Re: [Linaro-mm-sig] [RFC] ARM DMA mapping TODO, v1

[Arnd Bergmann]

On Monday 02 May 2011, David Brown wrote:
> I'll confirm this from the Qualcomm side as well.  You cannot have
> multiple inconsistent mappings of the same page without having difficult
> to find problems.

I believe Catalin was referring to the case where you have only
one nonconsistent (cacheable) mapping plus multiple consistent
(cacheable) mappings. I don't think anyone has suggested doing
DMA to a page that has multiple nonconsistent mappings with
virtually indexed caches.

> The spec clarifications appear to give ways of dealing with it if it
> happens, and bounds on what can go wrong, but I wouldn't call it
> something we want to do normally.  Corrupt data is arguably less of a
> problem than nasal demons, but still a problem.

Anything that has a theoretical chance of corrupting data is not an
option, but I'd really like to see what the clarified spec says
about this. Even if there is a way to legally leave a page for
dma_alloc_coherent in the linear mapping, it might turn out to
be harder to do than using highmem pages or unmapping supersections
at run time as was suggested.

	Arnd
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/