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From:   Ard Biesheuvel <ard.biesheuvel@linaro.org>
Date:   Fri, 25 Jan 2019 12:35:15 +0100
Message-ID: <CAKv+Gu_faPAf+0NBTNM03EUH4-_0vWbPfDQneshps5vbaU4R_Q@mail.gmail.com>
Subject: Re: [PATCH] drm: enable uncached DMA optimization for ARM and arm64
To:     "Koenig, Christian" <christian.koenig@amd.com>
Cc:     Alex Deucher <alexdeucher@gmail.com>,
        "Daenzer, Michel" <Michel.Daenzer@amd.com>,
        Carsten Haitzler <Carsten.Haitzler@arm.com>,
        Maxime Ripard <maxime.ripard@bootlin.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        dri-devel <dri-devel@lists.freedesktop.org>,
        "Zhang, Jerry" <Jerry.Zhang@amd.com>,
        Christoph Hellwig <hch@infradead.org>,
        David Airlie <airlied@linux.ie>,
        "Huang, Ray" <Ray.Huang@amd.com>,
        amd-gfx list <amd-gfx@lists.freedesktop.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Will Deacon <will.deacon@arm.com>,
        "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 Fri, 25 Jan 2019 at 12:30, Christian K=C3=B6nig
<ckoenig.leichtzumerken@gmail.com> wrote:
>
> Am 25.01.19 um 09:43 schrieb Ard Biesheuvel:
> > On Thu, 24 Jan 2019 at 15:01, Alex Deucher <alexdeucher@gmail.com> wrot=
e:
> >> 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 device=
s
> >>>>> only, but permits an optimization to be enabled in some cases, wher=
e
> >>>>> 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 implementati=
on
> >>>>> of the PCIe NoSnoop TLP attribute by the platform, otherwise the GP=
U
> >>>>> will use cached mappings nonetheless. On x86 platforms, this does n=
ot
> >>>>> seem to matter, as uncached CPU mappings will snoop the caches in a=
ny
> >>>>> case. However, on ARM and arm64, enabling this optimization on a
> >>>>> platform where NoSnoop is ignored results in loss of coherency, whi=
ch
> >>>>> 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 all
> >
> > Should I resend the patch with the subject corrected?
>
> I will update the subject line and push it upstream through
> drm-misc-next if nobody objects.
>

Wonderful, thanks.