linux-kernel - Re: [PATCH] Set bit 22 in the PL310 (cache controller) AuxCtlr register

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Message-ID: <20121112105641.GC2346@mudshark.cambridge.arm.com>
Date:	Mon, 12 Nov 2012 10:56:41 +0000
From:	Will Deacon <will.deacon@....com>
To:	Shiraz Hashim <shiraz.hashim@...com>
Cc:	Russell King <linux@....linux.org.uk>,
	"spear-devel@...t.st.com" <spear-devel@...t.st.com>,
	"alain.pasteur@...com" <alain.pasteur@...com>,
	"Joerg.Wienand@....de" <Joerg.Wienand@....de>,
	"amit.goel@...com" <amit.goel@...com>,
	Catalin Marinas <Catalin.Marinas@....com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	m.szyprowski@...sung.com
Subject: Re: [PATCH] Set bit 22 in the PL310 (cache controller) AuxCtlr
 register

On Mon, Nov 12, 2012 at 06:45:47AM +0000, Shiraz Hashim wrote:
> On Fri, Nov 09, 2012 at 09:54:01AM +0000, Will Deacon wrote:
> > On Fri, Nov 09, 2012 at 04:01:52AM +0000, Shiraz Hashim wrote:
> > > From: Catalin Marinas <catalin.marinas@....com>
> > > 
> > > Clearing bit 22 in the PL310 Auxiliary Control register (shared
> > > attribute override enable) has the side effect of transforming Normal
> > > Shared Non-cacheable reads into Cacheable no-allocate reads.
> > > 
> > > Coherent DMA buffers in Linux always have a Cacheable alias via the
> > > kernel linear mapping and the processor can speculatively load cache
> > > lines into the PL310 controller. With bit 22 cleared, Non-cacheable
> > > reads would unexpectedly hit such cache lines leading to buffer
> > > corruption.
> > 
> > Is this still the case with recent kernels? I thought the dma-mapping/cma
> > work avoided the cacheable alias, but perhaps I'm mistaken.
> 
> I haven't used CMA but DMA mappings are still normal memory
> non-cacheable.

Ok, so trawling through the list reveals we only have this issue for normal
DMA mappings and not with CMA:

  http://lists.infradead.org/pipermail/linux-arm-kernel/2012-October/124276.html

I wonder whether we shouldn't just fix that, rather than work around it with
a PL310-specific hack?

Will

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