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.\" $NetBSD: bus_dma.9,v 1.25 2002/10/14 13:43:16 wiz Exp $
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.Dd August 31, 2006
.Dt BUS_DMA 9
.Os
.Sh NAME
.Nm bus_dma ,
.Nm bus_dma_tag_create ,
.Nm bus_dma_tag_destroy ,
.Nm bus_dmamap_create ,
.Nm bus_dmamap_destroy ,
.Nm bus_dmamap_load ,
.Nm bus_dmamap_load_mbuf ,
.Nm bus_dmamap_load_mbuf_sg ,
.Nm bus_dmamap_load_uio ,
.Nm bus_dmamap_unload ,
.Nm bus_dmamap_sync ,
.Nm bus_dmamem_alloc ,
.Nm bus_dmamem_free
.Nd Bus and Machine Independent DMA Mapping Interface
.Sh SYNOPSIS
.In machine/bus.h
.Ft int
.Fn 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_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"
.Ft int
.Fn bus_dma_tag_destroy "bus_dma_tag_t dmat"
.Ft int
.Fn bus_dmamap_create "bus_dma_tag_t dmat" "int flags" "bus_dmamap_t *mapp"
.Ft int
.Fn bus_dmamap_destroy "bus_dma_tag_t dmat" "bus_dmamap_t map"
.Ft int
.Fn 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"
.Ft int
.Fn 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"
.Ft int
.Fn 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"
.Ft int
.Fn 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"
.Ft void
.Fn bus_dmamap_unload "bus_dma_tag_t dmat" "bus_dmamap_t map"
.Ft void
.Fn bus_dmamap_sync "bus_dma_tag_t dmat" "bus_dmamap_t map" \
"op"
.Ft int
.Fn bus_dmamem_alloc "bus_dma_tag_t dmat" "void **vaddr" \
"int flags" "bus_dmamap_t *mapp"
.Ft void
.Fn bus_dmamem_free "bus_dma_tag_t dmat" "void *vaddr" \
"bus_dmamap_t map"
.Sh 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.
.Pp
The
.Nm
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.
.Sh STRUCTURES AND TYPES
.Bl -tag -width indent
.It Vt 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.
.It Vt bus_dma_filter_t
Client specified address filter having the format:
.Bl -tag -width indent
.It Ft int
.Fn "client_filter" "void *filtarg" "bus_addr_t testaddr"
.El
.Pp
Address filters can be specified during tag creation to allow
for devices whose DMA address restrictions cannot be specified
by a single window.
The
.Fa filtarg
is client specified during tag creation to be passed to all
invocations of the callback.
The
.Fa testaddr
argument contains a potential starting address of a DMA mapping.
The filter function operates on the set of addresses from
.Fa testaddr
to
.Ql trunc_page(testaddr) + PAGE_SIZE - 1 ,
inclusive.
The filter function should return zero for any mapping in this range
that can be accommodated by the device and non-zero otherwise.
.It Vt bus_dma_segment_t
A machine-dependent type that describes individual
DMA segments.
.Bd -literal
        bus_addr_t      ds_addr;
        bus_size_t      ds_len;
.Ed
.Pp
The
.Fa ds_addr
field contains the device visible address of the DMA segment, and
.Fa 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.
.It Vt 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
.Dv NULL
on some platforms under certain conditions,
it should never be assumed that it will be
.Dv NULL
in all cases.
.It Vt bus_dmamap_callback_t
Client specified callback for receiving mapping information resulting from
the load of a
.Vt bus_dmamap_t
via
.Fn bus_dmamap_load .
Callbacks are of the format:
.Bl -tag -width indent
.It Ft void
.Fn "client_callback" "void *callback_arg" "bus_dma_segment_t *segs" \
"int nseg" "int error"
.El
.Pp
The
.Fa callback_arg
is the callback argument passed to dmamap load functions.
The
.Fa segs
and
.Fa nseg
parameters describe an array of
.Vt 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
.Fa error
parameter.
More information on the use of callbacks can be found in the
description of the individual dmamap load functions.
.It Vt bus_dmamap_callback2_t
Client specified callback for receiving mapping information resulting from
the load of a
.Vt bus_dmamap_t
via
.Fn bus_dmamap_load_uio
or
.Fn bus_dmamap_load_mbuf .
