Talking to u-boot on Buster

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When booting Buster on a Pi3B+ it seems that u-boot finishes
before there's any chance of text input on the console.

How does one make u-boot pause and accept input so the
boot can be redirected from microSD to USB when starting
Buster?

Thanks for reading,

bob prohaska


Re: Talking to u-boot on Buster
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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:
https://andrei.gherzan.ro/linux/uboot-on-rpi/

Once you have u-boot, it's possible to get the kernel from USB or network,
for example via TFTP:
https://www.denx.de/wiki/DULG/Manual
- 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

Re: Talking to u-boot on Buster
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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



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