BUS_DMA(9) BSD Kernel Developer's Manual 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_dmatag_subregion,
bus_dmatag_destroy — Bus and Machine Independent DMA Mapping Interface
SYNOPSIS
#include <sys/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 lwp *l, 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, void **kvap, int flags);
void
bus_dmamem_unmap(bus_dma_tag_t tag, void *kva, size_t size);
paddr_t
bus_dmamem_mmap(bus_dma_tag_t tag, bus_dma_segment_t *segs, int nsegs,
off_t off, int prot, int flags);
int
bus_dmatag_subregion(bus_dma_tag_t tag, bus_addr_t min_addr,
bus_addr_t max_addr, bus_dma_tag_t *newtag, int flags);
void
bus_dmatag_destroy(bus_dma_tag_t tag);
DESCRIPTION
Provide a bus- and machine-independent "DMA mapping interface."
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 exis‐
tence 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 pro‐
gramming 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. 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 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 condi‐
tion 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. 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 fail‐
ure.
bus_dmamap_destroy(tag, dmam)
Frees all resources associated with a given DMA handle. Argu‐
ments 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, l, 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.
l 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
lwp l'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_STREAMING By default, the bus_dma API assumes
that there is coherency between mem‐
ory and the device performing the DMA
transaction. Some platforms, how‐
ever, 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 per‐
forming 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 fea‐
ture, 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-depen‐
dent back-end that indicates the map‐
ping 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-depen‐
dent back-end that indicates the map‐
ping 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 incom‐
plete 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 fail‐
ure. 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 vir‐
tual 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 buf‐
fers are in user or kernel virtual address space is done inter‐
nally, 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 seg‐
ments 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 han‐
dle was created with BUS_DMA_ALLOCNOW.
bus_dmamap_unload() will not perform any implicit synchroniza‐
tion 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 syn‐
chronization. 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 syn‐
chronization 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_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 opera‐
tions, 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 mem‐
ory 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 appro‐
priate 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. 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 alloca‐
tion of small data structures. Arguments are as follows:
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 hard‐
ware 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 mem‐
ory 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_STREAMING Adjusts, if necessary, the size,
alignment, and boundary constrains
to conform to the platform-depen‐
dent 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 bound‐
ary constraints would already sat‐
isfy the platform's requirements,
this flag is silently ignored.
The BUS_DMA_STREAMING flag will
never relax the constraints speci‐
fied 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 fail‐
ure.
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 vir‐
tual 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-depen‐
dent functionality.
BUS_DMA_COHERENT This flag is a hint to machine-depen‐
dent code. If possible, map the memory
in such a way as it will be DMA coher‐
ent. This may include mapping the
pages into uncached address space or
setting the cache-inhibit bits in page
table entries. If DMA coherent map‐
pings 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 deter‐
mined 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 opera‐
tion. All calls to bus_dmamap_sync()
must still be made. This flag is pro‐
vided only as an optimization hint to
machine-dependent code.
Also note that this flag only applies
to coherency between the CPU and mem‐
ory. 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-depen‐
dent code. If possible, map the
uncached memory. This flag may be use‐
ful in the case that the memory cache
causes unexpected behavior of the
device.
Behavior is undefined if invalid arguments are passed to
bus_dmamem_map().
Returns 0 on success, or an error code indicating mode of fail‐
ure.
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 argu‐
ments 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-depen‐
dent 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. The arguments are as follows:
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 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_dmatag_destroy(tag)
Free a tag created by bus_dmatag_subregion().
SEE ALSObus_space(9), mb(9)
Jason Thorpe, "A Machine-Independent DMA Framework for NetBSD",
Proceedings of the FREENIX Track: 1998 USENIX Annual Technical
Conference, USENIX Association,
http://www.usenix.org/publications/library/proceedings/usenix98/freenix/thorpe_dma.pdf,
1-12, June 15-19, 1998.
HISTORY
The bus_dma interface appeared in NetBSD 1.3.
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 Chris Demetriou,
Charles Hannum, Ross Harvey, Matthew Jacob, Jonathan Stone, and Matt
Thomas.
BSD July 8, 2011 BSD
