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

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Date:	Tue, 30 Jun 2015 12:05:22 +0100
From:	Catalin Marinas <catalin.marinas@....com>
To:	Lorenzo Nava <lorenx4@...il.com>
Cc:	arnd@...db.de, 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 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.

> @@ -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)

Apart from the above, you can add:

Reviewed-by: Catalin Marinas <catalin.marinas@....com>
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