USB mass storage and ARM cache coherency

[Shilimkar, Santosh]

> -----Original Message-----
> From: Oliver Neukum [mailto:oliver at neukum.org]
> Sent: Tuesday, February 16, 2010 7:17 PM
> To: Shilimkar, Santosh
> Cc: Russell King - ARM Linux; 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 14:40:45 schrieb Shilimkar, Santosh:
> > > > 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.
> 
> Why would you be unable to map a buffer in the hcd driver when you know
> that you'll use DMA?
Probably it can be. The USB stack has the dma maintenance code at common 
place for all controllers and hence we were just trying to see if there is 
way to handle that way.

We shall check this possibility

Regards,
Santosh