down from the parent driver via <bus>_attach_args.

size

The amount of memory to allocate.

alignment

Each segment in the allocated memory will be aligned to this value. If the alignment is less than a hardware page size, it will be rounded up to the hardware page size. This value must be a power of two.

boundary

Each segment in the allocated memory must not cross this boundary (relative to zero). This value must be a power of two. A boundary value less than the size of the allocation is invalid. If no boundary condition needs to be observed, a boundary argument of 0 should be used.

segs

An array of bus_dma_segment_t's, filled in as memory is allocated, representing the opaque addresses of the memory chunks.

nsegs

Specifies the number of segments in segs, and this is the maximum number of segments that the allocated memory may contain.

rsegs

Used to return the actual number of segments the memory contains.

flags

BUS_DMA_WAITOK

It is safe to wait (sleep) for resources during this call.

BUS_DMA_NOWAIT

It is not safe to wait (sleep) for resources during this call.

BUS_DMA_STREAMING

Adjusts, if necessary, the size, alignment, and boundary constraints to conform to the platform-dependent requirements for the use of the BUS_DMA_STREAMING flag with the bus_dmamap_load() function. If the platform does not support the BUS_DMA_STREAMING feature, or if the size, alignment, and boundary constraints would already satisfy the platform's requirements, this flag is silently ignored. The BUS_DMA_STREAMING flag will never relax the constraints specified in the call.

BUS_DMA_BUS[1-4]

These flags are placeholders, and may be used by busses to provide bus-dependent functionality.

All pages allocated by bus_dmamem_alloc() will be wired down until they are freed by bus_dmamem_free().

Behavior is undefined if invalid arguments are passed to bus_dmamem_alloc().

Returns 0 on success, or an error code indicating mode of failure.

bus_dmamem_free(tag, segs, nsegs)

Frees memory previously allocated by bus_dmamem_alloc(). Any mappings will be invalidated.

tag

This is the bus_dma_tag_t passed down from the parent driver via <bus>_attach_args.

segs

The array of bus_dma_segment_t's filled in by bus_dmamem_alloc().

nsegs

The number of segments in segs.

Behavior is undefined if invalid arguments are passed to bus_dmamem_free().

If given valid arguments, bus_dmamem_free() always succeeds.

bus_dmamem_map(tag, segs, nsegs, size, kvap, flags)

Maps memory allocated with bus_dmamem_alloc() into kernel virtual address space.

tag

This is the bus_dma_tag_t passed down from the parent driver via <bus>_attach_args.

segs

The array of bus_dma_segment_t's filled in by bus_dmamem_alloc(), representing the memory regions to map.

nsegs

The number of segments in segs.

size

The size of the mapping.

kvap

Filled in to specify the kernel virtual address where the memory is mapped.

flags

BUS_DMA_WAITOK

It is safe to wait (sleep) for resources during this call.

BUS_DMA_NOWAIT

It is not safe to wait (sleep) for resources during this call.

BUS_DMA_BUS[1-4]

These flags are placeholders, and may be used by busses to provide bus-dependent functionality.

BUS_DMA_COHERENT

This flag is a hint to machine-dependent code. If possible, map the memory in such a way as it will be DMA coherent. This may include mapping the pages into uncached address space or setting the cache-inhibit bits in page table entries. If DMA coherent mappings are impossible, this flag is silently ignored.

Later, when this memory is loaded into a DMA map, machine-dependent code will take whatever steps are necessary to determine if the memory was mapped in a DMA coherent fashion. This may include checking if the kernel virtual address lies within uncached address space or if the cache-inhibit bits are set in page table entries. If it is determined that the mapping is DMA coherent, state may be placed into the DMA map for use by later calls to bus_dmamap_sync().

Note that a device driver must not rely on BUS_DMA_COHERENT for correct operation. All calls to bus_dmamap_sync() must still be made. This flag is provided only as an optimization hint to machine-dependent code.

Also note that this flag only applies to coherency between the CPU and memory. Coherency between memory and the device is controlled with a different flag. See the description of the bus_dmamap_load() function.

BUS_DMA_NOCACHE

This flag is a hint to machine-dependent code. If possible, map the memory uncached. This flag may be useful in the case that the memory cache causes unexpected behavior of the device.

Exclusive with BUS_DMA_PREFETCHABLE.

BUS_DMA_PREFETCHABLE

This flag is a hint to machine-dependent code. If possible, map the memory prefetchable/write-combining.

Exclusive with BUS_DMA_NOCACHE.

Behavior is undefined if invalid arguments are passed to bus_dmamem_map().

Returns 0 on success, or an error code indicating mode of failure.

bus_dmamem_unmap(tag, kva, size)

Unmaps memory previously mapped with bus_dmamem_map(), freeing the kernel virtual address space used by the mapping.

tag

This is the bus_dma_tag_t passed down from the parent driver via <bus>_attach_args.

kva

The kernel virtual address of the mapped memory.

size

The size of the mapping.

Behavior is undefined if invalid arguments are passed to bus_dmamem_unmap().

If given valid arguments, bus_dmamem_unmap() always succeeds.

bus_dmamem_mmap(tag, segs, nsegs, off, prot, flags)

Provides support for user mmap(2)'ing of DMA-safe memory. This function is to be called by a device driver's (*d_mmap)() entry point, which is called by the device pager for each page to be mapped.

tag

This is the bus_dma_tag_t passed down from the parent driver via <bus>_attach_args.

segs

The array of bus_dma_segment_t's filled in by bus_dmamem_alloc(), representing the memory to be mmap(2)'ed.

nsegs

The number of elements in the segs array.

off

The offset of the page in DMA memory which is to be mapped.

prot

The protection codes for the mapping.

flags

BUS_DMA_WAITOK

It is safe to wait (sleep) for resources during this call.

BUS_DMA_NOWAIT

It is not safe to wait (sleep) for resources during this call.

BUS_DMA_BUS[1-4]

These flags are placeholders, and may be used by busses to provide bus-dependent functionality.

BUS_DMA_COHERENT

See bus_dmamem_map() above for a description of this flag.

BUS_DMA_NOCACHE

See bus_dmamem_map() above for a description of this flag.

Behavior is undefined if invalid arguments are passed to bus_dmamem_mmap().

Returns -1 to indicate failure. Otherwise, returns an opaque value to be interpreted by the device pager.

bus_dmatag_subregion(tag, min_addr, max_addr, newtag, flags)

Given a bus_dma_tag_t create a new bus_dma_tag_t with a limited bus address space. This function should not normally be used, but is useful for devices that do not support the full address space of the parent bus. Not all ports implement this method; on ports where it is unavailable, EOPNOTSUPP is returned.

tag

This is the bus_dma_tag_t to subregion.

min_addr

The smallest address this new tag can address.

max_addr

The largest address this new tag can address.

newtag

Pointer filled in with the address of the new bus_dma_tag_t.

flags

BUS_DMA_WAITOK

It is safe to wait (sleep) for resources during this call.

BUS_DMA_NOWAIT

It is not safe to wait (sleep) for resources during this call.

The address range min_addr to max_addr is inclusive of both addresses.

bus_dmatag_destroy(tag)

Free a tag created by bus_dmatag_subregion().