[GSoC - x86_64] Console / serial communication

[Amaan Cheval]

Hi!

In the last thread about using FreeBSD's bootloader to bring UEFI
support to our port, Chris said this:

> It has been a couple of years but I think FreeBSD contains some of the Intel
> code to interface to UEFI and via this you can get to the UEFI console. This
> should be easy but it comes with a side effect.
>
> UEFI boots in graphics mode and so it's console on a PC is a slow scroll one. On
> boards like a Minnow using the UEFI console has the advantage of being able to
> support any redirection UEFI has enabled such as a serial port. The disadvantage
> of this is performance and overhead. In time this may be a boot option.
>
> What I am not sure is the boundary between UEFI and the kernel and what is
> enabled or available when the kernel is loaded.

Source: https://lists.rtems.org/pipermail/devel/2018-June/022136.html

According to my understanding, the efi_console is not available to the
kernel - only to the loader.efi (the PE image of the bootloader, which
eventually loads the ELF kernel).

(In brief, my reasoning for this belief is that efi_console is defined
and initialized only through the bootloader code (in stand/efi/main.c,
when cons_probe is called). The efi_console relies on UEFI's
BootServices, which won't be accessible when the ELF kernel is loaded,
because ExitBootServices() is called before loading the kernel (as
part of bi_load).)

Given that information, it makes more sense to actually port a console
driver that the ELF kernel uses, not that the bootloader uses.

I'll look into how FreeBSD implements their UART console
(https://github.com/freebsd/freebsd/blob/1cfbfa1fae9926303f69532a97a5a766fa672582/sys/dev/uart/uart_tty.c),
and look into possibly reusing RTEMS' existing drivers for UART
(NS16550?) as applicable.

I'm not sure I know enough about serial programming _yet_, so do let
me know if you think you have any resources that would be useful or if
you'd like to fine-tune my plan!

P.S. - Since we last disussed it, the context initialization and
switching are working, and the port does now make it to the user's
Init task! :D

Cheers