Hi Russel,
On Fri, 2018-05-18 at 18:50 +0100, Russell King - ARM Linux wrote:
It's necessary. Take a moment to think carefully about this:
dma_map_single(, dir)
dma_sync_single_for_cpu(, dir)
dma_sync_single_for_device(, dir)
dma_unmap_single(, dir)
In the case of a DMA-incoherent architecture, the operations done at each
stage depend on the direction argument:
map for_cpu for_device unmap
TO_DEV writeback none writeback none
TO_CPU invalidate invalidate* invalidate invalidate*
BIDIR writeback invalidate writeback invalidate
* - only necessary if the CPU speculatively prefetches.
I think invalidation of DMA buffer is required on for_cpu(TO_CPU) even
if CPU doesn't preferch - what if we reuse the same buffer for multiple
reads from DMA device?
The multiple invalidations for the TO_CPU case handles different
conditions that can result in data corruption, and for some CPUs, all
four are necessary.
I would agree that map()/unmap() a quite a special cases and so depending
on direction we need to execute in them either for_cpu() or for_device()
call-backs depending on direction.
As for invalidation in case of for_device(TO_CPU) I still don't see
a rationale behind it. Would be interesting to see a real example where
we benefit from this.
This is what is implemented for 32-bit ARM, depending on the CPU
capabilities, as we have DMA incoherent devices and we have CPUs that
speculatively prefetch data, and so may load data into the caches while
DMA is in operation.
Things get more interesting if the implementation behind the DMA API has
to copy data between the buffer supplied to the mapping and some DMA
accessible buffer:
map for_cpu for_device unmap
TO_DEV copy to dma none copy to dma none
TO_CPU none copy to cpu none copy to cpu
BIDIR copy to dma copy to cpu copy to dma copy to cpu
So, in both cases, the value of the direction argument defines what you
need to do in each call.
Interesting enough in your seond table (which describes more complicated
case indeed) you set "none" for for_device(TO_CPU) which looks logical
to me.
So IMHO that's what make sense:
---------------------------->8-----------------------------
map for_cpu for_device unmap
TO_DEV writeback none writeback none
TO_CPU none invalidate none invalidate*
BIDIR writeback invalidate writeback invalidate*
---------------------------->8-----------------------------
* is the case for prefetching CPU.
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