Does booting from USB flash drive alter OTP memory forever ?

[maicod]

in this article: http://www.makeuseof.com/tag/make-raspb ... -boot-usb/ I read that making the Rpi3 boot from an USB flash drive alters the OTP memory through editing config.txt. As OTP stands for One Time Programmable, does that mean I can't disable the boot from USB setting anymore ?

[DougieLawson]

Yes. OTP == one time programmable. Once an OTP bit is set it can never be reset.

It's adding

Code: Select all

program_usb_boot_mode=1

to /boot/config.txt that sets the bit USB/PXE boot bit in OTP byte 17.

[maicod]

Yes. OTP == one time programmable. Once an OTP bit is set it can never be reset.

It's adding

Code: Select all

program_usb_boot_mode=1

to /boot/config.txt that sets the bit USB/PXE boot bit in OTP byte 17.

What implications does that have in functionality ? What if I ever want to boot from MicroSD card again by using

Code: Select all

program_usb_boot_mode=0

or such ?

[PeterO]

This may help:

https://github.com/raspberrypi/document ... ootflow.md

PeterO

[DougieLawson]

That's a syntactically invalid directive so will be 100% completely, totally ignored.

Once a OTP is set it can NEVER be reset, at all ever, for all of future time.

All three of my Raspberry Pi 3Bs boot perfectly OK from an SDCard with that OTP set. The bigger problem was when I left a USB stick (with no bootcode.bin on it) in one of those machines - that resulted in a boot failure even though one of the USB devices was bootable.

[maicod]

This may help:

https://github.com/raspberrypi/document ... ootflow.md

PeterO

Thanks. Very thorough information but just a little bit too complicated to exactly understand what the implication is when the OTP bit for usb_boot_mode is set.

[maicod]

That's a syntactically invalid directive so will be 100% completely, totally ignored.

thats ok. I just thought it would work like that but apparently it does not

Once a OTP is set it can NEVER be reset, at all ever, for all of future time.

clear

All three of my Raspberry Pi 3Bs boot perfectly OK from an SDCard with that OTP set. The bigger problem was when I left a USB stick (with no bootcode.bin on it) in one of those machines - that resulted in a boot failure even though one of the USB devices was bootable.

After having set the OTP USB boot bit I could live with having to remove USB sticks if I wanted to boot from MicroSD but what concerns me is if it would take longer before booting from MicroSD starts, because maybe it tries to boot from USB sticks (not present) first. Did you notice a delay since you set the OTP USB boot bit or not ?

[DougieLawson]

The time from entering shutdown -r now to the new kernel running is less than eight seconds. I don't think there's any value in trying to save microseconds from that incredibly short timespan.

[maicod]

The time from entering shutdown -r now to the new kernel running is less than eight seconds. I don't think there's any value in trying to save microseconds from that incredibly short timespan.

you're right !