-----Original Message-----
From: Oliver Neukum [mailto:oliver at neukum.org]
Sent: Tuesday, February 16, 2010 7:03 PM
To: Russell King - ARM Linux
Cc: Shilimkar, Santosh; Catalin Marinas; Pavel Machek; Greg KH; Matthew Dharm; Sergei Shtylyov; Ming
Lei; Sebastian Siewior; linux-usb at vger.kernel.org; linux-kernel; linux-arm-kernel; Mankad, Maulik
Ojas
Subject: Re: USB mass storage and ARM cache coherency

Am Dienstag, 16. Februar 2010 10:39:46 schrieb Russell King - ARM Linux:

quoted

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.

This means don't use dma at all which will almost kill the performance.

Regards,
Santosh