BUS_DMA(9)

NAME
bus_dma, bus_dma_tag_create, bus_dma_tag_destroy, bus_dmamap_create, bus_dmamap_destroy, bus_dmamap_load, bus_dmamap_load_ccb, bus_dmamap_load_mbuf, bus_dmamap_load_mbuf_segment, bus_dmamap_load_mbuf_defrag, bus_dmamap_load_uio, bus_dmamap_unload, bus_dmamap_sync, bus_dmamem_alloc, bus_dmamem_coherent, bus_dmamem_coherent_any, bus_dmamem_free - Bus and Machine Independent DMA Mapping Interface

SYNOPSIS
#include <sys/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_size_t maxsize, int nsegments, bus_size_t maxsegsz, int flags, 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_ccb(bus_dma_tag_t dmat, bus_dmamap_t map, union ccb *ccb, 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_segment(bus_dma_tag_t dmat, bus_dmamap_t map, struct mbuf *mbuf, bus_dma_segment_t *segs, int maxsegs, int *nsegs, int flags);
int
bus_dmamap_load_mbuf_defrag(bus_dma_tag_t dmat, bus_dmamap_t map, struct mbuf **mbuf, bus_dma_segment_t *segs, int maxsegs, 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);
int
bus_dmamem_alloc(bus_dma_tag_t dmat, void **vaddr, int flags, bus_dmamap_t *mapp);
int
bus_dmamem_coherent(bus_dma_tag_t parent, bus_size_t alignment, bus_size_t boundary, bus_addr_t lowaddr, bus_addr_t highaddr, bus_size_t maxsize, int flags, bus_dmamem_t *dmem);
void *
bus_dmamem_coherent_any(bus_dma_tag_t parent, bus_size_t alignment, bus_size_t maxsize, int flags, bus_dma_tag_t *dtag, bus_dmamap_t *dmap, bus_addr_t *busaddr);
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, bus_dmasync_op_t op);
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

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.

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.

bus_dmamap_t
    A machine-dependent opaque type describing an individual mapping. Multiple DMA maps can be associated with one DMA tag.

bus_dmamem_t
    A machine-dependent type that describes DMA memory created by bus_dmamem_coherent().

        	bus_dma_tag_t	dmem_tag;
        	bus_dmamap_t	dmem_map;
        	void		*dmem_addr;
        	bus_addr_t	dmem_busaddr;

    The dmem_tag field contains the DMA tag of the DMA memory and dmem_map field contains the DMA map of the DMA memory. The dmem_addr field points to the allocated DMA memory in kernel virtual address space. The dmem_busaddr field contains the device visible address of the DMA memory.

bus_dmamap_callback_t
    Client specified callback for receiving mapping information resulting from the load of a bus_dmamap_t via bus_dmamap_load() 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)

    The callback_arg is the callback argument passed to dmamap load functions. The segs and nseg parameters describe an array of 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.

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

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 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 DMA engine's point of view:

    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_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.

FUNCTIONS

bus_dma_tag_create(parent, alignment, boundary, lowaddr, highaddr, maxsize, nsegments, maxsegsz, flags, *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 bus_dma_segment_t. The boundary must be either a power of 2 or 0. ‘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 address 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 below 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.
    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_SPACE_UNRESTRICTED may be specified for the tag intended to be used as the parent. BUS_SPACE_UNRESTRICTED must not be specified for the tags which will be used to create maps. For tags which will be used to create maps, this argument must be less than 16384 on x86_64.
    maxsegsz
        Maximum size, in bytes, of a segment in any DMA mapped region associated with dmat.
    flags
        Are as follows:
        BUS_DMA_ALLOCNOW
            Allocate the minimum resources necessary to guarantee that all map load operations associated with this tag may not block. If sufficient resources are not available, ENOMEM is returned.
        BUS_DMA_WAITOK
            Indicates that it is OK to wait for resources. However, unlike kmalloc(9), it is not guaranteed that the resource allocation will succeed. This flag is the default one, if BUS_DMA_NOWAIT is not supplied.
        BUS_DMA_NOWAIT
            If the resource allocation request cannot be immediately fulfilled, ENOMEM is returned.
        BUS_DMA_ONEBPAGE
            Allocte one bounce page at most, even if the maxsize indicates that multiple bounce pages are needed.
        BUS_DMA_ALIGNED
            Indicates that all memory to be loaded into the DMA maps associated with this DMA tag is properly aligned according to alignment constraint. No resources, e.g. bounce pages, will be allocated due to the alignment constraint. If unaligned memory was loaded into the DMA maps associated with this DMA tag, system will panic.
        BUS_DMA_PRIVBZONE
            Uses a private bounce zone instead of a shared one. A private bounce zone will vanish if the DMA tag is destroyed.
        BUS_DMA_ALLOCALL
            Allocate all required resources (mainly the bounce buffer). If any allocation fails, bus_dma_tag_create() fails.
        BUS_DMA_PROTECTED
            All of the functions called with the DMA tag are already protected by the caller, so the bus_dma code need not protect the internal data structures.
    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_WAITOK
            Indicates that it is OK to wait for resources. However, unlike kmalloc(9), it is not guaranteed that the resource allocation will succeed. This flag is the default one, if BUS_DMA_NOWAIT is not supplied.
        BUS_DMA_NOWAIT
            If the resource allocation request cannot be immediately fulfilled, ENOMEM is returned.
        BUS_DMA_ONEBPAGE
            Allocte one bounce page at most, even if the maxsize used to create the dmat indicates that multiple bounce pages are needed.
    mapp
        Pointer to a 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, ...)
    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. DMA maps created from DMA tags that are allocated with the BUS_DMA_ALLOCNOW flag will never return this status for a load operation.
    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 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_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.

