linux-kernel - Re: [PATCH] drm: enable uncached DMA optimization for ARM and arm64

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Date:   Thu, 24 Jan 2019 13:32:15 +0100
From:   Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:     "Koenig, Christian" <Christian.Koenig@....com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "Deucher, Alexander" <Alexander.Deucher@....com>,
        "Zhou, David(ChunMing)" <David1.Zhou@....com>,
        "Huang, Ray" <Ray.Huang@....com>,
        "Zhang, Jerry" <Jerry.Zhang@....com>,
        "Daenzer, Michel" <Michel.Daenzer@....com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <maxime.ripard@...tlin.com>,
        Sean Paul <sean@...rly.run>,
        Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Will Deacon <will.deacon@....com>,
        Christoph Hellwig <hch@...radead.org>,
        Robin Murphy <robin.murphy@....com>,
        amd-gfx list <amd-gfx@...ts.freedesktop.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Carsten Haitzler <Carsten.Haitzler@....com>
Subject: Re: [PATCH] drm: enable uncached DMA optimization for ARM and arm64

On Thu, 24 Jan 2019 at 13:31, Koenig, Christian
<Christian.Koenig@....com> wrote:
>
> Am 24.01.19 um 13:06 schrieb Ard Biesheuvel:
> > The DRM driver stack is designed to work with cache coherent devices
> > only, but permits an optimization to be enabled in some cases, where
> > for some buffers, both the CPU and the GPU use uncached mappings,
> > removing the need for DMA snooping and allocation in the CPU caches.
> >
> > The use of uncached GPU mappings relies on the correct implementation
> > of the PCIe NoSnoop TLP attribute by the platform, otherwise the GPU
> > will use cached mappings nonetheless. On x86 platforms, this does not
> > seem to matter, as uncached CPU mappings will snoop the caches in any
> > case. However, on ARM and arm64, enabling this optimization on a
> > platform where NoSnoop is ignored results in loss of coherency, which
> > breaks correct operation of the device. Since we have no way of
> > detecting whether NoSnoop works or not, just disable this
> > optimization entirely for ARM and arm64.
> >
> > Cc: Christian Koenig <christian.koenig@....com>
> > Cc: Alex Deucher <alexander.deucher@....com>
> > Cc: David Zhou <David1.Zhou@....com>
> > Cc: Huang Rui <ray.huang@....com>
> > Cc: Junwei Zhang <Jerry.Zhang@....com>
> > Cc: Michel Daenzer <michel.daenzer@....com>
> > Cc: David Airlie <airlied@...ux.ie>
> > Cc: Daniel Vetter <daniel@...ll.ch>
> > Cc: Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>
> > Cc: Maxime Ripard <maxime.ripard@...tlin.com>
> > Cc: Sean Paul <sean@...rly.run>
> > Cc: Michael Ellerman <mpe@...erman.id.au>
> > Cc: Benjamin Herrenschmidt <benh@...nel.crashing.org>
> > Cc: Will Deacon <will.deacon@....com>
> > Cc: Christoph Hellwig <hch@...radead.org>
> > Cc: Robin Murphy <robin.murphy@....com>
> > Cc: amd-gfx list <amd-gfx@...ts.freedesktop.org>
> > Cc: dri-devel <dri-devel@...ts.freedesktop.org>
> > Reported-by: Carsten Haitzler <Carsten.Haitzler@....com>
> > Signed-off-by: Ard Biesheuvel <ard.biesheuvel@...aro.org>
>
> The subject line should probably read "disable uncached...".
>

Ugh, of course ...

> With that fixed the patch is Reviewed-by: Christian König
> <christian.koenig@....com>.
>

Thanks.