linux-kernel - Re: [PATCH 6/7] ARM: NOMMU: Set ARM_DMA_MEM

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Date:   Tue, 21 Feb 2017 12:57:59 +0000
From:   Robin Murphy <robin.murphy@....com>
To:     Vladimir Murzin <vladimir.murzin@....com>,
        linux-arm-kernel@...ts.infradead.org
Cc:     kbuild-all@...org, linux@...linux.org.uk,
        akpm@...ux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 6/7] ARM: NOMMU: Set ARM_DMA_MEM_BUFFERABLE for M-class
 cpus

On 15/02/17 09:59, Vladimir Murzin wrote:
> Now, we have dedicated non-cacheable region for consistent DMA
> operations. However, that region can still be marked as bufferable by
> MPU, so it'd be safer to have barriers by default.

Makes sense - plenty of cases want their DMA buffers to still be
write-combining (e.g. framebuffers have already been mentioned here),
for which strongly-ordered mappings won't do. Plus you don't exactly
have a choice if you've no MPU and have fixed Normal attributes for your
RAM region.

Reviewed-by: Robin Murphy <robin.murphy@....com>

> Tested-by: Benjamin Gaignard <benjamin.gaignard@...aro.org>
> Tested-by: Andras Szemzo <sza@....hu>
> Tested-by: Alexandre TORGUE <alexandre.torgue@...com>
> Signed-off-by: Vladimir Murzin <vladimir.murzin@....com>
> ---
>  arch/arm/mm/Kconfig | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/arm/mm/Kconfig b/arch/arm/mm/Kconfig
> index 0b79f12..64a1465c 100644
> --- a/arch/arm/mm/Kconfig
> +++ b/arch/arm/mm/Kconfig
> @@ -1029,7 +1029,7 @@ config ARM_L1_CACHE_SHIFT
>  
>  config ARM_DMA_MEM_BUFFERABLE
>  	bool "Use non-cacheable memory for DMA" if (CPU_V6 || CPU_V6K) && !CPU_V7
> -	default y if CPU_V6 || CPU_V6K || CPU_V7
> +	default y if CPU_V6 || CPU_V6K || CPU_V7 || CPU_V7M
>  	help
>  	  Historically, the kernel has used strongly ordered mappings to
>  	  provide DMA coherent memory.  With the advent of ARMv7, mapping
>