adb on a pc only works when the adb daemon is running on the phone. It must be able to talk to that.
I've not seen phones respond to it in the bootloader personally, only when running the system with USB debugging on - and all that does is run adbd - the daemon.
I've not seen phones respond to it in the bootloader personally, only when running the system with USB debugging on - and all that does is run adbd - the daemon.