Boot order: change so USB is tried first

[b99]

I have a Pi 3B + running headless inside a case which leaves only the power, Ethernet, and 2 USB ports accessible. When a mistake in dhcpcd.conf cut off ssh access, I had to disassemble to case to remove the SD card to correct. What I'd like to do is set it up so a similar situation in the future could be fixed by booting from USB. Is there a way set the boot sequence to boot from a USB if it is plugged in even if the SD card is inserted?

[BespokeTech]

I don't think this is possible, the boot up sequence is stored in the boot rom which as far as I am aware cannot be changed.

But... it basically looks for a file called bootcode.bin on the SD card, if it cannot find it then it will move onto a USB boot. So as you are only talking about a loss of network scenario as oppose to a complete failure to boot from the SD card, you could write a little script which runs off the SD card image which watches for a USB stick being plugged in and then it could look for a certain file on this usb stick say for example "changeboot". If it finds the required file then the script renames the bootcode.bin file on the SD card and reboots the PI. In theory it should then boot from usb!

[b99]

Thanks! That's very creative. Then I'd need to remember to fix bootcode.bin before the reboot. I'd prefer a neater option if anyone is aware of one.

[BespokeTech]

Thanks! That's very creative. Then I'd need to remember to fix bootcode.bin before the reboot. I'd prefer a neater option if anyone is aware of one.

Well the USB stick you use to recover the situation could just have a startup script that renames the bootcode.bin file on the SD card back to it's original name.

For a detailed write up on how the boot modes work....

https://www.raspberrypi.org/blog/pi-3-b ... rage-boot/

All of the boot code has to fit into 32kB so I don't think it is possible to do any fancy boot sequence ordering unfortunately.

Thinking about what I proposed, the script that checks for the presence of a USB stick with the "changeboot" file on it only needs to run once during the bootup of the SD image. So in summary the process would be.

1. Lose network connectivity to PI (Booted from SD card)

2. Insert USB stick containing a bootable image and also "changeboot" empty text file.

3. Power cycle the PI, the SD card will boot as normal and run the script to check for the presence of the USB stick. If it finds it then, rename the bootcode.bin file and reboot.

4. PI should then boot from the USB stick, and as I said above you could have a boot up script that renames the original bootcode.bin on the SD card.

[fruitoftheloom]

Thanks! That's very creative. Then I'd need to remember to fix bootcode.bin before the reboot. I'd prefer a neater option if anyone is aware of one.

https://www.raspberrypi.org/documentati ... bootmodes/

[HawaiianPi]

You can change the boot device by setting an additional OTP bit and pulling certain GPIO pins high or low. This does not change the boot order, it tells the Pi to boot from either SD or USB, depending on the state of the GPIO pins (you could add a switch to the case to force USB boot).

https://www.raspberrypi.org/documentati ... ootflow.md

[b99]

Thanks all for the replies. I am going to try and implement BespokeTechs solution.

[epoch1970]

The solution presented forces to boot twice in order to start the system in the nominal case, USB present:
- SD boot: if USB present -> bootcode.hide + reboot
- USB boot: -> bootcode.bin
It will be a little slow and FAT is not the most resilient file system.

Another possibility is to boot in uboot on the SD and choose the final target dynamically from a script. Uboot is often used in OTA update scenarios (rootfs A/B) so besides scripting it has a few features allowing to fail safe(r).

[b99]

Thanks! That's very creative. Then I'd need to remember to fix bootcode.bin before the reboot. I'd prefer a neater option if anyone is aware of one.

Well the USB stick you use to recover the situation could just have a startup script that renames the bootcode.bin file on the SD card back to it's original name.

For a detailed write up on how the boot modes work....

https://www.raspberrypi.org/blog/pi-3-b ... rage-boot/

All of the boot code has to fit into 32kB so I don't think it is possible to do any fancy boot sequence ordering unfortunately.

Thinking about what I proposed, the script that checks for the presence of a USB stick with the "changeboot" file on it only needs to run once during the bootup of the SD image. So in summary the process would be.

1. Lose network connectivity to PI (Booted from SD card)

2. Insert USB stick containing a bootable image and also "changeboot" empty text file.

3. Power cycle the PI, the SD card will boot as normal and run the script to check for the presence of the USB stick. If it finds it then, rename the bootcode.bin file and reboot.

4. PI should then boot from the USB stick, and as I said above you could have a boot up script that renames the original bootcode.bin on the SD card.

For stage 3 I'm using the following script:

Code: Select all

#!/bin/bash

if [ $(find /media/pi/ -name changeboot) ]; then
    echo "Boot usb inserted"

    mv /boot/bootcode.bin /boot/bootcode.bak

fi

This works because Raspbian automounts USB drives in /media/pi.

I manually executed the script and rebooted and the Pi booted from the USB. So far, great.

For stage 4, it doesn't seem like Raspbian automounts the SD card so I'll have to have the script do it. The problem is finding the name of the boot partition. It is currently /dev/mmcblk0p1 as it has been every time I've looked in the past. Will it always be so?

[BespokeTech]

For stage 4, it doesn't seem like Raspbian automounts the SD card so I'll have to have the script do it. The problem is finding the name of the boot partition. It is currently /dev/mmcblk0p1 as it has been every time I've looked in the past. Will it always be so?

Yes they will stay the same....

mmcblk0p1 - Boot partition of the SD Card. (fat)
mmcblk0p2 - Main raspbian partition on the SD card (ext4)

[BespokeTech]

With regards to auto mounting the SD card, have a look at this...

http://www.embeddedpi.com/documentation ... ng-sd-card

[b99]

With regards to auto mounting the SD card, have a look at this...

http://www.embeddedpi.com/documentation ... ng-sd-card

Since this is all only for a rescue scenario, I think just having the script on the USB mount the SD card is simpler.

Thanks for all of your help, I've successfully implemented your proposal.

On the Pi:

$ cat /usr/local/bin/usbbootcheck.sh

Code: Select all

#!/bin/bash

if [ $(find /media/pi/ -name changeboot) ]; then
    echo "Boot usb inserted"
    sudo mv /boot/bootcode.bin /boot/bootcode.bak

fi

$ crontab -e

Code: Select all

@reboot sleep 15 && sudo sh /usr/local/bin/usbbootcheck.sh

It wasn't working without the sleep 15, I'm guessing cron was running the script before the USB was mounted.


On the USB:

$ cat /usr/local/bin/restorebootcode.sh

Code: Select all

#! /bin/bash

mount /dev/mmcblk0p1 /mnt/sdcard

mv /mnt/sdcard/bootcode.bak /mnt/sdcard/bootcode.bin

$ crontab -e

Code: Select all

@reboot sleep 15 && sudo sh /usr/local/bin/restorebootcode.sh

[BespokeTech]

Nice