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

[Ard Biesheuvel]

On Thu, 24 Jan 2019 at 13:31, Koenig, Christian
<Christian.Koenig at 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 at amd.com>
> > Cc: Alex Deucher <alexander.deucher at amd.com>
> > Cc: David Zhou <David1.Zhou at amd.com>
> > Cc: Huang Rui <ray.huang at amd.com>
> > Cc: Junwei Zhang <Jerry.Zhang at amd.com>
> > Cc: Michel Daenzer <michel.daenzer at amd.com>
> > Cc: David Airlie <airlied at linux.ie>
> > Cc: Daniel Vetter <daniel at ffwll.ch>
> > Cc: Maarten Lankhorst <maarten.lankhorst at linux.intel.com>
> > Cc: Maxime Ripard <maxime.ripard at bootlin.com>
> > Cc: Sean Paul <sean at poorly.run>
> > Cc: Michael Ellerman <mpe at ellerman.id.au>
> > Cc: Benjamin Herrenschmidt <benh at kernel.crashing.org>
> > Cc: Will Deacon <will.deacon at arm.com>
> > Cc: Christoph Hellwig <hch at infradead.org>
> > Cc: Robin Murphy <robin.murphy at arm.com>
> > Cc: amd-gfx list <amd-gfx at lists.freedesktop.org>
> > Cc: dri-devel <dri-devel at lists.freedesktop.org>
> > Reported-by: Carsten Haitzler <Carsten.Haitzler at arm.com>
> > Signed-off-by: Ard Biesheuvel <ard.biesheuvel at linaro.org>
>
> The subject line should probably read "disable uncached...".
>

Ugh, of course ...

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

Thanks.