USB mass storage and ARM cache coherency

[Benjamin Herrenschmidt]

On Wed, 2010-02-17 at 09:55 +0000, Russell King - ARM Linux wrote:
> Nope.  It's to do with mapping a buffer for DMA, and then doing PIO
> reads/writes to it.
> 
> With speculative prefetches, you have to deal with cache coherency with
> hardware DMA on DMA unmap.  If you've written to the buffer in violation
> of the DMA API buffer ownership rules, then your writes get thrown away
> resulting in immediate data corruption. 

Right, and this exact same problem will bite some embedded powerpc
too I suppose :-)

Hrm... actually not :-) We don't do the invalidate at unmap time
today because we know 44x have such a broken prefetcher that we disable
it ... interesting considering that there are machines around that
do non-coherent DMA with 750's style chips who -do- have a prefetcher...
damn, we have a bug :-)

In any case, same problem here.

See my reply to Oliver. Basically, the problem boils down to the
dma_map/unmap being done at the wrong layer. The driver should
simply not do these if it's going to do PIO over that range.

Cheers,
Ben.