Hi Thierry, I happened upon this thread while looking into another thread [1]. > 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. While I'm not an expert on IOMMUs, I have a decent high level understanding of the problem you are trying to solve. > 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. I'm not sure if this RFC will solve the splash screen issue across SoCs ([1] seems to have a different issue and might not have memory-regions). > 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. You are in luck! I added sync_state() driver callbacks [2] exactly for cases like this. Heck, I even listed IOMMUs as an example use case. :) sync_state() works even with modules if one enables of_devlink [3] kernel parameter (which already supports iommus DT bindings). I'd be happy to answer any question you have on sync_state() and of_devlink. > 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. Good foresight. As [1] shows, identity mapping doesn't solve it in a generic way. How about actually reading the current settings/mappings and just inheriting that instead of always doing a 1:1 identity mapping? And then those "inherited" mappings can be dropped when you get a sync_state(). What's wrong with that option? Cheers, Saravana [1] https://lore.kernel.org/linux-arm-msm/20200108091641.GA15147@willie-the-truck/ [2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/driver-api/driver-model/driver.rst#n172 [3] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/admin-guide/kernel-parameters.txt#n3239