.Pp
Callback2s are of the format:
.Bl -tag -width indent
.It Ft void
.Fn "client_callback2" "void *callback_arg" "bus_dma_segment_t *segs" \
"int nseg" "bus_size_t mapsize" "int error"
.El
.Pp
Callback2's behavior is the same as
.Vt bus_dmamap_callback_t
with the addition that the length of the data mapped is provided via
.Fa mapsize .
.It Vt 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
.Vt 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
.Fn bus_dmamap_sync
description below for more details on how to use these operations.
.Pp
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
.Nm .
.Bl -tag -width ".Dv BUS_DMASYNC_POSTWRITE"
.It Dv BUS_DMASYNC_PREREAD
Perform any synchronization required prior to an update of host memory by the
device.
.It Dv BUS_DMASYNC_PREWRITE
Perform any synchronization required after an update of host memory by the CPU
and prior to device access to host memory.
.It Dv BUS_DMASYNC_POSTREAD
Perform any synchronization required after an update of host memory by the
device and prior to CPU access to host memory.
.It Dv BUS_DMASYNC_POSTWRITE
Perform any synchronization required after the device access to host memory.
.El
.It Vt 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
.Dv BUS_DMA_LOCK
and immediately after with
.Dv 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:
.Bl -tag -width indent
.It Ft void
.Fn "lockfunc" "void *lockfunc_arg" "bus_dma_lock_op_t op"
.El
.Pp
Two
.Vt lockfunc
implementations are provided for convenience.
.Fn busdma_lock_mutex
performs standard mutex operations on the sleep mutex provided via the
.Fa lockfuncarg .
passed into
.Fn bus_dma_tag_create .
.Fn dflt_lock
will generate a system panic if it is called.
It is substituted into the tag when
.Fa lockfunc
is passed as
.Dv NULL
to
.Fn bus_dma_tag_create .
.It Vt bus_dma_lock_op_t
Operations to be performed by the client-specified
.Fn lockfunc .
.Bl -tag -width ".Dv BUS_DMA_UNLOCK"
.It Dv BUS_DMA_LOCK
Acquires and/or locks the client locking primitive.
.It Dv BUS_DMA_UNLOCK
Releases and/or unlocks the client locking primitive.
.El
.El
.Sh FUNCTIONS
.Bl -tag -width indent
.It Fn 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:
.Bl -tag -width ".Fa filtfuncarg"
.It Fa parent
Indicates restrictions between the parent bridge, CPU memory, and the
device.
Each device must use a master parent tag by calling
.Fn bus_get_dma_tag .
.It Fa 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
.Em 1
for byte alignment.
Hardware requiring DMA transfers to start on a multiple of 4K
would specify
.Em 4096 .
.It Fa 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
.Vt bus_dma_segment_t .
The boundary must be a power of 2 and must be no smaller than the
maximum segment size.
.Ql 0
indicates that there are no boundary restrictions.
.It Fa lowaddr , highaddr
Bounds of the window of bus address space that
.Em 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
.Dv BUS_SPACE_MAXADDR
and a lowaddr of
.Dv BUS_SPACE_MAXADDR_32BIT .
Similarly a device that can only dma to addresses bellow 16MB would
specify a highaddr of
.Dv BUS_SPACE_MAXADDR
and a lowaddr of
.Dv 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
.Ql safe memory
is used to bounce requests that would otherwise conflict with
the exclusion window.
.It Fa filtfunc
Optional filter function (may be
.Dv NULL )
to be called for any attempt to
map memory into the window described by
.Fa lowaddr
and
.Fa highaddr .
A filter function is only required when the single window described
by
.Fa lowaddr
and
.Fa highaddr
cannot adequately describe the constraints of the device.
The filter function will be called for every machine page
that overlaps the exclusion window.
.It Fa filtfuncarg
Argument passed to all calls to the filter function for this tag.
May be
.Dv NULL .
.It Fa maxsize
Maximum size, in bytes, of the sum of all segment lengths in a given
DMA mapping associated with this tag.
.It Fa nsegments
Number of discontinuities (scatter/gather segments) allowed
in a DMA mapped region.
If there is no restriction,
.Dv BUS_SPACE_UNRESTRICTED
may be specified.
.It Fa maxsegsz
Maximum size, in bytes, of a segment in any DMA mapped region associated
with
.Fa dmat .
.It Fa flags
Are as follows:
.Bl -tag -width ".Dv BUS_DMA_ALLOCNOW"
.It Dv BUS_DMA_ALLOCNOW
Pre-allocate enough resources to handle at least one map load operation on
this tag.
