Add overridable wait hook

[geky]

From ARMmbed#10693

Changed to fit into ARMmbed#10104.

I understand hal_sleep exists, and it's useful for the target, but it can't be used to catch sleep calls by the application (similar to mbed_target_override_serial vs mbed_override_serial).

While we're on the topic of naming, I went with mbed_override_sleep_hook to match the above serial names, but am open to other suggestions.

This is a follow from ARMmbed#10693

[kjbracey]

For your use case, wouldn't you actually want to replace the hal sleep? (Like the override_console replaces the target_override_console). The replacement would of course be free to pass the call onto the hal/target, conditionally.

For example, if you do some of your work, decide it's now finished so you need to signal a thread, you would not want to actually go into sleep, you'd want to leave idle immediately.

Also, unfortunately, sleep is entered with interrupts disabled in a normal idle situation - race-free checking of "do I need to sleep" requires this. So this approach isn't going to quite work - it's too low level.

But I think we can slot this into the ARMmbed#10104 "do_sleep" framework, into the "preparation" section which always has interrupts enabled, prior to making the actual IRQ off/sleep bit. That would also handle the "not idle any more" case, as it would precede the "sleep predicate" check.

[kjbracey]

One more thought - what sort of work are we talking about here? Very short-running, or long-running?

There's a fundamental choice to make - is this hook run

• a) as part of the idle thread while the kernel is active, so it can be pre-empted by high-priority threads, or

• b) while the kernel is suspended - in effect making it the highest-priority thing in the system below interrupts, until it chooses to yield.

b) is a bit scary - it's a priority inversion of the idle thread. Would be fine for something very short-running, but if you're intending to do real work, it seems like you need a). Which is effectively the "idle hook" we already have, except we don't have a nice chaining mechanism to let you move on to standard sleep.