Debugging ARM kernels using fast interrupts

Not really.

fiq_debugger deliberately does not implement commands that could be used to hijack the phone and, within kgdb/kdb, the purpose of the reduced capability patchset is to achieve the same thing by disabling commands that could be to hijack the phone.

For example, arbitrary memory reads can be banned to prevent fishing for cryptographic keys. Similarly breakpoints and single stepping can also be banned, which is an interesting case. Breakpointing might initially appear to be "passive observation" it can, in fact, be used to single step decryption routines such that the control flow might reveal the key.

> I wonder why they did not make the UART listen to the touchscreen
> instead. Just swipe with the right fingerprint and you are set.

Managing the touchscreen requires too much of the phone to be working correctly. The purpose of both fiq_debugger and kgdb/kdb+FIQ is to provide debug mechanisms that still function when significant portions of the kernel are not working correctly. For this reason they need to use very simple hardware.