Provided by: freebsd-manpages_12.2-1_all 
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.
OVERVIEW
A tag structure (bus_dma_tag_t) is used to describe the properties of a group of related DMA
transactions. One way to view this is that a tag describes the limitations of a DMA engine. For
example, if a DMA engine in a device is limited to 32-bit addresses, that limitation is specified by a
parameter when creating the tag for that device. Similarly, a tag can be marked as requiring buffers
whose addresses are aligned to a specific boundary.
Some devices may require multiple tags to describe DMA transactions with differing properties. For
example, a device might require 16-byte alignment of its descriptor ring while permitting arbitrary
alignment of I/O buffers. In this case, the driver must create one tag for the descriptor ring and a
separate tag for I/O buffers. If a device has restrictions that are common to all DMA transactions in
addition to restrictions that differ between unrelated groups of transactions, the driver can first
create a “parent” tag that decribes the common restrictions. The per-group tags can then inherit these
restrictions from this “parent” tag rather than having to list them explicitly when creating the per-
group tags.
A mapping structure (bus_dmamap_t) represents a mapping of a memory region for DMA. On systems with I/O
MMUs, the mapping structure tracks any I/O MMU entries used by a request. For DMA requests that require
bounce pages, the mapping tracks the bounce pages used.
To prepare for one or more DMA transactions, a mapping must be bound to a memory region by calling one of
the bus_dmamap_load() functions. These functions configure the mapping which can include programming
entries in an I/O MMU and/or allocating bounce pages. An output of these functions (either directly or
indirectly by invoking a callback routine) is the list of scatter/gather address ranges a consumer can
pass to a DMA engine to access the memory region. When a mapping is no longer needed, the mapping must
be unloaded via bus_dmamap_unload().
Before and after each DMA transaction, bus_dmamap_sync() must be used to ensure that the correct data is
used by the DMA engine and the CPU. If a mapping uses bounce pages, the sync operations copy data
between the bounce pages and the memory region bound to the mapping. Sync operations also handle
architecture-specific details such as CPU cache flushing and CPU memory operation ordering.
STATIC VS DYNAMIC
bus_dma handles two types of DMA transactions: static and dynamic. Static transactions are used with a
long-lived memory region that is reused for many transactions such as a descriptor ring. Dynamic
transactions are used for transfers to or from transient buffers such as I/O buffers holding a network
packet or disk block. Each transaction type uses a different subset of the bus_dma API.
Static Transactions
Static transactions use memory regions allocated by bus_dma. Each static memory region is allocated by
calling bus_dmamem_alloc(). This function requires a valid tag describing the properties of the DMA
transactions to this region such as alignment or address restrictions. Multiple regions can share a
single tag if they share the same restrictions.
bus_dmamem_alloc() allocates a memory region along with a mapping object. The associated tag, memory
region, and mapping object must then be passed to bus_dmamap_load() to bind the mapping to the allocated
region and obtain the scatter/gather list.
It is expected that bus_dmamem_alloc() will attempt to allocate memory requiring less expensive sync
operations (for example, implementations should not allocate regions requiring bounce pages), but sync
operations should still be used. For example, a driver should use bus_dmamap_sync() in an interrupt
handler before reading descriptor ring entries written by the device prior to the interrupt.
When a consumer is finished with a memory region, it should unload the mapping via bus_dmamap_unload()
and then release the memory region and mapping object via bus_dmamem_free().
Dynamic Transactions
Dynamic transactions map memory regions provided by other parts of the system. A tag must be created via
bus_dma_tag_create() to describe the DMA transactions to and from these memory regions, and a pool of
mapping objects must be allocated via bus_dmamap_create() to track the mappings of any in-flight
transactions.
When a consumer wishes to schedule a transaction for a memory region, the consumer must first obtain an
unused mapping object from its pool of mapping objects. The memory region must be bound to the mapping
object via one of the bus_dmamap_load() functions. Before scheduling the transaction, the consumer
should sync the memory region via bus_dmamap_sync() with one or more of the “PRE” flags. After the
transaction has completed, the consumer should sync the memory region via bus_dmamap_sync() with one or
more of the “POST” flags. The mapping can then be unloaded via bus_dmamap_unload(), and the mapping
object can be returned to the pool of unused mapping objects.
When a consumer is no longer scheduling DMA transactions, the mapping objects should be freed via
bus_dmamap_destroy(), and the tag should be freed via bus_dma_tag_destroy().
STRUCTURES AND TYPES
bus_dma_tag_t
A machine-dependent (MD) opaque type that describes the characteristics of a group 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 DMA tag, and initializes it according to the arguments provided:
parent A parent tag from which to inherit restrictions. The restrictions passed in other
arguments can only further tighten the restrictions inherited from the parent tag.
All tags created by a device driver must inherit from the tag returned by
bus_get_dma_tag() to honor restrictions between the parent bridge, CPU memory, and
the device.
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
require 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.
BUS_DMA_COHERENT Indicate that the DMA engine and CPU are cache-coherent. Cached
memory may be used to back allocations created by
bus_dmamem_alloc(). For bus_dma_tag_create(), the
BUS_DMA_COHERENT flag is currently implemented on arm64.
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, arm64 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.
Debian August 11, 2018 BUS_DMA(9)