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

["Laszlo Ersek" <lersek@redhat.com>]

On 9/28/23 13:24, Peter Maydell wrote:
> 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,

Right -- I wasn't aware. I can see GetSerialConsolePortAddress() in
"ArmVirtPkg/Library/Fdt16550SerialPortHookLib/EarlyFdt16550SerialPortHookLib.c",
which is used in "ArmVirtPkg/ArmVirtKvmTool.dsc".

> 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.

OK -- I think the proposal might still work if we replaced "UART found
second" with "first non-stdout-path UART", and "UART found first" with
"stdout-path UART".

Laszlo




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