Re: USB mass storage and ARM cache coherency

[Oliver Neukum]

Am Dienstag, 16. Februar 2010 10:39:46 schrieb Russell King - ARM Linux:
> However, because ARM CPUs can now speculatively prefetch, just leaving it
> at that results in corruption of buffers used for DMA.  So we have to
> invalidate DMA_FROM_DEVICE and DMA_BIDIRECTIONAL buffers on unmap to
> ensure coherency with DMA operations.
> 
> If the CPU writes to a DMA_FROM_DEVICE buffer between map and unmap, the
> writes can sit in the cache, and on unmap, they will be discarded.
> 
> Cleaning the cache on unmap is not an option; that too can lead to DMA
> buffer corruption in the DMA case.

I am afraid for these controllers the controller driver must be responsible
for all DMA and cache issues. Indicating the exact requirements to the
upper layer would be a battle already lost.
so the safe choice is not to set has_dma and the generic layer will leave
the issue to the lower level.

	Regards
		Oliver
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