On Wed, May 30, 2018 at 02:30:51PM +0100, Robin Murphy wrote: > On 30/05/18 14:00, Thierry Reding wrote: > > On Wed, May 30, 2018 at 11:30:25AM +0100, Robin Murphy wrote: > > > On 30/05/18 09:03, Thierry Reding wrote: > > > > 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. > > > > > > I wonder if it might make sense to have a hook in iommu_attach_device() to > > > notify the arch DMA API code when moving devices between unmanaged and DMA > > > ops domains? That seems like it might be the most low-impact way to address > > > the overall problem long-term. > > > > > > > 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) > > > > > > Wouldn't CONFIG_ARM_DMA_USE_IOMMU be even more appropriate? > > > > Not necessarily. arm_dma_iommu_detach_device() is always defined on ARM, > > only with CONFIG_ARM_DMA_USE_IOMMU=n it will be empty. So this check is > > a guard to make sure we don't call the function when it isn't available, > > but it may still not do anything. > > Calling a function under condition A, which only does anything under > condition B, when B depends on A, is identical in behaviour to only calling > the function under condition B, except needlessly harder to follow. > > > > > + /* make sure we can use the IOMMU exclusively */ > > > > + arm_dma_iommu_detach_device(dev); > > > > > > As before, I would just use the existing infrastructure the same way the > > > Exynos DRM driver currently does in __exynos_iommu_attach() (albeit without > > > then reattaching to another DMA ops mapping). > > > > That's pretty much what I initially did and which was shot down by > > Christoph. As I said earlier, at this point I don't really care what > > color the shed will be. Can you and Christoph come to an agreement > > on what it should be? > > What I was getting at is that arm_iommu_detach_device() already *is* the > exact function Christoph was asking for, it just needs a minor fix instead > of adding explicit set_dma_ops() fiddling at its callsites which only > obfuscates the fact that it's supposed to be responsible for resetting the > device's DMA ops already. It still has the downside of callers having to explicitly check for the existence of a mapping, otherwise they'll cause a warning to be printed to the kernel log. That's not all that bad, though. I'll prepare version 4 with those changes. Thierry
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