USB mass storage and ARM cache coherency

[Russell King - ARM Linux]

On Thu, Feb 18, 2010 at 07:37:00AM +1100, Benjamin Herrenschmidt wrote:
> On Wed, 2010-02-17 at 15:27 +0000, Catalin Marinas wrote:
> > We do the same on ARM. The problem with most (all) HCD drivers that do
> > PIO is that they copy the data to the transfer buffer but there is no
> > call in this driver to flush_dcache_page(). The upper mass storage or
> > filesystem layers don't call this function either, so there isn't
> > anything that would set the PG_arch1 bit. 
> 
> Actually, clear it :-)
> 
> I suppose that's one thing that needs to be fixed in the drivers.

No, because that'd probably bugger up the Sparc64 method of delaying
flush_dcache_page.

This method works as follows:

- a page cache page is allocated - this has PG_arch_1 clear.

- IO happens on it and it's placed into the page cache.  PG_arch_1 is
  still clear.

- someone calls read()/write() which accesses the page.  The generic
  file IO layers call flush_dcache_page() in response to read()/write()
  fs calls.  flush_dcache_page() spots that the page is not yet mapped
  into userspace, and sets PG_arch_1 to mark the fact that the kernel
  mapping is dirty.

- when someone maps the page, we check PG_arch_1 in update_mmu_cache.
  If PG_arch_1 is set, we flush the kernel mapping.

Clearly, if we go around having drivers clearing PG_arch_1, this is going
to break horribly.