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More clarification on how bus_dmamap_sync works. Fix bogus text concerning

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bus_dmamem_alloc.
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scottl 2005-08-31 16:55:00 +00:00
parent 6417f6af98
commit 1c08d39d7b
1 changed files with 25 additions and 11 deletions
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36
share/man/man9/bus_dma.9
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@ -243,24 +243,28 @@ The
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 CPU's
point of view, where a read implies data coming from the device to the CPU, and
a write implies data going from the CPU to the device.
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.
Alternately, the operations can be thought of in terms of driver operations,
where reading a network packet or storage sector coresponds to a read operation
in busdma.
.Bl -tag -width BUS_DMASYNC_POSTWRITE
.It Dv BUS_DMASYNC_PREREAD
Perform any synchronization required prior to an update of memory by the DMA
operation.
Perform any synchronization required prior to an update of host memory by the
DMA read operation.
.It Dv BUS_DMASYNC_PREWRITE
Perform any synchronization required after an update of memory by the CPU
Perform any synchronization required after an update of host memory by the CPU
and prior to DMA write operations.
.It Dv BUS_DMASYNC_POSTREAD
Perform any synchronization required after DMA read operations and prior to
CPU access to the memory.
CPU access to host memory.
.It Dv BUS_DMASYNC_POSTWRITE
Perform any synchronization required after DMA write operations.
.El
@ -652,7 +656,7 @@ 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"
.It Fn bus_dmamem_alloc "dmat" "**vaddr" "flags" "*mapp"
Allocates memory that is mapped into KVA at the address returned
in
.Fa vaddr
@ -689,7 +693,9 @@ these operations.
Causes the allocated memory to be set to all zeros.
.El
.It Fa mapp
Pointer to storage for the returned DMA map.
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
@ -701,11 +707,19 @@ 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
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.
section still apply and should be used to achieve portability on architecutures
without coherent buses.
.Pp
Returns
.Er ENOMEM