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References: <20190124120658.30288-1-ard.biesheuvel@linaro.org>
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From:   Alex Deucher <alexdeucher@gmail.com>
Date:   Thu, 24 Jan 2019 09:01:37 -0500
Message-ID: <CADnq5_PETD6umm9Df7OZ9BdaV5MZutmhN+HTAnVkcRLg5=TNWw@mail.gmail.com>
Subject: Re: [PATCH] drm: enable uncached DMA optimization for ARM and arm64
To:     Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc:     "Koenig, Christian" <Christian.Koenig@amd.com>,
        "Zhou, David(ChunMing)" <David1.Zhou@amd.com>,
        Maxime Ripard <maxime.ripard@bootlin.com>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Carsten Haitzler <Carsten.Haitzler@arm.com>,
        David Airlie <airlied@linux.ie>,
        Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
        dri-devel <dri-devel@lists.freedesktop.org>,
        "Daenzer, Michel" <Michel.Daenzer@amd.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Will Deacon <will.deacon@arm.com>,
        Christoph Hellwig <hch@infradead.org>,
        "Zhang, Jerry" <Jerry.Zhang@amd.com>,
        "Huang, Ray" <Ray.Huang@amd.com>,
        amd-gfx list <amd-gfx@lists.freedesktop.org>,
        Daniel Vetter <daniel@ffwll.ch>,
        Michael Ellerman <mpe@ellerman.id.au>,
        "Deucher, Alexander" <Alexander.Deucher@amd.com>,
        Sean Paul <sean@poorly.run>,
        "linux-arm-kernel@lists.infradead.org" 
        <linux-arm-kernel@lists.infradead.org>,
        Robin Murphy <robin.murphy@arm.com>
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On Thu, Jan 24, 2019 at 9:00 AM Ard Biesheuvel
<ard.biesheuvel@linaro.org> wrote:
>
> On Thu, 24 Jan 2019 at 13:31, Koenig, Christian
> <Christian.Koenig@amd.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@amd.com>
> > > Cc: Alex Deucher <alexander.deucher@amd.com>
> > > Cc: David Zhou <David1.Zhou@amd.com>
> > > Cc: Huang Rui <ray.huang@amd.com>
> > > Cc: Junwei Zhang <Jerry.Zhang@amd.com>
> > > Cc: Michel Daenzer <michel.daenzer@amd.com>
> > > Cc: David Airlie <airlied@linux.ie>
> > > Cc: Daniel Vetter <daniel@ffwll.ch>
> > > Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
> > > Cc: Maxime Ripard <maxime.ripard@bootlin.com>
> > > Cc: Sean Paul <sean@poorly.run>
> > > Cc: Michael Ellerman <mpe@ellerman.id.au>
> > > Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
> > > Cc: Will Deacon <will.deacon@arm.com>
> > > Cc: Christoph Hellwig <hch@infradead.org>
> > > Cc: Robin Murphy <robin.murphy@arm.com>
> > > Cc: amd-gfx list <amd-gfx@lists.freedesktop.org>
> > > Cc: dri-devel <dri-devel@lists.freedesktop.org>
> > > Reported-by: Carsten Haitzler <Carsten.Haitzler@arm.com>
> > > Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
> >
> > The subject line should probably read "disable uncached...".
> >
>
> Ugh, of course ...
>
> > With that fixed the patch is Reviewed-by: Christian K=C3=B6nig
> > <christian.koenig@amd.com>.
> >

Same:
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>

>
> Thanks.
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