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

[Gijs Peskens]

On 12/31/24 15:53, Peter Robinson wrote:
>
>
> 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.
Can you try by setting the bootdelay to -2? That did fix it for me, 
though autoboot is no longer interruptable.

As stated, I suspect my CM5 is generating spurious signals on the debug 
uart when it's not connected. Is there a config option that changes 
autoboot interruption to only happen on a specific key combination? i.e. 
control+c ?
That might fix it sufficiently for me.

Gijs