linux-kernel - Re: Re: USB mass storage and ARM cache coherency

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Message-ID: <1266423544.3666.5.camel@e102109-lin.cambridge.arm.com>
Date:	Wed, 17 Feb 2010 16:19:04 +0000
From:	Catalin Marinas <catalin.marinas@....com>
To:	benh@...nel.crashing.org
Cc:	mdharm-kernel@...-eyed-alien.net, linux-usb@...r.kernel.org,
	linux@....linux.org.uk, x0082077@...com, sshtylyov@...mvista.com,
	tom.leiming@...il.com, bigeasy@...utronix.de, oliver@...kum.org,
	linux-kernel@...r.kernel.org, santosh.shilimkar@...com,
	pavel@....cz, greg@...ah.com, linux-arm-kernel@...ts.infradead.org
Subject: Re: Re: USB mass storage and ARM cache coherency

SORRY - one more message to apologise for the multiple reposts (and
automatically appended legal disclaimer). I've been moved to Exchange
2007 and trying to use Evolution + Exchange-MAPI. It looks like it went
terribly wrong.

Catalin

On Wed, 2010-02-17 at 15:40 +0000, Catalin Marinas wrote:
> On Wed, 2010-02-17 at 20:05 +1100, Benjamin Herrenschmidt wrote:
> > On Tue, 2010-02-16 at 09:22 +0100, Oliver Neukum wrote:
> > > This seems wrong to me. Buffers for control transfers may be
> > > transfered
> > > by DMA, so the caches must be flushed on architectures whose caches
> > > are not coherent with respect to DMA.
> > >=20
> > > Would you care to elaborate on the exact nature of the bug you are
> > > fixing?
> >=20
> > I missed part of this thread, so forgive me if I'm a bit off here, but
> > if the problem is indeed I$/D$ cache coherency vs. PIO transfers, then
> > this is a long solved issue on other archs such as ppc (and I _think_
> > sparc).
> 
> The thread I started was indeed regarding I/D cache coherency and PIO.
> But it diverged into DMA issues a few days ago (should have been a new
> thread).
> 
> > The way we do it, at least on powerpc which is PIPT, is to keep track on
> > a per-page basis, whether a given page is clean for execution using
> > PG_arch1 bit. This bit is cleared when a new page is popped into the
> > page cache, and we clear it from flush_dcache_page() iirc (you may want
> > to dbl check I don't have the code at hand right now, or rather, I do
> > but I'm to lazy to look right now :-)
> 
> 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.
> 
> --=20
> Catalin
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