Re: [edk2-devel] EDK2 ArmVirtQemu behaviour with multiple UARTs

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

On Thu, 28 Sept 2023 at 08:54, Laszlo Ersek <lersek@redhat.com> wrote:
>
> On 9/21/23 14:02, ardb at kernel.org (Ard Biesheuvel) wrote:
> > EDK2's DEBUG output is extremely noisy, so being able to redirect this
> > output to a different UART would be very useful.
> >
> > The stdout-path is the intended console, and so we should honour that.
> > This also means that we should parse aliases. But the console is
> > actually configurable [persistenly] via the UEFI menu, and so it would
> > be nice if we could take advantage of this flexibility. This means in
> > principle that the UARTs should be represented via different device
> > paths (which would include the base address so they are
> > distinguishable) with perhaps a magical alias which is the default and
> > is tied to whatever stdout-path points to. This way, all the logic we
> > introduce is spec compliant and reusable on physical platforms with
> > multiple UARTs.

> > What we might do is use stdout-path as well, unless a certain DT alias
> > exist perhaps? We should probably align here with other projects,
> > although this a distinction of the same nature may not exist there.
> >
>
> Alias parsing in edk2 would be a bit too complicated for my taste. :)
>
> I see the following two problems with the current state (based on
> Peter's captures, using the original UART order in the DTB, i.e.,
> <https://people.linaro.org/~peter.maydell/uart0.txt> and
> <https://people.linaro.org/~peter.maydell/uart1.txt>):
>
> (1) The DEBUG output switches from one UART to the other when we reach
> the DXE_CORE (in this case, from UART0 to UART1, but the precise numbers
> aren't the problem, the switchover is),
>
> (2) The UEFI console (which is used by the setup browser, the UEFI
> shell, grub, etc) is on UART1, while the kernel stuff is on UART0.
>
> Here's what I'd propose:
>
> - if there is only one UART in the DTB, no change
>
> - otherwise, direct all DEBUG messages to the UART found *second* via
> forward traversal in the DTB (let's call this UART1), and include the
> UART found *first* via forward traversal in the DTB (let's call this
> UART0) in the UEFI console. Furthermore, do not expose UART1 in the UEFI
> protocol database *at all* (don't install devpath protocol / SerialIo
> protocol); make it effectively hidden hardware (similarly how the x86
> QEMU debug console, IO Port 0x402, is not exposed at all). Let the
> system think there is only one UART (UART0), and treat UART1 as a
> "bespoke", custom debug device only. This also ensures that existent
> higher level products such as libvirt, which may only handle UART0 at
> the moment, will expose the interactive console (UEFI and Linux) to the
> user, and at worst the firmware debug log will not be captured.

The 16550 version of the QEMU-specific EDK uart-location code (used when
running it under kvmtool) already honours stdout-path, so I'm not sure
why we wouldn't want to be consistent with that. I'm not really a fan of
anything that depends on ordering of nodes in the DTB -- it is pretty
fragile in my experience. The DTB spec provides a mechanism to
correctly identify which UART to use, so I think that there would
need to be a really strong reason not to do it that way.

thanks
-- PMM


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