bus_dma(9)

NAME

bus_dma, bus_dma_tag_create, bus_dma_tag_destroy,
bus_dmamap_create,
bus_dmamap_destroy, bus_dmamap_load, bus_dmamap_load_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_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);
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_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 limita
tions.

STRUCTURES AND TYPES

bus_dma_tag_t
A machine-dependent (MD) opaque type that describes
the characteristics of DMA transactions. DMA tags are orga
nized into a
hierarchy, with each child tag inheriting the re
strictions 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 cre
ation to allow
for devices whose DMA address restrictions cannot
be specified
by a single window. The filtarg is client speci
fied during tag
creation to be passed to all invocations of the
callback. The
testaddr argument contains a potential starting ad
dress 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 for
any mapping in
this range that can be accommodated by the device
and non-zero
otherwise.
bus_dma_segment_t
A machine-dependent type that describes individual
DMA segments.

bus_addr_t ds_addr;
bus_size_t ds_len;
The ds_addr field contains the device visible ad
dress 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 indi
vidual mapping. One map is used for each memory allocation
that will be
loaded. Maps can be reused once they have been un
loaded. 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 in
formation
resulting from the load of a bus_dmamap_t via
bus_dmamap_load().
Callbacks are of the format:
void client_callback(void *callback_arg,
bus_dma_segment_t
*segs, int nseg, int error)
The callback_arg is the callback argument passed to
dmamap load
functions. The segs and nseg parameters describe
an array of
bus_dma_segment_t structures that represent the
mapping. This
array is only valid within the scope of the call
back function.
The success or failure of the mapping is indicated
by the error
parameter. More information on the use of call
backs can be
found in the description of the individual dmamap
load functions.
bus_dmamap_callback2_t
Client specified callback for receiving mapping in
formation
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 op
erations 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. Alternately, 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
DMA read operation.
BUS_DMASYNC_PREWRITE Perform any synchronization
required
after an update of host mem
ory by the CPU
and prior to DMA write oper
ations.
BUS_DMASYNC_POSTREAD Perform any synchronization
required
after DMA read operations
and prior to
CPU access to host memory.
BUS_DMASYNC_POSTWRITE Perform any synchronization
required
after DMA write operations.
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 op
eration 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)
Two lockfunc implementations are provided for con
venience.
busdma_lock_mutex() performs standard mutex opera
tions on the
sleep mutex provided via the lockfuncarg. passed
into
bus_dma_tag_create(). 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().
bus_dma_lock_op_t
Operations to be performed by the client-specified
lockfunc().
BUS_DMA_LOCK Acquires and/or locks the client
locking primi
tive.
BUS_DMA_UNLOCK Releases and/or unlocks the client
locking prim
itive.

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 initial
izes it according to the arguments provided:
parent Indicates restrictions between the
parent bridge,
CPU memory, and the device. May be
NULL, if no
DMA restrictions are to be inherited.
alignment Alignment constraint, in bytes, of
any mappings
created using this tag. The align
ment 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 indi
cates 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 dma to addresses bellow 16MB
would specify a highaddr of BUS_SPACE_MAXADDR
and a lowaddr
of BUS_SPACE_MAXADDR_24BIT. Some im
plementations
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 exclu
sion window.
filtfuncarg Argument passed to all calls to the
filter func
tion for this tag. May be NULL.
maxsize Maximum size, in bytes, of the sum of
all segment
lengths in a given DMA mapping asso
ciated with
this tag.
nsegments Number of discontinuities (scat
ter/gather seg
ments) allowed in a DMA mapped re
gion. If there
is no restriction, BUS_SPACE_UNRE
STRICTED 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 re
turned. 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 ma
nipulate 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 avail
able 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 The value of this argument is currently
undefined and
should be specified as `0'.
mapp Pointer to a bus_dmamap_t where the re
sulting DMA map
will be stored.
Returns ENOMEM if sufficient memory is not avail
able 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 map
ping.
buf A kernel virtual address pointer to a con
tiguous (in
KVA) buffer, to be mapped into device visi
ble 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 de
ferred 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 ap
propriately 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 de
ferred in case
of insufficient mapping re
sources, and
instead should return imme
diately with
an appropriate error.
Return values to the caller are as follows:
0 The callback has been called and com
pleted. The
status of the mapping has been deliv
ered 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. To ensure that ordering
is guaranteed,
all subsequent load requests will also
be deferred
until all callbacks have been pro
cessed.
ENOMEM The load request has failed due to in
sufficient
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_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 ad
dress space.
Beside the error values listed for
bus_dmamap_load(), EINVAL
will be returned if the size of the mbuf chain ex
ceeds 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 seg
ments 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 as
sumed 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 at
tempting a map
load operation. Pages may be locked using vs
lock(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 map
ping with the
CPU visible memory referenced by that mapping. Ar
guments 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 descrip
tion of the
acceptable values for op.
bus_dmamap_sync() is the method used to ensure that
CPU and
device DMA access to shared memory is coherent.
For example,
the CPU might be used to setup the contents of a
buffer that is
to be DMA'ed into a device. To ensure that the da
ta are visible
via the device's mapping of that memory, the buffer
must be
loaded and a dma sync operation of BUS_DMASYNC_PRE
READ must be
performed. Additional sync operations must be per
formed after
every CPU write to this memory if additional DMA
reads are to be
performed. Conversely, for the DMA write case, the
buffer must
be loaded, and a dma sync operation of
BUS_DMASYNC_PREWRITE must
be performed. The CPU will only be able to see the
results of
this DMA write once the DMA has completed and a
BUS_DMASYNC_POSTWRITE operation has been performed.
If DMA read and write operations are not preceded
and followed
by the appropriate synchronization operations, be
havior is undefined.
bus_dmamem_alloc(dmat, **vaddr, flags, *mapp)
Allocates memory that is mapped into KVA at the ad
dress returned
in vaddr that is permanently loaded into the newly
created
bus_dmamap_t returned via mapp. Arguments are as
follows:
dmat DMA tag describing the constraints of
the DMA map
ping.
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 al
lowed to wait
for resources. If re
sources are not
available, ENOMEM is re
turned.
BUS_DMA_COHERENT
Attempt to map this mem
ory 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 en
gine, frequently. Use of this
flag does not
remove the requirement
of using
bus_dmamap_sync, but it
may reduce
the cost of performing
these operations.
BUS_DMA_ZERO Causes the allocated
memory to be set
to all zeros.
mapp Pointer to a bus_dmamap_t where the re
sulting DMA map
will be stored.
The size of memory to be allocated is maxsize as
specified in
dmat.
The current implementation of bus_dmamem_alloc()
will allocate
all requests as a single segment.
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 de
stroyed.
Although an explicit load is not required for each
access to the
memory referenced by the returned map, the synchro
nization
requirements as described in the bus_dmamap_sync()
section still
apply and should be used to achieve portability on
architecutures without coherent buses.
Returns ENOMEM if sufficient memory is not avail
able 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, as dis
cussed above.
All void routines will succeed if provided with valid argu
ments.

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 re
move 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 Re
search 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 contribu
tions 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.
BSD December 5, 2005

BSD

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