Re: USB mass storage and ARM cache coherency

From: Catalin Marinas
Date: Wed Mar 03 2010 - 05:21:56 EST

On Tue, 2010-03-02 at 23:33 +0000, Benjamin Herrenschmidt wrote:
> On Tue, 2010-03-02 at 17:47 +0000, Catalin Marinas wrote:
> >
> > Actually, option 2 still has an issue - does not easily work on SMP
> > systems where cache maintenance operations aren't broadcast in hardware.
> > In this case (ARM11MPCore), flush_dcache_page() is implemented
> > non-lazily so that the flushing happens on the same processor that
> > dirtied the cache. But since with some drivers there is no call to this
> > function, it wouldn't make any difference.
> Also, option 1 would not solve the icache issue which has the same
> problem related to IPIs.

Correct. But that's true for both options.

It would have been simpler if we had software TLBs.

> You -really- need to spank some HW folks here :-)

I think they got the message :). Cortex-A9 does it properly.

> > A solution is to do something like read-for-ownership before flushing
> > the D-cache in update_mmu_cache() (or set_pte_at()).
> You might also want to experiment with not clearing PG_arch_1 in
> flush_dcache_page(). I'm not 100% convinced it is necessary and that may
> reduce the amount of flushing needed.

Could a file map page be swapped out (and the mapping removed), then the
page cache page modified (i.e. NFS filesystem) and flush_dcache_page()

> Another thing is, on powerpc, we only do the cleaning when we try to
> execute from the pages. IE. We basically "filter out" exec permission
> when pages are not clean. At least on processors that support per-page
> exec permission. You may want to consider something like that as well.

For non-aliasing VIPT, I think that's a fair optimisation.



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