linux-kernel - Re: [RFC PATCH] drm: disable WC optimization for cache coherent devices on non-x86

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Message-ID: <CAKv+Gu8OkbHwY5UBxahpQpM_gKh2331pQ6o-4YyJdLurbFNHvw@mail.gmail.com>
Date:   Mon, 21 Jan 2019 18:59:10 +0100
From:   Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:     Michel Dänzer <michel@...nzer.net>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Will Deacon <will.deacon@....com>,
        David Zhou <David1.Zhou@....com>,
        Maxime Ripard <maxime.ripard@...tlin.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        David Airlie <airlied@...ux.ie>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Michel Daenzer <michel.daenzer@....com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        amd-gfx@...ts.freedesktop.org, Junwei Zhang <Jerry.Zhang@....com>,
        Huang Rui <ray.huang@....com>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Daniel Vetter <daniel@...ll.ch>,
        Michael Ellerman <mpe@...erman.id.au>,
        Alex Deucher <alexander.deucher@....com>,
        Sean Paul <sean@...rly.run>,
        Christian Koenig <christian.koenig@....com>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [RFC PATCH] drm: disable WC optimization for cache coherent
 devices on non-x86

On Mon, 21 Jan 2019 at 18:55, Michel Dänzer <michel@...nzer.net> wrote:
>
> On 2019-01-21 5:30 p.m., Ard Biesheuvel wrote:
> > On Mon, 21 Jan 2019 at 17:22, Christoph Hellwig <hch@...radead.org> wrote:
> >
> >> Until that happens we should just change the driver ifdefs to default
> >> the hacks to off and only enable them on setups where we 100%
> >> positively know that they actually work.  And document that fact
> >> in big fat comments.
> >
> > Well, as I mentioned in my commit log as well, if we default to off
> > unless CONFIG_X86, we may break working setups on MIPS and Power where
> > the device is in fact non-cache coherent, and relies on this
> > 'optimization' to get things working.
>
> FWIW, the amdgpu driver doesn't rely on non-snooped transfers for
> correct basic operation (the scenario Christian brought up is a very
> specialized use-case), so that shouldn't be an issue.
>

The point is that this is only true for x86.

On other architectures, the use of non-cached mappings on the CPU side
means that you /do/ rely on non-snooped transfers, since if those
transfers turn out not to snoop inadvertently, the accesses are
incoherent with the CPU's view of memory.