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

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Message-ID: <1267740711.30572.5.camel@e102109-lin.cambridge.arm.com>
Date:	Thu, 04 Mar 2010 22:11:51 +0000
From:	Catalin Marinas <catalin.marinas@....com>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	Paul Mundt <lethal@...ux-sh.org>,
	James Bottomley <James.Bottomley@...senPartnership.com>,
	Pavel Machek <pavel@....cz>,
	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>,
	linux@....linux.org.uk, mdharm-kernel@...-eyed-alien.net,
	linux-usb@...r.kernel.org, x0082077@...com,
	sshtylyov@...mvista.com, tom.leiming@...il.com,
	bigeasy@...utronix.de, oliver@...kum.org,
	linux-kernel@...r.kernel.org, santosh.shilimkar@...com,
	greg@...ah.com, linux-arm-kernel@...ts.infradead.org
Subject: Re: USB mass storage and ARM cache coherency

On Thu, 2010-03-04 at 21:37 +0000, Benjamin Herrenschmidt wrote:
> On Thu, 2010-03-04 at 18:07 +0000, Catalin Marinas wrote:
> > I'm not familiar with SH but for PIO devices the flushing shouldn't be
> > more aggressive. For the DMA devices, Russell suggested that we mark
> > the page as clean (set PG_dcache_clean) in the DMA API to avoid the
> > default flushing.
> 
> I really like that idea, as I said earlier, but I'm worried about the I$
> side of things. IE. What I'm trying to say is that I can't see how to do
> that optimisation without ending up with missing I$ invalidations or
> doing way too many of them, unless we have a separate bit to track I$
> state.

But does this optimisation really matter? I think with careful checking
in set_pte_at(), you are not going to invalidate the I-cache more than
necessary. If the original page wasn't pte_present() you would need to
do the I-cache invalidation. The other cases where set_pte_at() is
called for LRU (pte_young) or COW (pte_write) we can avoid the extra
invalidation.

-- 
Catalin

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