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 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_segment(dmat, map, mbuf, *segs, maxsegs, *nsegs, flags)
    It is like bus_dmamap_load_mbuf() without callback. Segmentation information are saved in the segs and nsegs if the loading is successful. The maxsegs, which indicates the number of elements in the segs, must be set by the caller and must be at least 1 and at most equal the nsegments used to create the dmat. The flags must have BUS_DMA_NOWAIT turned on.

    This function will not block. When system is short of DMA resources, this function will return ENOMEM, instead of EINPROGRESS.

bus_dmamap_load_mbuf_defrag(dmat, map, *mbuf, *segs, maxsegs, *nsegs, flags)
    This function is like bus_dmamap_load_mbuf_segment(), but it will call m_defrag() on the *mbuf and try reloading, if low level code indicates too many fragments in the *mbuf; the mbuf will be updated under this situation. However, *mbuf would not be freed by this function, even if m_defrag() failed.

    Return ENOBUFS, if the calling of m_defrag() failed.

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 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.

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 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 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.
        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. On x86_64, the BUS_DMA_NOCACHE flag results in the Strong Uncacheable PAT to be set for the allocated virtual address range.
    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_coherent(parent, alignment, boundary, lowaddr, highaddr, maxsize, flags, *dmem)
    This is a convenient function to create one segment of DMA memory. It combines following bus_dma function calls:

        	bus_dma_tag_create(..., dtag);
        	bus_dmamem_alloc(*dtag, vaddr, ..., dmap);
        	bus_dmamap_load(*dtag, *dmap, *vaddr, ..., \
        			callback, busaddr, ...);

    The final results of the above function calls are: DMA tag, DMA map, DMA memory's kernel virtual address and its device visible address. bus_dmamem_coherent() saves the results in *dmem.

    The parent, alignment, boundary, lowaddr and highaddr will be passed to bus_dma_tag_create() as they are. The maxsize will be passed to bus_dma_tag_create() as its maxsize and maxsegsz and ‘1’ will be passed to bus_dma_tag_create() as its nsegments. When bus_dmamem_alloc() is called, flags will be first or'ed with BUS_DMA_COHERENT then passed to it. The final results of the above three functions, i.e. DMA tag, DMA map, DMA memory's kernel virtual address and its device visible address, are saved in *dmem. If any of the three functions failed, this function will return the error code and the *dmem should not be used.

bus_dmamem_coherent_any(parent, alignment, maxsize, flags, *dtag, *dmap, *busaddr)
    This function is a simplified version of bus_dmamem_coherent() with its boundary set to ‘0’, lowaddr set to BUS_SPACE_MAXADDR and highaddr set to BUS_SPACE_MAXADDR. The parent usually should not be NULL.

    Return the DMA memory's kernel virtual address. The DMA tag, DMA map and device visible address are returned in *dtag, *dmap, and *busaddr. If this function failed, NULL will be returned; *dtag, *dmap, and *busaddr are left unchanged.

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

All void routines will succeed if provided with valid arguments.

SEE ALSO
devclass(9), device(9), driver(9), rman(9) Jason R. Thorpe, A Machine-Independent DMA Framework for NetBSD, Proceedings of the Summer 1998 USENIX Technical Conference, USENIX Association, June 1998.

HISTORY
The bus_dma interface first appeared in NetBSD 1.3.

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

AUTHORS
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 -nosplit Chris Demetriou, Charles Hannum, Ross Harvey, Matthew Jacob, Jonathan Stone, and Matt Thomas.

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