Re: [PATCH v3 6/7] ARM: NOMMU: Set ARM_DMA_MEM_BUFFERABLE for M-class cpus

[Arnd Bergmann]

On Fri, Mar 10, 2017 at 10:23 AM, Vladimir Murzin
<vladimir.murzin@xxxxxxx> 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.
>
> Tested-by: Benjamin Gaignard <benjamin.gaignard@xxxxxxxxxx>
> Tested-by: Andras Szemzo <sza@xxxxxx>
> Tested-by: Alexandre TORGUE <alexandre.torgue@xxxxxx>
> Reviewed-by: Robin Murphy <robin.murphy@xxxxxxx>
> Signed-off-by: Vladimir Murzin <vladimir.murzin@xxxxxxx>
> ---
>  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 d731f28..7dd46ae 100644
> --- a/arch/arm/mm/Kconfig
> +++ b/arch/arm/mm/Kconfig
> @@ -1050,7 +1050,7 @@ 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

The patch doesn't seem to match the description: I would have expected
this to be user-selectable on CPU_V7M as we do on V6, but it is enabled
unconditionally.

Can you either modify the description to explain why we now need this on
all ARMv7M, or add a '|| CPU_V7M' for the 'bool' line to make it optional?

Would it be better to leave the default as disabled on CPU_V7M and
require users to enable it manually? That way we don't regress the
performance of readl/writel on platforms that don't need this.

     Arnd