USB mass storage and ARM cache coherency

[Russell King - ARM Linux]

On Mon, Mar 01, 2010 at 10:39:14AM +0000, Catalin Marinas wrote:
> On Sun, 2010-02-28 at 05:01 +0000, James Bottomley wrote:
> > But the point of all of this is that I cache invalidation doesn't appear
> > anywhere in the I/O path ... so  if we're getting I/D incoherency,
> > there's some problem in the mm code (or there's a missing arch
> > assumption ... like I cache gets moved in more aggressively than we
> > expect).  Parisc is very sensitive to I/D incoherency, so we'd notice if
> > there were a serious generic problem here.
> 
> On ARM PIPT, it's probably because flush_cache_page isn't implemented.
> But as I said above, given the speculative fetches I don't think it
> would help much (well, it would work a bit better but not a complete
> fix).

Not quite.  flush_cache_page() is called when we unmap or replace a page
in userspace, which is completely the wrong place to do I-cache coherency
when you have speculatively loaded caches - or even D-cache coherency if
your cache behaves as a speculatively loaded PIPT or non-aliasing VIPT.

Flushing the I-cache after a page has been in userspace does nothing to
ensure that there aren't any I-cache lines associated with that page
when you next come to map it into userspace.