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

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Date:   Thu, 7 Feb 2019 10:33:37 -0500
From:   Alex Deucher <alexdeucher@...il.com>
To:     Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:     "Koenig, Christian" <christian.koenig@....com>,
        Will Deacon <will.deacon@....com>,
        Carsten Haitzler <Carsten.Haitzler@....com>,
        Maxime Ripard <maxime.ripard@...tlin.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        David Airlie <airlied@...ux.ie>,
        "Daenzer, Michel" <Michel.Daenzer@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Christoph Hellwig <hch@...radead.org>,
        "Zhang, Jerry" <Jerry.Zhang@....com>,
        "Huang, Ray" <Ray.Huang@....com>,
        amd-gfx list <amd-gfx@...ts.freedesktop.org>,
        "Deucher, Alexander" <Alexander.Deucher@....com>,
        Sean Paul <sean@...rly.run>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        Robin Murphy <robin.murphy@....com>
Subject: Re: [PATCH] drm: enable uncached DMA optimization for ARM and arm64

On Thu, Feb 7, 2019 at 10:20 AM Ard Biesheuvel
<ard.biesheuvel@...aro.org> wrote:
>
> On Wed, 6 Feb 2019 at 19:38, Christian König
> <ckoenig.leichtzumerken@...il.com> wrote:
> >
> > Am 06.02.19 um 18:23 schrieb Ard Biesheuvel:
> > > On Fri, 25 Jan 2019 at 11:35, Ard Biesheuvel <ard.biesheuvel@...aro.org> wrote:
> > >> On Fri, 25 Jan 2019 at 12:30, Christian König
> > >> <ckoenig.leichtzumerken@...il.com> wrote:
> > >>> Am 25.01.19 um 09:43 schrieb Ard Biesheuvel:
> > >>>> On Thu, 24 Jan 2019 at 15:01, Alex Deucher <alexdeucher@...il.com> wrote:
> > >>>>> On Thu, Jan 24, 2019 at 9:00 AM Ard Biesheuvel
> > >>>>> <ard.biesheuvel@...aro.org> wrote:
> > >>>>>> 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>.
> > >>>>>>>
> > >>>>> Same:
> > >>>>> Reviewed-by: Alex Deucher <alexander.deucher@....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.
> > > Hi Christian,
> > >
> > > Are you still planning to merge this for v5.1?
> >
> > My bad, only pushed this to our internal branch, but forgot out
> > drm-misc-next.
> >
> > Fixed now, thanks for the reminder.
> >
>
> Thanks,
>
> Does anyone mind if I propose this patch for backporting to v4.19 or
> earlier once it gets merged for v5.1?

Go for it.  I was going to suggest that this should probably go to stable.

Alex