From mboxrd@z Thu Jan  1 00:00:00 1970
From: catalin.marinas@arm.com (Catalin Marinas)
Date: Thu, 04 Mar 2010 22:11:51 +0000
Subject: USB mass storage and ARM cache coherency
In-Reply-To: <1267738660.22204.77.camel@pasglop>
References: <20100226210030.GC23933@n2100.arm.linux.org.uk>
	<1267316072.23523.1842.camel@pasglop>
	<1267333263.2762.11.camel@mulgrave.site>
	<20100302211049V.fujita.tomonori@lab.ntt.co.jp>
	<1267549527.15401.78.camel@e102109-lin.cambridge.arm.com>
	<20100303215437.GF2579@ucw.cz>
	<1267709756.6526.380.camel@e102109-lin.cambridge.arm.com>
	<20100304135128.GA12191@atrey.karlin.mff.cuni.cz>
	<1267712512.31654.176.camel@mulgrave.site>
	<1267716578.6526.483.camel@e102109-lin.cambridge.arm.com>
	<20100304154103.GA9384@linux-sh.org>
	<1267726049.6526.543.camel@e102109-lin.cambridge.arm.com>
	<1267738660.22204.77.camel@pasglop>
Message-ID: <1267740711.30572.5.camel@e102109-lin.cambridge.arm.com>
To: linux-arm-kernel@lists.infradead.org
List-Id: linux-arm-kernel.lists.infradead.org

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