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

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Date:	Wed, 17 Feb 2010 20:05:43 +1100
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Oliver Neukum <oliver@...kum.org>
Cc:	"Shilimkar, Santosh" <santosh.shilimkar@...com>,
	Matthew Dharm <mdharm-kernel@...-eyed-alien.net>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	Ming Lei <tom.leiming@...il.com>,
	"Mankad, Maulik Ojas" <x0082077@...com>,
	Sergei Shtylyov <sshtylyov@...mvista.com>,
	Catalin Marinas <catalin.marinas@....com>,
	Sebastian Siewior <bigeasy@...utronix.de>,
	"linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Pavel Machek <pavel@....cz>, Greg KH <greg@...ah.com>,
	linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>
Subject: Re: USB mass storage and ARM cache coherency

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.
> 
> Would you care to elaborate on the exact nature of the bug you are
> fixing?

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 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 :-)

Any page with that not set is mapped into userspace with execute
permission disabled. We do the flush and set PG_arch1 on the first exec
fault to that page.

Cheers,
Ben.

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