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

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Date:	Fri, 3 Jul 2015 10:03:06 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Catalin Marinas <catalin.marinas@....com>
Cc:	Lorenzo Nava <lorenx4@...il.com>, 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 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?

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