If sufficient resources are not available,
.Er ENOMEM
is returned.
This should not be used for tags that only describe buffers that will be
allocated with
.Fn 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.
.El
.It Fa lockfunc
Optional lock manipulation function (may be
.Dv NULL )
to be called when busdma
needs to manipulate a lock on behalf of the client.
If
.Dv NULL
is specified,
.Fn dflt_lock
is used.
.It Fa lockfuncarg
Optional argument to be passed to the function specified by
.Fa lockfunc .
.It Fa dmat
Pointer to a bus_dma_tag_t where the resulting DMA tag will
be stored.
.El
.Pp
Returns
.Er ENOMEM
if sufficient memory is not available for tag creation
or allocating mapping resources.
.It Fn bus_dma_tag_destroy "dmat"
Deallocate the DMA tag
.Fa dmat
that was created by
.Fn bus_dma_tag_create .
.Pp
Returns
.Er EBUSY
if any DMA maps remain associated with
.Fa dmat
or
.Ql 0
on success.
.It Fn bus_dmamap_create "dmat" "flags" "*mapp"
Allocates and initializes a DMA map.
Arguments are as follows:
.Bl -tag -width ".Fa nsegments"
.It Fa dmat
DMA tag.
.It Fa flags
The value of this argument is currently undefined and should be
specified as
.Ql 0 .
.It Fa mapp
Pointer to a
.Vt bus_dmamap_t
where the resulting DMA map will be stored.
.El
.Pp
Returns
.Er ENOMEM
if sufficient memory is not available for creating the
map or allocating mapping resources.
.It Fn bus_dmamap_destroy "dmat" "map"
Frees all resources associated with a given DMA map.
Arguments are as follows:
.Bl -tag -width ".Fa dmat"
.It Fa dmat
DMA tag used to allocate
.Fa map .
.It Fa map
The DMA map to destroy.
.El
.Pp
Returns
.Er EBUSY
if a mapping is still active for
.Fa map .
.It Fn bus_dmamap_load "dmat" "map" "buf" "buflen" "*callback" \
"callback_arg" "flags"
Creates a mapping in device visible address space of
.Fa buflen
bytes of
.Fa buf ,
associated with the DMA map
.Fa map .
This call will always return immediately and will not block for any reason.
Arguments are as follows:
.Bl -tag -width ".Fa buflen"
.It Fa dmat
DMA tag used to allocate
.Fa map .
.It Fa map
A DMA map without a currently active mapping.
.It Fa buf
A kernel virtual address pointer to a contiguous (in KVA) buffer, to be
mapped into device visible address space.
.It Fa buflen
The size of the buffer.
.It Fa callback Fa 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.
.It Fa flags
Are as follows:
.Bl -tag -width ".Dv BUS_DMA_NOWAIT"
.It Dv BUS_DMA_NOWAIT
The load should not be deferred in case of insufficient mapping resources,
and instead should return immediately with an appropriate error.
.El
.El
.Pp
Return values to the caller are as follows:
.Bl -tag -width ".Er EINPROGRESS"
.It 0
The callback has been called and completed.
The status of the mapping has been delivered to the callback.
.It Er 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.
To ensure that ordering is guaranteed, all subsequent load requests will also
be deferred until all callbacks have been processed.
.It Er ENOMEM
The load request has failed due to insufficient resources, and the caller
specifically used the
.Dv BUS_DMA_NOWAIT
flag.
.It Er EINVAL
The load request was invalid.
The callback has been called and has been provided the same error.
This error value may indicate that
.Fa dmat ,
.Fa map ,
.Fa buf ,
or
.Fa callback
were invalid, or
.Fa buflen
was larger than the
.Fa maxsize
argument used to create the dma tag
.Fa dmat .
.El
.Pp
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:
.Bl -tag -width ".Er EINPROGRESS"
.It 0
The mapping was successful and the
.Fa dm_segs
callback argument contains an array of
.Vt bus_dma_segment_t
elements describing the mapping.
This array is only valid during the scope of the callback function.
.It Er 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.
.El
.It Fn bus_dmamap_load_mbuf "dmat" "map" "mbuf" "callback2" "callback_arg" \
"flags"
This is a variation of
.Fn bus_dmamap_load
which maps mbuf chains
for DMA transfers.
