Talking to u-boot on Buster

The default Pi boot process uses firmware running on the GPU and loaded from SD card (except Pi 4 where there's flash ROM), so the ARM isn't operational until the kernel is loaded. This isn't u-boot. It is possible to load a 'kernel' which happens to be the u-boot binary, and then u-boot selects where to load the next stage from.

AIUI the default Raspbian doesn't use u-boot. It is possible to put it on the SD card, in which case it provides a serial console. For example:

Once you have u-boot, it's possible to get the kernel from USB or network, for example via TFTP:

- you would tell u-boot to run a script stored on the SD card. And then that kernel would boot and have its root fs come from USB or NFS or whatever.

Theo

Sounds like the key is having a serial console. I don't, on the machine in question, and can't easily set one up.

On FreeBSD machines that _do_ have a serial console it's possible to interrupt u-boot by hitting the spacebar and tell it to look for a USB mass storage device with: usb reset Once found, the command run bootcmd_USB0 will boot from the USB device.

I was hoping to do something similar for the machine running RaspiOS, which is isolated with no place to set up a serial console, just hdmi and keyboard. It's still possible to simply insert the microSD card and let it take over, like an old-fashioned boot floppy. I guess that will have to do for now.

Thanks for writing!

bob prohaska

Isn't USB booting a feature that was recently introduced for the Raspberry Pi 4 only?

Not at all:

hth,

bob prohaska