Merge tag 'v4.19.29' into dev-4.19dev-4.19

This is the 4.19.29 stable release

Signed-off-by: Joel Stanley <joel@jms.id.au>

1 files changed, 18 insertions, 0 deletions

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