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

[Ard Biesheuvel]

On Thu, 24 Jan 2019 at 13:31, Koenig, Christian
<Christian.Koenig@xxxxxxx> 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@xxxxxxx>
> > Cc: Alex Deucher <alexander.deucher@xxxxxxx>
> > Cc: David Zhou <David1.Zhou@xxxxxxx>
> > Cc: Huang Rui <ray.huang@xxxxxxx>
> > Cc: Junwei Zhang <Jerry.Zhang@xxxxxxx>
> > Cc: Michel Daenzer <michel.daenzer@xxxxxxx>
> > Cc: David Airlie <airlied@xxxxxxxx>
> > Cc: Daniel Vetter <daniel@xxxxxxxx>
> > Cc: Maarten Lankhorst <maarten.lankhorst@xxxxxxxxxxxxxxx>
> > Cc: Maxime Ripard <maxime.ripard@xxxxxxxxxxx>
> > Cc: Sean Paul <sean@xxxxxxxxxx>
> > Cc: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
> > Cc: Benjamin Herrenschmidt <benh@xxxxxxxxxxxxxxxxxxx>
> > Cc: Will Deacon <will.deacon@xxxxxxx>
> > Cc: Christoph Hellwig <hch@xxxxxxxxxxxxx>
> > Cc: Robin Murphy <robin.murphy@xxxxxxx>
> > Cc: amd-gfx list <amd-gfx@xxxxxxxxxxxxxxxxxxxxx>
> > Cc: dri-devel <dri-devel@xxxxxxxxxxxxxxxxxxxxx>
> > Reported-by: Carsten Haitzler <Carsten.Haitzler@xxxxxxx>
> > Signed-off-by: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>
>
> The subject line should probably read "disable uncached...".
>

Ugh, of course ...

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

Thanks.