Re: [RFC 0/2] iommu: arm-smmu: Add support for early direct mappings

Hi Thierry,

I happened upon this thread while looking into another thread [1].

From: Thierry Reding <redacted>

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/ (local)
[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

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