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 <[email protected]>
Cc: Alex Deucher <[email protected]>
Cc: David Zhou <[email protected]>
Cc: Huang Rui <[email protected]>
Cc: Junwei Zhang <[email protected]>
Cc: Michel Daenzer <[email protected]>
Cc: David Airlie <[email protected]>
Cc: Daniel Vetter <[email protected]>
Cc: Maarten Lankhorst <[email protected]>
Cc: Maxime Ripard <[email protected]>
Cc: Sean Paul <[email protected]>
Cc: Michael Ellerman <[email protected]>
Cc: Benjamin Herrenschmidt <[email protected]>
Cc: Will Deacon <[email protected]>
Cc: Christoph Hellwig <[email protected]>
Cc: Robin Murphy <[email protected]>
Cc: amd-gfx list <[email protected]>
Cc: dri-devel <[email protected]>
Reported-by: Carsten Haitzler <[email protected]>
Signed-off-by: Ard Biesheuvel <[email protected]>
Reviewed-by: Christian König <[email protected]>
Reviewed-by: Alex Deucher <[email protected]>
Link: https://patchwork.kernel.org/patch/10778815/
Signed-off-by: Christian König <[email protected]>
Ard Biesheuvel