From: Thierry Reding <treding@xxxxxxxxxx> Depending on the kernel configuration, early ARM architecture setup code may have attached the GPU to a DMA/IOMMU mapping that transparently uses the IOMMU to back the DMA API. Tegra requires special handling for IOMMU backed buffers (a special bit in the GPU's MMU page tables indicates the memory path to take: via the SMMU or directly to the memory controller). Transparently backing DMA memory with an IOMMU prevents Nouveau from properly handling such memory accesses and causes memory access faults. As a side-note: buffers other than those allocated in instance memory don't need to be physically contiguous from the GPU's perspective since the GPU can map them into contiguous buffers using its own MMU. Mapping these buffers through the IOMMU is unnecessary and will even lead to performance degradation because of the additional translation. One exception to this are compressible buffers which need large pages. In order to enable these large pages, multiple small pages will have to be combined into one large (I/O virtually contiguous) mapping via the IOMMU. However, that is a topic outside the scope of this fix and isn't currently supported. An implementation will want to explicitly create these large pages in the Nouveau driver, so detaching from a DMA/IOMMU mapping would still be required. Signed-off-by: Thierry Reding <treding@xxxxxxxxxx> --- Changes in v3: - clarify the use of IOMMU mapping for compressible buffers - squash multiple patches into this drivers/gpu/drm/nouveau/nvkm/engine/device/tegra.c | 5 +++++ 1 file changed, 5 insertions(+) diff --git a/drivers/gpu/drm/nouveau/nvkm/engine/device/tegra.c b/drivers/gpu/drm/nouveau/nvkm/engine/device/tegra.c index 78597da6313a..d0538af1b967 100644 --- a/drivers/gpu/drm/nouveau/nvkm/engine/device/tegra.c +++ b/drivers/gpu/drm/nouveau/nvkm/engine/device/tegra.c @@ -105,6 +105,11 @@ nvkm_device_tegra_probe_iommu(struct nvkm_device_tegra *tdev) unsigned long pgsize_bitmap; int ret; +#if IS_ENABLED(CONFIG_ARM) + /* make sure we can use the IOMMU exclusively */ + arm_dma_iommu_detach_device(dev); +#endif + if (!tdev->func->iommu_bit) return; -- 2.17.0 _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/dri-devel
