port/rp2: Make tickless, remove 1ms timeout when idle. - #12837

The main motivation for doing this was to reduce the latency when the system is woken by a USB interrupt. The best_effort_wfe_or_timeout() function calls into the pico-sdk dynamic timer framework which sets up a new dynamic timer instance each time, and then has to tear it down before continuing after a WFE.

Testing Python interrupt latency it seems to improved by about 12us (from average of 46us to 34us running a Pin IRQ). C-based "scheduled nodes" should see even lower latency.

Test code was:

import machine
from machine import Pin

i = Pin(2, Pin.IN, Pin.PULL_UP)
o = Pin(3, Pin.OUT)

def handler(_):
    o(1)
    print('!')
    o(0)

i.irq(handler, Pin.IRQ_FALLING)

while True:
    machine.idle()

With latency measured using logic analyzer:

(Channel 0 is "i", Channel is "o". Please excuse noisy CH0, was triggering with a button. Time 0 is the first falling edge of channel 0.)

The risk with this change is that it exposes cases where a driver expects execution to continue without an interrupt. Alternative is to enable the standard ARM SysTick interrupt to wake the CPU at tick intervals. (There is a branch with SysTick enabled and working, but testing shows tickless is just as functional.)

This work was funded through GitHub Sponsors.