U-Boot autoboot interrupt because of uart

[roee.douek]

Hey everyone,

I’m running into a frustrating issue and hoping someone has dealt with this before.

Setup: I’ve got a Raspberry Pi Zero 2W running converted RPI-OS with Mender and U-Boot. I’m using a custom HAT that connects a sensor to the default GPIO pins.

The problem: When I power on the Pi, my UART device immediately starts sending data. This causes U-Boot to think someone is pressing keys and stops the autoboot process. The Pi just sits there waiting for input instead of booting normally.

What I’m wondering:

• Is it safe/possible to set bootdelay=0 or mess with U-Boot’s autoboot settings without breaking Mender updates?
• Any other ideas for getting around this?

Would really appreciate any advice from folks who’ve dealt with similar problem or have ideas!

Thanks!

[TheYoctoJester]

Hi @roee.douek,

Thanks for reaching out. There’s a couple of aspects here, hopefully some might be helpful to you.

• I guess that all other UARTs are already occupied, otherwise you can move the sensor connection.
• adding power switching to the sensor via GPIO can also be a good idea depending on situation.
• setting bootdelay=0 will not be a problem for Mender. However according to the u-boot configuration, -2 will be a better choice: u-boot/boot/Kconfig at 8f85a7345ed5df70a155f0630da72970eb01d87a · u-boot/u-boot · GitHub
• there are more options to automatic booting in u-boot, like requiring specific hashes or passwords. Check out u-boot/boot/Kconfig at 8f85a7345ed5df70a155f0630da72970eb01d87a · u-boot/u-boot · GitHub and all that follows for inspiration.
• You could also patch u-boot to just never check if the boot should be halted.

The one thing you should keep in mind is, depending on the sensor and its datagram format, the boot messages might confuse the sensor, or eventually, in weird cases, the sensor data might accidentally match such a password or hash sequence. So to be absolutely sure if you totally cannot move the UART and cannot power cycle the sensor, you would have to patch u-boot into being completely silent (and deaf).

Greetz,
Josef