NAME¶
bus_dma
,
bus_dma_tag_create
,
bus_dma_tag_destroy
,
bus_dmamap_create
,
bus_dmamap_destroy
,
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
,
bus_dmamem_alloc
,
bus_dmamem_free
—
Bus and Machine Independent DMA Mapping
Interface
SYNOPSIS¶
#include
<machine/bus.h>
int
bus_dma_tag_create
(
bus_dma_tag_t
parent,
bus_size_t
alignment,
bus_addr_t
boundary,
bus_addr_t
lowaddr,
bus_addr_t
highaddr,
bus_dma_filter_t
*filtfunc,
void
*filtfuncarg,
bus_size_t
maxsize,
int
nsegments,
bus_size_t
maxsegsz,
int
flags,
bus_dma_lock_t
*lockfunc,
void
*lockfuncarg,
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_bio
(
bus_dma_tag_t
dmat,
bus_dmamap_t
map,
struct bio
*bio,
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_sg
(
bus_dma_tag_t
dmat,
bus_dmamap_t
map,
struct mbuf
*mbuf,
bus_dma_segment_t
*segs,
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);
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,
op);
int
bus_dmamem_alloc
(
bus_dma_tag_t
dmat,
void
**vaddr,
int
flags,
bus_dmamap_t
*mapp);
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_filter_t
- 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.
- bus_dma_segment_t
- A machine-dependent type that describes individual DMA segments. 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 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. 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.
- bus_dmamap_callback_t
- Client specified callback for receiving mapping information resulting from
the load of a 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)
The callback_arg is the callback argument
passed to dmamap load functions. The segs
and nseg arguments 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 argument. 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. 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.
BUS_DMASYNC_POSTWRITE
- Perform any synchronization required after device access to host
memory.
- 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.
Two 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.
- bus_dma_lock_op_t
- Operations to be performed by the client-specified
lockfunc
().
BUS_DMA_LOCK
- Acquires and/or locks the client locking primitive.
BUS_DMA_UNLOCK
- Releases and/or unlocks the client locking primitive.
FUNCTIONS¶
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
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_SPACE_MAXADDR
and a
lowaddr of
BUS_SPACE_MAXADDR_32BIT
. Similarly
a device that can only perform 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.
- 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_SPACE_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 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 request's 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 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 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 bus_size_t
argument is also passed to the callback routine, which contains the mbuf
chain's 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
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
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 CPU's and device's 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 device's 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 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 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.
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 on failure as discussed above.
All
void routines will succeed if provided with
valid arguments.
LOCKING¶
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.
SEE ALSO¶
devclass(9),
device(9),
driver(9),
rman(9),
vslock(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
Chris Demetriou,
Charles Hannum,
Ross Harvey,
Matthew Jacob,
Jonathan Stone, and
Matt Thomas.
The
bus_dma
interface in
FreeBSD benefits from the contributions of
Justin T. Gibbs,
Peter Wemm,
Doug Rabson,
Matthew N. Dodd,
Sam Leffler,
Maxime Henrion,
Jake Burkholder,
Takahashi Yoshihiro,
Scott Long and many others.
This manual page was written by
Hiten M.
Pandya and
Justin T. Gibbs.