Re: USB mass storage and ARM cache coherency

From: Catalin Marinas
Date: Thu Mar 04 2010 - 12:34:36 EST

On Thu, 2010-03-04 at 16:30 +0000, Russell King - ARM Linux wrote:
> On Fri, Mar 05, 2010 at 12:41:03AM +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.
> Indeed; the ARM approach was basically taken from Sparc64.
> The general critera (from memory) seems to be:
> - a virtual indexed aliasing cache (whether it be VIVT or VIPT aliasing)
> - write allocate caches show the problem better than read allocate only
> - using a block device for the filesystem
> - mmap'ing a page and immediately accessing the last few cache lines in
> that page

It actually triggers easily with a non-aliasing VIPT cache (can't even
start /sbin/init). The main condition is for the caches to be in
write-allocate mode (and the processor to support this, i.e. Cortex-A9).

A simple test is to use an ext2/3 filesystem (cramfs, jffs2 etc.
wouldn't do since they call flush_dcache_page) on a compact flash card
using the pata_platform driver (and without commit 2d68b7fe55d9e19).

Other forms of triggering this is to use something like slram + ext2/3.


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