rpi: Compute Module 5 doesn't autoboot without debug UART

[Simon Glass]

Hi Gijs,

On Wed, 25 Dec 2024 at 02:59, Gijs Peskens <gijs.peskens at munisense.com> wrote:
>
> I'm working on getting a compute module working with Yocto using U-Boot
> as bootloader, which mostly works thanks to previous work to get RPi 5
> working.
> However I'm running into a strange issue: if I turn on the CM5 without
> the debug UART (which requires soldering a header onto the CM5)
> attached, the Pi doesn't boot beyond showing the submarine splash
> screen, if I boot it with the console attached it autoboots correctly.
> If I attach the UART after the boot halted and press enter I get a
> U-Boot console, and giving the boot command boots it normally.
> And setting bootdelay=-2 in the env allows it to correctly boot.
>
> Which leads me to believe there is perhaps a spurious input registered
> by U-Boot which interrupts autobooting.
>
> Is there a command that allows me to extract why U-Boot didn't continue
> autobooting?
>
> And anyone have a hint how this could perhaps be fixed in a more elegant
> way (allowing interruptable autoboot).
>
> The hardware I'm testing on is the CM5 devkit, I needed to make small
> changes to yocto to get it working:
> https://github.com/agherzan/meta-raspberrypi/pull/1397
>
> No changes were needed in U-Boot to get it to boot
>

I see the same thing. I wonder if we need to be able to silently
disable the UART. I made a few attempts at [1] and [2]. Tom
specifically mentioned the rpi at [3].

At least for the case where I saw it in my lab, the board continued to
boot OK, even though no console output appeared on the display. I
haven't actually made time to investigate this though.

Regards,
Simon

[1] https://patchwork.ozlabs.org/project/uboot/patch/20231101180447.99361-1-sjg@chromium.org/
[2] https://patchwork.ozlabs.org/project/uboot/list/?series=433605
[3] https://patchwork.ozlabs.org/project/uboot/cover/20241120153554.860710-1-sjg@chromium.org/