Re: USB mass storage and ARM cache coherency

[Catalin Marinas <catalin.marinas@arm.com>]

Hi,

On Mon, 2010-02-08 at 06:55 +0000, Pavel Machek wrote:
> > > So, let's put this in the HCD drivers and be done with it.
> >
> > The patch below is what fixes the I-D cache incoherency issues on ARM. I
> > don't particularly like the solution but it seems to be the only one
> > available.
> 
> Really? It looks like arm should just flush the caches when mapping
> executable page to the userspace.... you can't expect all the drivers
> to be modified like that...

We could of course flush the caches every time we get a page fault but
that's far from optimal, especially since DMA-capable drivers to do not
pollute the D-cache and don't need this extra flushing. Note that the
recent ARM processors have PIPT caches but separate for I and D and it's
the PIO drivers that pollute the D-cache.

The kernel API provides flush_dcache_page() to be called every time the
kernel writes to a page cache page. This is further optimised for
working in pair with update_mmu_cache() to delay the flushing until the
actual page is mapped into user space and this latter function is called
(which in general is not a cache maintenance function).

The problem with some PIO drivers and a filesystems like ext2 is that
there is no call to flush_dcache_page() when getting data into a page
cache page. Since the page isn't marked as dirty (PG_arch_1), a
subsequent call to update_mmu_cache() as a result of a page fault
doesn't flush the caches.

There is a flush_icache_page() function called from __do_fault(),
however, Documentation/cachetlb.txt states that all the functionality of
this function can be implemented in flush_dcache_page() and
update_mmu_cache(), hence this function is a no-op.

Please suggest a better solution that does not involve modifying generic
Linux code.

> Plus it does unneccessary flushes on x86, etc...

On x86, it should indeed be conditionally compiled based on
ARCH_IMPLEMENTS_FLUSH_DCACHE_PAGE.

Regards.

-- 
Catalin