4.20-stable review patch. If anyone has any objections, please let me know.
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[ Upstream commit e02f5c1bb2283cfcee68f2f0feddcc06150f13aa ]
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@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 Reviewed-by: Christian König christian.koenig@amd.com Reviewed-by: Alex Deucher alexander.deucher@amd.com Link: https://patchwork.kernel.org/patch/10778815/ Signed-off-by: Christian König christian.koenig@amd.com Signed-off-by: Sasha Levin sashal@kernel.org --- include/drm/drm_cache.h | 18 ++++++++++++++++++ 1 file changed, 18 insertions(+)
diff --git a/include/drm/drm_cache.h b/include/drm/drm_cache.h index bfe1639df02d..97fc498dc767 100644 --- a/include/drm/drm_cache.h +++ b/include/drm/drm_cache.h @@ -47,6 +47,24 @@ static inline bool drm_arch_can_wc_memory(void) return false; #elif defined(CONFIG_MIPS) && defined(CONFIG_CPU_LOONGSON3) return false; +#elif defined(CONFIG_ARM) || defined(CONFIG_ARM64) + /* + * 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. + */ + return false; #else return true; #endif