25.09.2020 15:39, Robin Murphy пишет: ... >> Yes, my understanding that this is what Robin suggested here: >> >> https://lore.kernel.org/linux-iommu/cb12808b-7316-19db-7413-b7f852a6f8ae@xxxxxxx/ >> > > Just to clarify, what I was talking about there is largely orthogonal to > the issue here. That was about systems with limited translation > resources letting translation be specifically opt-in by IOMMU-aware > drivers. It probably *would* happen to obviate the issue of disrupting > live DMA at boot time on these particular Tegra platforms, but we still > need something like Thierry's solution in general, since IOMMU drivers > may have no other way to determine whether devices are active at boot > and they have to take care to avoid breaking anything - e.g. SMMUv3 will > at a bare minimum need to set up *some* form of valid stream table entry > for the relevant device(s) right at the beginning where we first probe > and reset the SMMU itself, regardless of what happens with domains and > addresses later down the line. Yes, I only meant that yours suggestion also should be useful here. Anyways, thank you for the clarification :) I agree that the Thierry's proposal is good! But it needs some more thought yet because it's not very applicable to the current devices. >>> The primary goal here is to move towards using the DMA API rather than >>> the IOMMU API directly, so we don't really have the option of replacing >>> with an explicitly created domain. Unless we have code in the DMA/IOMMU >>> code that does this somehow. >>> >>> But I'm not sure what would be a good way to mark certain devices as >>> needing an identity domain by default. Do we still use the reserved- >>> memory node for that? >> >> The reserved-memory indeed shouldn't be needed for resolving the >> implicit IOMMU problem since we could mark certain devices within the >> kernel IOMMU driver. >> >> I haven't got around to trying to implement the implicit IOMMU support >> yet, but I suppose we could implement the def_domain_type() hook in the >> SMMU driver and then return IOMMU_DOMAIN_IDENTITY for the Display/VDE >> devices. Then the Display/VDE drivers will take over the identity domain >> and replace it with the explicit domain. > > FWIW I've already cooked up identity domain support for tegra-gart; I > was planning on tackling it for tegra-smmu as well for the next version > of my arm default domains series (which will be after the next -rc1 now > since I'm just about to take some long-overdue holiday). Very nice! Maybe we will have some more food for the discussion by the time you'll return. Have a good time!
