freebsd-skq/share/man/man9/bus_dma.9

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.\" $NetBSD: bus_dma.9,v 1.25 2002/10/14 13:43:16 wiz Exp $
.\"
.Dd May 28, 2003
.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_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_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_uio "bus_dma_tag_t dmat" "bus_dmamap_t map" \
"struct uio *uio" "bus_dmamap_callback2_t *callback" "void *callback_arg" \
"int flags"
.Ft int
.Fn bus_dmamem_alloc "bus_dma_tag_t dmat" "void **vaddr" \
"int flags" "bus_dmamap_t *mapp"
.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 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 compact
.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 compact
.It Ft int
.Fn "client_filter" "void *filtarg" "bus_addr_t testaddr"
.El
.sp
Address filters can be specified during tag creation to allow
for devices who's 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
.sp
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.
Multiple DMA maps can be associated with one DMA tag.
.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 compact
.It Ft void
.Fn "client_callback" "void *callback_arg" "bus_dma_segment_t *segs" \
"int nseg" "int error"
.El
.sp
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 .
.sp
Callback2s are of the format:
.Bl -tag -width compact
.It Ft void
.Fn "client_callback2" "void *callback_arg" "bus_dma_segment_t *segs" \
"int nseg" "bus_size_t mapsize" "int error"
.El
.sp
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 it's 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.
All operations specified below are performed from the DMA engine's
point of view:
.Bl -tag -width BUS_DMASYNC_POSTWRITE
.It Dv BUS_DMASYNC_PREREAD
Perform any synchronization required after an update of memory by the CPU
but prior to DMA read operations.
.It Dv BUS_DMASYNC_PREWRITE
Perform any synchronization required after an update of memory by the CPU
but prior to DMA write operations.
.It Dv BUS_DMASYNC_PREREAD|BUS_DMASYNC_PREWRITE
Perform any synchronization required prior to a combination of DMA read
and write operations.
.It Dv BUS_DMASYNC_POSTREAD
Perform any synchronization required after DMA read operations, but prior
to CPU access of the memory.
.It Dv BUS_DMASYNC_POSTWRITE
Perform any synchronization required after DMA write operations, but prior
to CPU access of the memory.
.It Dv BUS_DMASYNC_POSTREAD|BUS_DMASYNC_POSTWRITE
Perform any synchronization required after a combination of DMA read
and write operations.
.El
.El
.Sh FUNCTIONS
.Bl -tag -width compact
.It Fn bus_dma_tag_create "parent" "alignment" "boundary" "lowaddr" \
"highaddr" "*filtfunc" "*filtfuncarg" "maxsize" "nsegments" "maxsegsz" \
"flags" "*dmat"
Allocates a device specific DMA tag, and initializes it according to
the arguments provided:
.Bl -tag -width *filtfuncarg -compact
.It Fa parent
Indicates restrictions between the parent bridge, CPU memory, and the
device.
May be NULL, if no DMA restrictions are to be inherited.
.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 either a power of 2 or 0.
.Ql 0
indicates that there are no boundary restrictions.
.It Fa lowaddr
.It Fa highaddr
Bounds of the window of bus address space that
.Em cannot
be directly accessed by the device.
The window contains all address 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 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 NULL.
.It Fa maxsegsz
Maximum size, in bytes, of a segment in any DMA mapped region associated
with
.Fa dmat .
.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 flags
Are as follows:
.Bl -tag -width "BUS_DMA_ALLOCNOW" -compact
.It Dv BUS_DMA_ALLOCNOW
Allocate the resources necessary to guarantee that all map load
operations associated with this tag will not block.
If sufficient resources are not available,
.Er ENOMEM
is returned.
.El
.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 nsegments -compact
.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 dmat -compact
.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" "..."
Creates a mapping in device visible address space of
.Fa buflen
bytes of
.Fa buf ,
associated with the DMA map
.Fa map.
Arguments are as follows:
.Bl -tag -width buflen -compact
.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.
.It Fa flags
The value of this argument is currently undefined, and should be
specified as
.Ql 0 .
.El
.Pp
Return values to the caller are as follows:
.Bl -tag -width EINPROGRESS -compact
.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.
DMA maps created from DMA tags that are allocated with
the
.Dv BUS_DMA_ALLOCNOW
flag will never return this status for a load operation.
.It Er EINVAL
The load request was invalid.
The callback has not, and will not be called.
This error value may indicate that
.Fa dmat ,
.Fa map ,
.Fa buf ,
or
.Fa callback
were invalid, or
.Fa buslen
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 EINPROGRESS -compact
.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.
.Pp
Mbuf chains are assumed to be in kernel virtual address space.
.Pp
Returns
.Er EINVAL
if the size of the mbuf chain exceeds the maximum limit of the
DMA tag.
.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 .
.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.
.It Fn bus_dmamap_unload "dmat" "map"
Unloads a DMA map.
Arguments are as follows:
.Bl -tag -width dmam -compact
.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 dmat -compact
.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
.Fn 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 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_PREREAD
must be performed.
Additional sync operations must be performed 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
.Dv 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
.Dv BUS_DMASYNC_POSTWRITE
operation has been performed.
.Pp
If DMA 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 alignment -compact
.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 BUS_DMA_NOWAIT -compact
.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.
.El
.It Fa mapp
Pointer to storage for the returned DMA map.
.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
Although no explicit loading is required to access the memory
referenced by the returned map, the synchronization requirements
as described in the
.Fn bus_dmamap_sync
section still apply.
.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 vaddr -compact
.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 SEE ALSO
.Xr devclass 9 ,
.Xr device 9 ,
.Xr driver 9 ,
.Xr rman 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 Jason R. Thorpe of the
Numerical Aerospace Simulation Facility, NASA Ames Research Center.
Additional input on the
.Nm
design was provided by Chris Demetriou, Charles Hannum, Ross Harvey,
Matthew Jacob, Jonathan Stone, and Matt Thomas.
.Pp
The
.Nm
interface in
.Fx
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.
.Pp
This manual page was written by Hiten M. Pandya and Justin T. Gibbs.