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

[Peter Robinson]

On Tue, 24 Dec 2024 at 14:00, 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
>

I don't think this is unique to the CM5, I've seen it on other RPi models,
I have a RPi4 8Gb model that exhibits the same behavior, yet other RPi4s I
have don't exhibit the behavious. In the case of the 8Gb I have a USB
serial TTL on it, it boots if the USB is plugged in, doesn't if not. I have
other v4s, both with and without the TTL and they boot fine, I've never
been able to get to the bottom of why.