From: Thierry Reding <treding@xxxxxxxxxx> On some platforms, the firmware will setup hardware to read from a given region of memory. One such example is a display controller that is scanning out a splash screen from physical memory. During Linux' boot process, the ARM SMMU will configure all contexts to fault by default. This means that memory accesses that happen by an SMMU master before its driver has had a chance to properly set up the IOMMU will cause a fault. This is especially annoying for something like the display controller scanning out a splash screen because the faults will result in the display controller getting bogus data (all-ones on Tegra) and since it repeatedly scans that framebuffer, it will keep triggering such faults and spam the boot log with them. In order to work around such problems, scan the device tree for IOMMU masters and set up a special identity domain that will map 1:1 all of the reserved regions associated with them. This happens before the SMMU is enabled, so that the mappings are already set up before translations begin. One thing that was pointed out earlier, and which I don't have a good idea on how to solve it, is that the early identity domain is not discarded. The assumption is that the standard direct mappings code of the IOMMU framework will replace the early identity domain once devices are properly attached to domains, but we don't have a good point in time when it would be safe to remove the early identity domain. One option that I can think of would be to create an early identity domain for each master and inherit it when that master is attached to the domain later on, but that seems rather complicated from an book- keeping point of view and tricky because we need to be careful not to map regions twice, etc. Any good ideas on how to solve this? It'd also be interesting to see if there's a more generic way of doing this. I know that something like this isn't necessary on earlier Tegra SoCs with the custom Tegra SMMU because translations are only enabled when the devices are attached to a domain. I'm not sure about other IOMMUs, but in the absence of a struct device, I suspect that we can't really do anything really generic that would work across drivers. Thierry Thierry Reding (2): iommu: arm-smmu: Extract arm_smmu_of_parse() iommu: arm-smmu: Add support for early direct mappings drivers/iommu/arm-smmu.c | 195 +++++++++++++++++++++++++++++++++++++-- drivers/iommu/arm-smmu.h | 2 + 2 files changed, 189 insertions(+), 8 deletions(-) -- 2.23.0
