USB mass storage and ARM cache coherency

[Benjamin Herrenschmidt]

On Tue, 2010-02-16 at 09:22 +0100, Oliver Neukum wrote:
> This seems wrong to me. Buffers for control transfers may be
> transfered
> by DMA, so the caches must be flushed on architectures whose caches
> are not coherent with respect to DMA.
> 
> Would you care to elaborate on the exact nature of the bug you are
> fixing?

I missed part of this thread, so forgive me if I'm a bit off here, but
if the problem is indeed I$/D$ cache coherency vs. PIO transfers, then
this is a long solved issue on other archs such as ppc (and I _think_
sparc).

The way we do it, at least on powerpc which is PIPT, is to keep track on
a per-page basis, whether a given page is clean for execution using
PG_arch1 bit. This bit is cleared when a new page is popped into the
page cache, and we clear it from flush_dcache_page() iirc (you may want
to dbl check I don't have the code at hand right now, or rather, I do
but I'm to lazy to look right now :-)

Any page with that not set is mapped into userspace with execute
permission disabled. We do the flush and set PG_arch1 on the first exec
fault to that page.

Cheers,
Ben.