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

[Ard Biesheuvel <ard.biesheuvel@linaro.org>]

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>
---
 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
-- 
2.20.1


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