SPI TX andRX buffer overlap

[Ned Forrester <nforrester-/d+BM93fTQY@public.gmane.org>]

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 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?

-- 
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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