A
.Vt bus_size_t
argument is also passed to the callback routine, which
contains the mbuf chain's packet header length.
The
.Dv BUS_DMA_NOWAIT
flag is implied, thus no callback deferral will happen.
.Pp
Mbuf chains are assumed to be in kernel virtual address space.
.Pp
Beside the error values listed for
.Fn bus_dmamap_load ,
.Er EINVAL
will be returned if the size of the mbuf chain exceeds the maximum limit of the
DMA tag.
.It Fn bus_dmamap_load_mbuf_sg "dmat" "map" "mbuf" "segs" "nsegs" "flags"
This is just like
.Fn bus_dmamap_load_mbuf
except that it returns immediately without calling a callback function.
It is provided for efficiency.
The scatter/gather segment array
.Va segs
is provided by the caller and filled in directly by the function.
The
.Va nsegs
argument is returned with the number of segments filled in.
Returns the same errors as
.Fn bus_dmamap_load_mbuf .
.It Fn bus_dmamap_load_uio "dmat" "map" "uio" "callback2" "callback_arg" "flags"
This is a variation of
.Fn bus_dmamap_load
which maps buffers pointed to by
.Fa uio
for DMA transfers.
A
.Vt bus_size_t
argument is also passed to the callback routine, which contains the size of
.Fa uio ,
i.e.
.Fa uio->uio_resid .
The
.Dv BUS_DMA_NOWAIT
flag is implied, thus no callback deferral will happen.
Returns the same errors as
.Fn bus_dmamap_load .
.Pp
If
.Fa uio->uio_segflg
is
.Dv UIO_USERSPACE ,
then it is assumed that the buffer,
.Fa uio
is in
.Fa "uio->uio_td->td_proc" Ns 's
address space.
User space memory must be in-core and wired prior to attempting a map
load operation.
Pages may be locked using
.Xr vslock 9 .
.It Fn bus_dmamap_unload "dmat" "map"
Unloads a DMA map.
Arguments are as follows:
.Bl -tag -width ".Fa dmam"
.It Fa dmat
DMA tag used to allocate
.Fa map .
.It Fa map
The DMA map that is to be unloaded.
.El
.Pp
.Fn bus_dmamap_unload
will not perform any implicit synchronization of DMA buffers.
This must be done explicitly by a call to
.Fn bus_dmamap_sync
prior to unloading the map.
.It Fn 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:
.Bl -tag -width ".Fa dmat"
.It Fa dmat
DMA tag used to allocate
.Fa map .
.It Fa map
The DMA mapping to be synchronized.
.It Fa op
Type of synchronization operation to perform.
See the definition of
.Vt bus_dmasync_op_t
for a description of the acceptable values for
.Fa op .
.El
.Pp
The
.Fn 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
.Dv BUS_DMASYNC_PREWRITE
must be performed.
If later CPU modifies this buffer again, another
.Dv BUS_DMASYNC_PREWRITE
sync operation must be performed before an additional
access to this memory by a device.
Conversely, a device updates the memory that is to be read by a CPU.
In this case, the buffer must be loaded, and a DMA sync operation of
.Dv BUS_DMASYNC_PREREAD
must be performed.
The CPU will only be able to see the results of this memory update
once the DMA operation has completed and a
.Dv BUS_DMASYNC_POSTREAD
sync operation has been performed.
.Pp
If read and write operations are not preceded and followed by the
appropriate synchronization operations, behavior is undefined.
.It Fn bus_dmamem_alloc "dmat" "**vaddr" "flags" "*mapp"
Allocates memory that is mapped into KVA at the address returned
in
.Fa vaddr
that is permanently loaded into the newly created
.Vt bus_dmamap_t
returned via
.Fa mapp .
Arguments are as follows:
.Bl -tag -width ".Fa alignment"
.It Fa dmat
DMA tag describing the constraints of the DMA mapping.
.It Fa vaddr
Pointer to a pointer that will hold the returned KVA mapping of
the allocated region.
.It Fa flags
Flags are defined as follows:
.Bl -tag -width ".Dv BUS_DMA_NOWAIT"
.It Dv BUS_DMA_WAITOK
The routine can safely wait (sleep) for resources.
.It Dv BUS_DMA_NOWAIT
The routine is not allowed to wait for resources.
If resources are not available,
.Dv ENOMEM
is returned.
.It Dv 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.
The
.Dv BUS_DMA_COHERENT
flag is currently implemented on sparc64 and arm.
