Re: SPI TX andRX buffer overlap

[David Brownell <david-b-yBeKhBN/0LDR7s880joybQ@public.gmane.org>]

On Thursday 13 November 2008, Ned Forrester wrote:
> David,
> 
> In DMA mode, pxa2xx_spi.c fails to detect the case where the tx and rx
> buffers overlap, and thus it performs dma_map_single incorrectly for
> that case.  I am working on a patch for that, but I have a question
> about overlapped buffers.
> 
> I know that it is legitimate for the tx and rx buffers to be the same;
> spidev passes identical tx and rx addresses, for example.  I plan to fix
> pxa2xx_spi so that it handles buffers having the same start address
> (completely overlapped), in addition to the currently handled case of
> completely non-overlapped buffers.  For shared buffers, I plan to call
> dma_map_single() and dma_unmap_single() once for the buffer with a
> parameter of DMA_BIDIRECTIONAL (let me know if that is not correct).

That seems OK ... albeit probably overkill.

Typically those DMA calls flush dcache to memory (data written to device)
or purge the cache (so entries get read afresh after the data gets read
from the device into memory), or both (bidirectional).

As always there's always something to make things complicated, and in
this case it's spelled IOMMU.  If one of those is in play, there can be
both cache operations *and& IOMMU mapping setup/teardown.  That's not
cheap ... but also not a factor on PXA, so far as I know.

 
> That said, I would like to know whether I can/should reject the case of
> overlapped buffers that do not have the same start address.  As I try to
> program for that case, the code is getting ugly.  It would be cleaner to
> detect overlapped but unequal buffers and refuse DMA in that case.
> Comments?

You shouldn't need to impose arbitrary limitations like that.  In fact,
I'm surprised you need to detect the bidirectional case explicitly.
Maybe the explanation for that is in some unread email.  :(

Can't you just dma_map_single( ... DMA_TO_DEVICE) first to flush the
caches, then dma_map_single( ... DMA_FROM_DEVICE) second to purge any
entries -- flushed or not -- that must not remain in the cache?

- Dave



> 
> -- 
> Ned Forrester                                       nforrester-/d+BM93fTQY@public.gmane.org
> Oceanographic Systems Lab                                  508-289-2226
> Applied Ocean Physics and Engineering Dept.
> Woods Hole Oceanographic Institution          Woods Hole, MA 02543, USA
> http://www.whoi.edu/sbl/liteSite.do?litesiteid=7212
> http://www.whoi.edu/hpb/Site.do?id=1532
> http://www.whoi.edu/page.do?pid=10079
> 
> 



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