ixp4xx dmabounce

[Russell King - ARM Linux]

On Tue, Sep 22, 2009 at 06:02:19PM -0400, Brian Walsh wrote:
> On Thu, Sep 17, 2009 at 5:53 PM, Russell King - ARM Linux
> <linux at arm.linux.org.uk> wrote:
> >
> > On Thu, Sep 17, 2009 at 05:02:59PM -0400, Brian Walsh wrote:
> > > Any ideas or suggestions?
> >
> > It's caused because we don't allow dma_free_coherent() to be called from
> > IRQ context (which is reasonable because it needs to flush TLBs across
> > all processors on SMP systems.)
> 
> I am not running on an SMP system so would this even be a problem?

Yes - because it's there to ensure that the API is used in a consistent
way.

> > Unfortunately, with the DMA bounce code enabled, this function does get
> > called from IRQ context, and so tends to spit out these warnings.
> >
> > I did have a patch which made dma_free_coherent() lazy, but it was
> > reported that the suffered disk corruption (though it was never
> > conclusive whether it was caused by the patch or not.)  Here's an
> > updated version of that patch.
> >
> 
> I did not see any data rate improvement using this patch over just commenting
> out the warning stack dump.  I am still seeing about half the data transfer rate
> using the high speed ehci USB controller over the full speed ohci USB
> controller.

I didn't suggest it would improve the data rate - only that it should
fix the stack dump.

I'd really like to get this patch properly tested and confirmed that it
does _not_ actually cause corruption, so that I can get it merged.