From mboxrd@z Thu Jan  1 00:00:00 1970
From: will.deacon@arm.com (Will Deacon)
Date: Mon, 12 Nov 2012 10:56:41 +0000
Subject: [PATCH] Set bit 22 in the PL310 (cache controller) AuxCtlr
 register
In-Reply-To: <20121112064547.GN32313@localhost.localdomain>
References: <1352433712-16364-1-git-send-email-shiraz.hashim@st.com>
 <20121109095400.GC2357@mudshark.cambridge.arm.com>
 <20121112064547.GN32313@localhost.localdomain>
Message-ID: <20121112105641.GC2346@mudshark.cambridge.arm.com>
To: linux-arm-kernel@lists.infradead.org
List-Id: linux-arm-kernel.lists.infradead.org

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@arm.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