linux-kernel - Re: [PATCH v5] arm DMA: Fix allocation from CMA for coherent DMA

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Message-ID: <CAE=W-e3Bweki8BBapPKOgFOGha8tM6wgq=AOhXQ2WUAPcqJkiw@mail.gmail.com>
Date:	Sat, 18 Jul 2015 13:54:42 +0200
From:	Lorenzo Nava <lorenx4@...il.com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	Catalin Marinas <catalin.marinas@....com>,
	Arnd Bergmann <arnd@...db.de>, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v5] arm DMA: Fix allocation from CMA for coherent DMA

On Fri, Jul 3, 2015 at 11:03 AM, Russell King - ARM Linux
<linux@....linux.org.uk> wrote:
> On Wed, Jul 01, 2015 at 12:12:51PM +0100, Catalin Marinas wrote:
>> On Tue, Jun 30, 2015 at 11:29:06PM +0200, Lorenzo Nava wrote:
>> > This patch allows the use of CMA for DMA coherent memory allocation.
>> > At the moment if the input parameter "is_coherent" is set to true
>> > the allocation is not made using the CMA, which I think is not the
>> > desired behaviour.
>> >
>> > Signed-off-by: Lorenzo Nava <lorenx4@...il.com>
>> > Reviewed-by: Catalin Marinas <catalin.marinas@....com>
>>
>> If Russell doesn't have any objections, you can send the patch to
>> his patch system. See here for more information:
>
> I'm left wondering whether this patch is really want Lorenzo wants.
> From my reading of it, while this has the effect of allocating from
> CMA for coherent devices, it's no different from the non-coherent
> case, because by calling __alloc_from_contiguous(), we end up
> remapping the allocated memory, removing the cacheability status
> from the allocated pages.
>
> This brings up an interesting point: presumably, it's been tested, and
> people are happy with the performance it's giving, inspite of it not
> returning cacheable memory... or maybe it hasn't been tested that much?
>
Hello Russell,

I was wondering if you have any other observations about this patch or
if you need any further investigation about some aspects.
It's not clear to me if the last comments from me and Catalin answer
your questions or if you see any other problem that we didn't
consider.

Thank you.
Lorenzo
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