BUS_DMA(9)

NAME
bus_dma, bus_dmamap_create, bus_dmamap_destroy, bus_dmamap_load, bus_dmamap_load_mbuf, bus_dmamap_load_uio, bus_dmamap_load_raw, bus_dmamap_unload, bus_dmamap_sync, bus_dmamem_alloc, bus_dmamem_free, bus_dmamem_map, bus_dmamem_unmap, bus_dmamem_mmap - Bus and Machine Independent DMA Mapping Interface

SYNOPSIS
#include <machine/bus.h>
int
bus_dmamap_create(bus_dma_tag_t tag, bus_size_t size, int nsegments, bus_size_t maxsegsz, bus_size_t boundary, int flags, bus_dmamap_t *dmamp);
void
bus_dmamap_destroy(bus_dma_tag_t tag, bus_dmamap_t dmam);
int
bus_dmamap_load(bus_dma_tag_t tag, bus_dmamap_t dmam, void *buf, bus_size_t buflen, struct proc *p, int flags);
int
bus_dmamap_load_mbuf(bus_dma_tag_t tag, bus_dmamap_t dmam, struct mbuf *chain, int flags);
int
bus_dmamap_load_uio(bus_dma_tag_t tag, bus_dmamap_t dmam, struct uio *uio, int flags);
int
bus_dmamap_load_raw(bus_dma_tag_t tag, bus_dmamap_t dmam, bus_dma_segment_t *segs, int nsegs, bus_size_t size, int flags);
void
bus_dmamap_unload(bus_dma_tag_t tag, bus_dmamap_t dmam);
void
bus_dmamap_sync(bus_dma_tag_t tag, bus_dmamap_t dmam, bus_addr_t offset, bus_size_t len, int ops);
int
bus_dmamem_alloc(bus_dma_tag_t tag, bus_size_t size, bus_size_t alignment, bus_size_t boundary, bus_dma_segment_t *segs, int nsegs, int *rsegs, int flags);
void
bus_dmamem_free(bus_dma_tag_t tag, bus_dma_segment_t *segs, int nsegs);
int
bus_dmamem_map(bus_dma_tag_t tag, bus_dma_segment_t *segs, int nsegs, size_t size, caddr_t *kvap, int flags);
void
bus_dmamem_unmap(bus_dma_tag_t tag, caddr_t kva, size_t size);
int
bus_dmamem_mmap(bus_dma_tag_t tag, bus_dma_segment_t *segs, int nsegs, int off, int prot, int flags);

DESCRIPTION
Provide a bus- and machine-independent "DMA mapping interface."

NOTES

All data structures, function prototypes, and macros will be defined by the port-specific header ~ <machine/bus.h>. Note that this document assumes the existence of types already defined by the current "bus.h" 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_mapsize;
        	int		dm_nsegs;
        	bus_dma_segment_t *dm_segs;

    The structure may have machine-dependent members and arbitrary layout. 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. Arguments are as follows:
    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.
    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
        Flags are defined as follows:
        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.

    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. Arguments are as follows:
    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. Arguments are as follows:
    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 process p's address space.
    flags
        are defined as follows:
        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.

    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.

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. The value of uio->uio_segflg will determine if the buffers are in user or kernel virtual address space. If the buffers are in user address space, the buffers are assumed to be in uio->uio_procp's address space.

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. Arguments are as follows:
    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.

    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. Arguments are as follows:
    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 operation to perform. The following DMA synchronization operations are defined:
        BUS_DMASYNC_PREREAD
            Perform any pre-read DMA cache and/or bounce operations.
        BUS_DMASYNC_POSTREAD
            Perform any post-read DMA cache and/or bounce operations.
        BUS_DMASYNC_PREWRITE
            Perform any pre-write DMA cache and/or bounce operations.
        BUS_DMASYNC_POSTWRITE
            Perform any post-write DMA cache and/or bounce operations.

        More than one operation may 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, e.g. a device -> memory operation is a READ and a memory -> device operation is a WRITE.

    bus_dmamp_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 reordered stores, bus_dmamap_sync() will always cause the store buffer to be flushed.

    This function exists so that multiple read and write transfers can be performed with the same buffer, and so that drivers can explicitly inform the bus_dma code when their data is 'ready' in its DMA buffer.

    An example of multiple read-write use of a single mapping might look like:

        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(...);

    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, ...)
    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 is not to assume that the addresses returned are valid in kernel virtual address space, or that the addresses returned are system physical addresses.

    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. Arguments are as follows:
    tag The 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.
    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
        Flags are defined as follows:
        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.

    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. Arguments are as follows:
    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. Arguments are as follows:
    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
        Flags are defined as follows:
        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 implementation of DMA coherent mappings is impossible, this is 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().

    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. The arguments are as follows:
    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. The arguments are as follows:
    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
        Flags are defined as follows:
        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.

    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.

SEE ALSO
bus_space(9)

AUTHOR
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.

HISTORY
The bus_dma interface appeared in NetBSD 1.3.