USB mass storage and ARM cache coherency

[benh@kernel.crashing.org (Benjamin Herrenschmidt)]

On Fri, 2010-02-26 at 21:00 +0000, Russell King - ARM Linux wrote:
> On Fri, Feb 26, 2010 at 04:25:21PM +0000, Catalin Marinas wrote:
> > For mmap'ed pages (and present in the page cache), is it guaranteed that
> > the HCD driver won't write to it once it has been mapped into user
> > space? If that's the case, it may solve the problem by just reversing
> > the meaning of PG_arch_1 on ARM and assume that a newly allocated page
> > has dirty D-cache by default.
> 
> I guess we could also set PG_arch_1 in the DMA API as well, to avoid the
> unnecessary D cache flushing when clean pages get mapped into userspace.

That's an interesting thought for us too. When doing I$/D$ coherency, we
have to fist flush the D$ and then invalidate the I$. If we could keep
track of D$ and I$ separately, we could avoid the first step in many
cases, including the DMA API trick you mentioned.

I wonder if it's time to get a PG_arch_2 :-)

Cheers,
Ben.