linux-kernel - Re: [PATCH 1/1] ARM: dma-mapping: Drop GFP

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Message-ID: <20131016104021.GC5403@mudshark.cambridge.arm.com>
Date:	Wed, 16 Oct 2013 11:40:21 +0100
From:	Will Deacon <will.deacon@....com>
To:	Hiroshi Doyu <hdoyu@...dia.com>
Cc:	"linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>,
	Nicolas Pitre <nicolas.pitre@...aro.org>,
	Russell King <linux@....linux.org.uk>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	steve.capper@....com
Subject: Re: [PATCH 1/1] ARM: dma-mapping: Drop GFP_COMP for DMA memory
 allocations

On Wed, Oct 16, 2013 at 11:19:13AM +0100, Hiroshi Doyu wrote:
> arm_iommu_alloc_attrs wants to split pages after allocation in order
> to reduce the memory footprint. This does not work well with GFP_COMP
> pages, so drop this flag before allocation.
> 
> ref: ea2e7057c0234cfb8b09467d8f137760d371fc72:
> 
>     ARM: 7172/1: dma: Drop GFP_COMP for DMA memory allocations
> 
>     dma_alloc_coherent wants to split pages after allocation in order to
>     reduce the memory footprint. This does not work well with GFP_COMP
>     pages, so drop this flag before allocation.
> 
> Signed-off-by: Hiroshi Doyu <hdoyu@...dia.com>
> ---
>  arch/arm/mm/dma-mapping.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/arch/arm/mm/dma-mapping.c b/arch/arm/mm/dma-mapping.c
> index f5e1a84..955dd3e 100644
> --- a/arch/arm/mm/dma-mapping.c
> +++ b/arch/arm/mm/dma-mapping.c
> @@ -1321,6 +1321,13 @@ static void *arm_iommu_alloc_attrs(struct device *dev, size_t size,
>  	struct page **pages;
>  	void *addr = NULL;
>  
> +	/* Following is a work-around (a.k.a. hack) to prevent pages
> +	 * with __GFP_COMP being passed to split_page() which cannot
> +	 * handle them.  The real problem is that this flag probably
> +	 * should be 0 on ARM as it is not supported on this
> +	 * platform--see CONFIG_HUGETLB_PAGE. */
> +	gfp &= ~(__GFP_COMP);

Right, except that we *do* have hugepage support on ARM, so this probably
needs fixing properly.

Will
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