linux-kernel - Re: [RFC PATCH v2] arm DMA: Fix allocation from CMA for coherent DMA

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Message-ID: <20150611142537.GA29305@e104818-lin.cambridge.arm.com>
Date:	Thu, 11 Jun 2015 15:26:57 +0100
From:	Catalin Marinas <catalin.marinas@....com>
To:	Lorenzo Nava <lorenx4@...il.com>
Cc:	linux-arm-kernel@...ts.infradead.org,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	linux-kernel@...r.kernel.org, Arnd Bergmann <arnd@...db.de>
Subject: Re: [RFC PATCH v2] arm DMA: Fix allocation from CMA for coherent DMA

On Wed, Jun 10, 2015 at 09:34:43PM +0200, Lorenzo Nava wrote:
> On Wed, Jun 10, 2015 at 6:28 PM, Catalin Marinas
> <catalin.marinas@....com> wrote:
> > On Wed, Jun 03, 2015 at 07:15:45PM +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@...xxxxx>
[...]
> > So while you allow __alloc_from_contiguous() to be called when
> > is_coherent, the memory returned is still non-cacheable. The reason is
> > that the "prot" argument passed to __dma_alloc() in
> > arm_coherent_dma_alloc() is pgprot_dmacoherent(PAGE_KERNEL) which means
> > Normal NonCacheable memory. The mmap seems to create a cacheable mapping
> > as vma->vm_page_prot is not passed through __get_dma_pgprot().
[...]
> Well the final scope of this patch is just to fix what in my opinion
> is an incorrect behaviour: the lack of use of CMA when the flag
> "is_coherent" is set.

But you still have to fix it properly: "is_coherent" means cacheable
memory which you don't get with your patch.

> Of course it still exists the problem of modify the attribute to make
> the memory cacheable, but it is something I would like to do in a
> second step (the patch you posted is of course a good starting point).

So between the first and the second step, you basically break
dma_alloc_coherent() by moving the allocation from
__alloc_simple_buffer() (returning cacheable memory) to
__alloc_from_contiguous() which changes the memory attributes to
whatever __get_dma_pgprot() returned (currently Normal Non-cacheable).

> I think that the current implementation maps memory keeping non
> cacheable attributes enable, because the 'attrs' parameter passed to
> arm_dma_mmap() has no WRITE_COMBINE attribute set (according to
> dma_mmap_coherent() in include/asm-generic/dma-mapping-common.h).

At least on ARMv7, WRITE_COMBINE and Normal Non-cacheable are the same.

> I also notice this patch that is pending "[PATCH v3]
> arm/mm/dma-mapping.c: Add arm_coherent_dma_mmap": it modifies the
> mapping of memory for coherent DMA. I want to understand if the merge
> of this patch requires any other modification to guarantee that
> coherent memory is allocated with cacheable attributes.

I think this patch will go in, it is already in linux-next.

-- 
Catalin
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