[PATCH 0/5] serial: Add MMIO & SPCR support for AWS EC2 metal instances

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

(Apologies if that got sent twice, there was an issue with my setup
yesterday causing it to be sent with the wrong From: line)

This series adds support for the serial console of AWS EC2 "metal" x86
instances to grub. This requires two improvements:

- Support for MMIO accesses to the 8250 serial port. This adds the basic
plumbing, a function to register an MMIO based port, and support for
configuring an MMIO port by using the string "mmio" followed by a hex
address for the --port directive. [Q: Should we instead add a --mmio
directive ? Can we make two such options mutually exclusive with the
current infrastructure ?]

- Support for setting up a default port using the ACPI SPCR table if
present. This series will make this happen if the command "serial" is
used without arguments. In that case, SPCR will be used if present,
otherwise grub will revert to com0 with default settings as before.

This work started originally from Matthias Lange series
https://marc.info/?l=grub-devel&m=148775823217022&w=2

However, I ended up rewriting most of it using a different approach
which I felt was less invasive and simpler, as I don't expect we will
be collecting more 'backends' for ns8250.c among other things.

I did not keep Matthias original support for OXSEMI PCI uart. This can
be fairly easily added on top, however, I believe a better approach
would be to define a syntax to the "serial" command to define a PCI UART
by seg/bus/dev/fn with options to set the base clock.

We could also add a table of "known" ones as we go based in vid/did
similar to what Linux does. None of this was necessary for my purpose
and I lack the ability/time to test this setup, but it would be easy
to add it on top of this series.

This was tested using SPCR on an actual c5.metal instance, and using
explicit instanciation via serial -p mmioXXXXXXXX on a modified qemu
hacked to create MMIO PCI serial ports.