On Wed, Apr 25, 2018 at 12:10:47PM +0200, 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. > > Signed-off-by: Thierry Reding <treding@xxxxxxxxxx> > --- > I had already sent this out independently to fix a regression that was > introduced in v4.16, but then Christoph pointed out that it should've > been sent to a wider audience and should use a core API rather than > calling into architecture code directly. > > I've added it to this series for easier reference and to show the need > for the new API. This is good stuff, I am struggling with something similar on ARM64. One problem that I wasn't able to fully solve cleanly was that for arm-smmu the SMMU HW resources are not released until the domain itself is destroyed and I never quite figured out a way to swap the default domain cleanly. This is a problem for the MSM GPU because not only do we run our own IOMMU as you do we also have a hardware dependency to use context bank 0 to asynchronously switch the pagetable during rendering. I'm not sure if this is a problem you have encountered. In any event, this code gets us a little bit further down the path and at least there is somebody else out there in the cold dark world that understands my pain. :) For your interest, here was my half-hearted attempt to avoid creating DMA domains in the first place based on a blacklist to try to spur a bit of discussion: https://patchwork.freedesktop.org/series/41573/ Jordan -- The Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project -- To unsubscribe from this list: send the line "unsubscribe linux-tegra" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
