RE: [PATCH 7/9] ARM: DMA: steal memory for DMA coherent mappings

Marek Szyprowski
2011-08-16T10:18:09Z

Hello,

On Friday, August 12, 2011 2:53 PM Arnd Bergmann wrote:

> On Friday 12 August 2011, Marek Szyprowski wrote:
> >
> > From: Russell King <rmk+kernel@arm.linux.org.uk>
> >
> > Steal memory from the kernel to provide coherent DMA memory to drivers.
> > This avoids the problem with multiple mappings with differing attributes
> > on later CPUs.
> >
> > Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
> > [m.szyprowski: rebased onto 3.1-rc1]
> > Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
>
> Hi Marek,
>
> Is this the same patch that Russell had to revert because it didn't
> work on some of the older machines, in particular those using
> dmabounce?

Yes.

> I thought that our discussion ended with the plan to use this only
> for ARMv6+ (which has a problem with double mapping) but not on ARMv5
> and below (which don't have this problem but might need dmabounce).

Ok, my fault. I've forgot to mention that this patch was almost ready
during Linaro meeting, but I didn't manage to post it that time. Of course
it doesn't fulfill all the agreements from that discussion.

I was only unsure if we should care about the case where CMA is not enabled
for ARMv6+ or not. This patch was prepared in assumption that
dma_alloc_coherent should work in both cases - with and without CMA.

Now I assume that for ARMv6+ the CMA should be enabled unconditionally.

Best regards
--
Marek Szyprowski
Samsung Poland R&D Center