Using Rust on non-Rust side of kernel

[Jarkko Sakkinen <jarkko@kernel.org>]

Hi

As of today can we possibly do this:

1. drivers/char/tpm (C code)
2. drivers/char/tpm/protocol (imported tpm2_protocol)

?

And then build FFI from C to Rust for building commands that we need
today etc.

There's one particular challenge where this could help: early boot code
for D-RTM (i.e., Trenchboot) as given my crate is just a thing in stack
with no deps, it could be linked also to that payload.

This would be much better integration step for TPM2 than having a
separate driver on Rust side. We could start with tpm2-cmd1/cmd2, then
move on to tpm2-space.c i.e. get all structural processing inside Rust.

tpm2_protocol is light on definitions and should not need any kernel
specific Rust shenanigans.

Consider it as value like integer but just a bit more complex internaal
represention but in the end it is just a value on stack.

My goal with tpm2_protocol is to have ACPICA alike model of imports as
the crate is driven by TCG spec updates and it is very likely to be
also used by TPM-RS (also via import style process).

BR, Jarkko