# Breaking away

(This page doesn't talk about the "break" word for loops. For this, you
want to read [doc/usage/flow].)

Infinite loops are tricky because, well, they never end! On a preemptive multi-
tasking system, the annoyance is minor as we can just kill the process being
stuck.

In Dusk OS, loops can only be broken through interrupts. To prevent against
accidental lock ups, you are probably interested in having some kind of "break
away" system.

The solution for breaking away in Dusk is to hook some kind of input to an
interrupt and have that interrupt return to "abort" instead of returning where
it's supposed to. Simple, elegant and can break away from all loops.

But not all input devices are compatible with this scheme! PS/2 keyboards are
fine, most UARTs are fine... but USB keyboards are not. Sure, USB controllers
do have interrupts to notify of transfer status change, but those transfers
always have to be initiated by the host controller. If your infinte loop doesn't
call on your USB polling method, then your USB keyboard cannot be asked whether
the "Break" key was pressed.

Therefore, on systems where the primary input is a USB keyboard, an alternate
input has to be set up for breaking away.

TODO: I just had an idea. Maybe that "unplugging the USB keyboard" could work.
On many USB controllers, this is going to generate an interrupt that is
independent from currently active transfers. If the keyboard is plugged directly
in a port, maybe that the logic "check if that unplugged device was the active
keyboard, if yes, break away" could be simple enough for being carried through
in an interrupt handler... But then, this would need to be coupled with a robust
system for being able to hot-plug the system's primary input through USB...

For example, on the Raspberry Pi, the proposed system [doc/hw/arm/rpi/break]
goes through GPIOs. You plug a push button to a GPIO pin with GND, hook an
interrupt handler to its falling edge event and break away in that event.
Therefore, one can "break away" by pushing that button. Different systems will
have different solutions to that problem.