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

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Date:	Fri, 05 Mar 2010 15:31:13 +1100
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>,
	mdharm-kernel@...-eyed-alien.net, linux-usb@...r.kernel.org,
	Catalin Marinas <catalin.marinas@....com>, x0082077@...com,
	sshtylyov@...mvista.com, tom.leiming@...il.com,
	bigeasy@...utronix.de, oliver@...kum.org,
	linux-kernel@...r.kernel.org,
	James Bottomley <James.Bottomley@...senPartnership.com>,
	santosh.shilimkar@...com, Pavel Machek <pavel@....cz>,
	greg@...ah.com, linux-arm-kernel@...ts.infradead.org
Subject: Re: USB mass storage and ARM cache coherency

On Thu, 2010-03-04 at 21:40 +0000, Russell King - ARM Linux wrote:
> On Fri, Mar 05, 2010 at 08:28:34AM +1100, Benjamin Herrenschmidt wrote:
> > I don't think there's a core or driver problem in this specific case. As
> > we discussed earlier, I believe the problem is that ARM considers a
> > fresh page out of the page cache as "clean" instead of "dirty", and
> > inverting that like we do on powerpc will fix their problem too.
> 
> The only concern is that it means we treat anonymous pages as dirty
> by default.
>
> That's quite sub-optimal since we take care (eg) on write faults to
> copy the page and take care of the cache issues while we do that -

If you do the cache handling inside your copy_user_highpage() then you
can just set PG_arch_1 stuff there.

> whether that be remapping the page to be coherent with the user
> address, or cleaning each cache line as we copy the data.
> 
> Of course, the simple solution is to also arrange for PG_arch_1 to be
> set in this case.

Right.

Cheers,
Ben.


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