linux-kernel - Re: [PATCH v4] arm DMA: Fix allocation from CMA for coherent DMA

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Date:	Tue, 30 Jun 2015 13:40:40 +0200
From:	Lorenzo Nava <lorenx4@...il.com>
To:	Catalin Marinas <catalin.marinas@....com>
Cc:	Arnd Bergmann <arnd@...db.de>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v4] arm DMA: Fix allocation from CMA for coherent DMA

On Tue, Jun 30, 2015 at 1:05 PM, Catalin Marinas
<catalin.marinas@....com> wrote:
>
> On Mon, Jun 29, 2015 at 07:10:45PM +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>
>
> It's nice to have a proper email address here.

oops, sorry I thought it was made to avoid spam. This is an easy fix :-)

>
> > @@ -680,9 +684,14 @@ void *arm_dma_alloc(struct device *dev, size_t size, dma_addr_t *handle,
> >  static void *arm_coherent_dma_alloc(struct device *dev, size_t size,
> >       dma_addr_t *handle, gfp_t gfp, struct dma_attrs *attrs)
> >  {
> > -     pgprot_t prot = __get_dma_pgprot(attrs, PAGE_KERNEL);
> > +     pgprot_t prot;
> >       void *memory;
> >
> > +     if (attrs == NULL)
> > +              prot  = PAGE_KERNEL;
> > +     else
> > +              prot  = __get_dma_pgprot(attrs, PAGE_KERNEL);
> > +
> >       if (dma_alloc_from_coherent(dev, size, handle, &memory))
> >               return memory;
>
> We have a slight inconsistency with commit 55af8a91640d (ARM: 8387/1:
> arm/mm/dma-mapping.c: Add arm_coherent_dma_mmap) where the attrs are not
> honoured for the coherent ops. That is, DMA_ATTRS_WRITE_COMBINE
> doesn't have any effect for mmap but with your patch it still has an
> effect on alloc.
>
> I think we could just ignore it, the WRITE_COMBINE attr is a hint that
> the arch may or may not implement and I haven't seen it used with
> coherent mappings. So in this case just pass PAGE_KERNEL to
> __dma_alloc() directly without calling __get_dma_pgprot().
>
> (it also looks like commit 55af8a91640d left an unused attrs argument
> for __arm_dma_mmap that can be removed as well)
>

Right: ok I just pass PAGE_KERNEL to the alloc function, so we are
coherent with the mmap.

> Apart from the above, you can add:
>
> Reviewed-by: Catalin Marinas <catalin.marinas@....com>

Thanks.
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