.It Dv BUS_DMA_ZERO
Causes the allocated memory to be set to all zeros.
.El
.It Fa mapp
Pointer to a
.Vt bus_dmamap_t
where the resulting DMA map will be stored.
.El
.Pp
The size of memory to be allocated is
.Fa maxsize
as specified in
.Fa dmat .
.Pp
The current implementation of
.Fn bus_dmamem_alloc
will allocate all requests as a single segment.
.Pp
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
.Fn bus_dmamem_free .
Maps are automatically handled by this function and should not be explicitly
allocated or destroyed.
.Pp
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
.Fn bus_dmamap_sync
section still apply and should be used to achieve portability on architecutures
without coherent buses.
.Pp
Returns
.Er ENOMEM
if sufficient memory is not available for completing
the operation.
.It Fn bus_dmamem_free "dmat" "*vaddr" "map"
Frees memory previously allocated by
.Fn bus_dmamem_alloc .
Any mappings
will be invalidated.
Arguments are as follows:
.Bl -tag -width ".Fa vaddr"
.It Fa dmat
DMA tag.
.It Fa vaddr
Kernel virtual address of the memory.
.It Fa map
DMA map to be invalidated.
.El
.El
.Sh 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,
.Er ENOMEM
is returned.
All
routines that are not of type,
.Vt void ,
will return 0 on success or an error
code, as discussed above.
.Pp
All
.Vt void
routines will succeed if provided with valid arguments.
.Sh LOCKING
Two locking protocols are used by
.Nm .
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
.Nm
and is not exposed to clients of the API.
.Pp
The second protocol involves protecting various resources stored in the tag.
Since almost all
.Nm
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
.Nm
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
.Fn bus_dmamap_load
callback function is called from a deferred context instead of the driver
context.
This means that certain
.Nm
functions must always be called with the same lock held that is specified in the
tag.
These functions include:
.Pp
.Bl -item -offset indent -compact
.It
.Fn bus_dmamap_load
.It
.Fn bus_dmamap_load_uio
.It
.Fn bus_dmamap_load_mbuf
.It
.Fn bus_dmamap_load_mbuf_sg
.It
.Fn bus_dmamap_unload
.It
.Fn bus_dmamap_sync
.El
.Pp
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.
.Pp
Certain
.Nm
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:
.Pp
.Bl -item -offset indent -compact
.It
.Fn bus_dma_tag_create
.It
.Fn bus_dmamap_create
.It
.Fn bus_dmamem_alloc
.El
.Pp
All other functions do not have a locking protocol and can thus be
called with or without and system or driver locks held.
.Sh SEE ALSO
.Xr devclass 9 ,
.Xr device 9 ,
.Xr driver 9 ,
.Xr rman 9 ,
.Xr vslock 9
.Pp
.Rs
.%A "Jason R. Thorpe"
.%T "A Machine-Independent DMA Framework for NetBSD"
.%J "Proceedings of the Summer 1998 USENIX Technical Conference"
.%Q "USENIX Association"
.%D "June 1998"
.Re
.Sh HISTORY
The
.Nm
interface first appeared in
.Nx 1.3 .
.Pp
The
.Nm
API was adopted from
.Nx
for use in the CAM SCSI subsystem.
The alterations to the original API were aimed to remove the need for
a
.Vt bus_dma_segment_t
array stored in each
.Vt bus_dmamap_t
while allowing callers to queue up on scarce resources.
.Sh AUTHORS
The
.Nm
interface was designed and implemented by
.An Jason R. Thorpe
of the Numerical Aerospace Simulation Facility, NASA Ames Research Center.
Additional input on the
.Nm
design was provided by
.An -nosplit
.An Chris Demetriou ,
.An Charles Hannum ,
.An Ross Harvey ,
.An Matthew Jacob ,
.An Jonathan Stone ,
and
.An Matt Thomas .
.Pp
The
.Nm
interface in
.Fx
benefits from the contributions of
.An Justin T. Gibbs ,
.An Peter Wemm ,
.An Doug Rabson ,
.An Matthew N. Dodd ,
.An Sam Leffler ,
.An Maxime Henrion ,
.An Jake Burkholder ,
.An Takahashi Yoshihiro ,
.An Scott Long
and many others.
.Pp
This manual page was written by
.An Hiten M. Pandya
and
.An Justin T. Gibbs .