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

Once upon a time, Peter Robinson <pbrobinson@gmail.com> said:
> > 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.

Heh, nice. I got back looking at it with the release of Fedora 41
(trying to test all my broken issues to close or update :) ). I'm on
the BZ, but dug some more to see if I was missing something.

> > 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.

Trying things, https://docs.u-boot.org/en/latest/usage/environment.html
says that -2 is "autoboot with no delay and not check for abort". But
it looks like maybe the EFI menu doesn't apply the values the same way?
It is a bit interesting that it always seems to get to the EFI menu now,
not abort at the u-boot prompt (like it used to). Although IIRC having
the EFI menu is relatively new?

Is there any way to edit the EFI variables to change the menu timeout
(including maybe a similar "don't check for abort" value), or does it
always use the bootdelay setting?

> > 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).

Okay, will keep a watch out.

> > 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.

Oh, okay. That explains things.

With one GPS HAT, I have a systemd unit that as the last thing on
shutdown will send a warm reset command to the GPS module, which takes
long enough to start sending NMEA sentences again that the boot process
has run.

But that's the one that I haven't been able to run a kernel past 6.6,
because something causes it to hard reset and stop at u-boot about 20-30
seconds after boot, possibly when the GPS starts sending PPS on GPIO?
That one does use a non-default GPIO pin (4, default for pps-gpio
overlay is 18).

--
Chris Adams <linux@cmadams.net>
--
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