linux-kernel - Re: [PATCH v3 6/7] ARM: NOMMU: Set ARM_DMA_MEM

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Date:   Sat, 22 Apr 2017 00:12:39 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Vladimir Murzin <vladimir.murzin@....com>
Cc:     Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Alexandre Torgue <alexandre.torgue@...com>,
        Russell King - ARM Linux <linux@...linux.org.uk>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        kbuild-all@...org,
        Benjamin Gaignard <benjamin.gaignard@...aro.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Robin Murphy <robin.murphy@....com>, sza@....hu
Subject: Re: [PATCH v3 6/7] ARM: NOMMU: Set ARM_DMA_MEM_BUFFERABLE for M-class cpus

On Wed, Apr 19, 2017 at 4:10 PM, Vladimir Murzin
<vladimir.murzin@....com> wrote:
> On 19/04/17 11:02, Arnd Bergmann wrote:

>> 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.
>>
>
> It is architectural vs implementation differences. Even though existing
> implementations rarely need this sticking with architecture (it permits memory
> re-ordering to happen in many cases) makes code more robust and save some
> debugging time when more sophisticated implementations go wild (see, for
> instance, 8e02676ffa69 "ARM: 8610/1: V7M: Add dsb before jumping in handler
> mode"). We can consider making it user selectable if performance regressions
> are reported.

I would expect the performance difference to be noticeable, even though
MMIO tends to be a rather slow operation, having tons of barriers in drivers
that don't do DMA but transfer data through a series of writel() can be
significant.

      Arnd