On Fri, Mar 26, 2021 at 06:29:28PM +0300, Dmitry Osipenko wrote:
> 25.03.2021 16:03, Thierry Reding пишет:
> > From: Thierry Reding <treding@nvidia.com>
> > 
> > Hi,
> > 
> > this is a set of patches that is the result of earlier discussions
> > regarding early identity mappings that are needed to avoid SMMU faults
> > during early boot.
> > 
> > The goal here is to avoid early identity mappings altogether and instead
> > postpone the need for the identity mappings to when devices are attached
> > to the SMMU. This works by making the SMMU driver coordinate with the
> > memory controller driver on when to start enforcing SMMU translations.
> > This makes Tegra behave in a more standard way and pushes the code to
> > deal with the Tegra-specific programming into the NVIDIA SMMU
> > implementation.
> 
> It is an interesting idea which inspired me to try to apply a somewhat similar thing to Tegra SMMU driver by holding the SMMU ASID enable-bit until display driver allows to toggle it. This means that we will need an extra small tegra-specific SMMU API function, but it should be okay.
> 
> I typed a patch and seems it's working good, I'll prepare a proper patch if you like it.

That would actually be working around the problem that this patch was
supposed to prepare for. The reason for this current patch series is to
make sure SMMU translation isn't enabled until a device has actually
been attached to the SMMU. Once it has been attached, the assumption is
that any identity mappings will have been created.

One Tegra SMMU that shouldn't be a problem because translations aren't
enabled until device attach time. So in other words this patch set is to
get Tegra186 and later to parity with earlier chips from this point of
view.

I think the problem that you're trying to work around is better solved
by establishing these identity mappings. I do have patches to implement
this for Tegra210 and earlier, though they may require additional work
if you have bootloaders that don't use standard DT bindings for passing
information about the framebuffer to the kernel.

Thierry