Re: [edk2-devel] [PATCH 0/9] ArmVirtPkg: support two PL011 UARTs

[Peter Maydell <peter.maydell@linaro.org>]

On Tue, 10 Oct 2023 at 16:33, Laszlo Ersek <lersek@redhat.com> wrote:
> On 10/10/23 09:43, Ard Biesheuvel wrote:
> > Thanks for looking into this - a cleanup was overdue here.
> >
> > I will take a look in more detail later, but one thing that occurred
> > to me when reading this overview is that having a separate DEBUG
> > serial port would permit us to
> >
> > a) remove it from the DT
>
> ... as in, hide it from Linux, I assume?
>
> > b) add a runtime mapping for it
> > c) keep using it after ExitBootServices
> >
> > This could be useful for debugging issues with the variable store etc.
> >
> > Not saying this is something to address in this series, but I'd like
> > to hear your take on this.
> >
>
> Sounds like a useful feature.
>
> I see four challenges:
>
>
> (1) We'd have to coordinate it with Peter. If we hide any one of the
> serial ports from Linux, that may not be what QEMU intends for Linux to
> happen. Linux currently ties getties to all serial ports -- via the
> serial* aliases, IIUC. Thus, some "positive identification" in the DT
> could be necessary (i.e., that edk2 was welcome to hide that port from
> Linux).

The potential awkwardness here is that what the guest thinks about
the serial ports depends on the ACPI table fragments which QEMU
provides. EDK2 would need to edit the table fragment to remove any
mention of the second UART if it wanted to hide it from the kernel.
I don't know how hard that would be in EDK2.

(As far as I'm aware usually a boot via EDK2 doesn't pass the
dtb on to Linux, though I guess there's no reason it can't.)

From QEMU's point of view, we provide two UARTs to the guest, and we
don't really care whether that means one is used by EDK2 and one by
Linux, or both are used as getty terminals by Linux, or whether the
Linux guest uses one serial as a terminal and leaves the other to its
userspace programs  -- it's all just guest software to us :-)

[snip other technical stuff]

> All in all, I think the implementation would be quite a steep divergence
> from, or on top of, this patch set. :)

I agree with this and with Ard's "not something to address in this
series" comment above; it doesn't sound like this is something that
needs to hold up the patchset we have currently.

Does anybody have time to review Laszlo's code? It would be nice
to be able to get this into the next EDK2 release.

thanks
-- PMM


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