Re: USB mass storage and ARM cache coherency

From: Benjamin Herrenschmidt
Date: Thu Mar 04 2010 - 16:36:43 EST

On Fri, 2010-03-05 at 00:41 +0900, Paul Mundt wrote:
> On Thu, Mar 04, 2010 at 03:29:38PM +0000, Catalin Marinas wrote:
> > On Thu, 2010-03-04 at 14:21 +0000, James Bottomley wrote:
> > > The thing which was discovered in this thread is basically that ARM is
> > > handling deferred flushing (for D/I coherency) in a slightly different
> > > way from everyone else ...
> >
> > Doing a grep for PG_dcache_dirty defined in terms of PG_arch_1 reveals
> > that MIPS, Parisc, Score, SH and SPARC do similar things to ARM. PowerPC
> > and IA-64 use PG_arch_1 as a clean rather than dirty bit.
> >
> SH used to use it as a PG_mapped which was roughly similar to the
> PG_dcache_clean approach, at which point things like flushing for the PIO
> case in the HCD wasn't necessary. It did result in rather aggressive over
> flushing though, which is one of the reasons we elected to switch to
> PG_dcache_dirty.
> Note that the PG_dcache_dirty semantics are also outlined in
> Documentation/cachetlb.txt for PG_arch_1 usage, so it's hardly esoteric.

Doing this way though is a lot more fragile... since page cache pages
are no longer dirty by default, you need to ensure that any driver
writing to one without DMA sets PG_arch_1, and as we've seen, this is
generally not the case (it's almost never the case actually).

Also, in the DMA case, you may not need to flush D$, but you -still-
need to invalidate I$, and unless you then get another bit for tracking
it, you end up doing a lot of over-invalidating of I$ no ?

Or am I missing a critical piece of the puzzle ?


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