[fedora-arm] Re: Disabling serial console in u-boot

Hi Chris,

> I have a couple of Raspberry Pi 4Bs with a couple of different GPS HATs.
> Both GPS HATs work similar - they use the main serial port pins to
> transmit the NMEA sentences, several every second once they have GPS
> sync. Since u-boot stops on serial input, this keeps them from booting
> unattended.

This a problem we've sadly had for ever [1], and it's been discussed
upstream in U-Boot too (I have tagged messages in my inbox somewhere).
There's not yet a good solution, although I was thinking about it the
other day.

> With a fresh Fedora 41 install, I see u-boot start and then get the EFI
> boot menu, which stops until I hit ENTER when the GPS HAT is attached.
>
> I tried setting bootdelay to -2 but that seems to stop the EFI boot menu
> from autobooting no matter what (e.g. pulled the HAT off and it still
> stopped there).

Why -2? That might be something worth looking into separately.

> I also changed stdin/stdout/stderr to remove serial, that didn't seem to
> have any effect.

No, it doesn't, U-boot very much wants a serial console, there has
been a little improvement upstream here though which should be in
2025.01 (I think).

> I assume people don't have these problems with the "official" Raspberry
> Pi OS - do they build u-boot with different options from Fedora that
> allow fully disabling the u-boot serial console mode?

They don't use U-Boot, they boot directly from the VC firmware to the
kernel using ancient interfaces.

This would be solved with the RPi5 as it uses a completely different
serial port for the console (and has about 16 serial interfaces on the
SoC) but of course that doesn't help with older devices.

[1] https://bugzilla.redhat.com/show_bug.cgi?id=1643714
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