8000 port/rp2: Make tickless, remove 1ms timeout when idle. by projectgus · Pull Request #12837 · micropython/micropython · GitHub
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port/rp2: Make tickless, remove 1ms timeout when idle. #12837

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merged 1 commit into from
Nov 3, 2023
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port/rp2: Make tickless, remove 1ms timeout when idle. #12837

merged 1 commit into from
Nov 3, 2023

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@projectgus
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@projectgus projectgus commented Oct 31, 2023 •
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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:

LA Capture of the above in pulseview

(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.

@dpgeorge
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dpgeorge commented Oct 31, 2023

Thanks. I think we should make this change and then if there are any cases of missed events, add SEV in the appropriate place.

@projectgus projectgus mentioned this pull request Nov 1, 2023
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@projectgus @dpgeorge
rp2: Remove 1ms timeout to make idle waiting tickless.
8c432ea 
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 be improved by about 12us
(from average of 46us to 34us running a Pin IRQ).  C-based "scheduled
nodes" should see even lower latency.

This work was funded through GitHub Sponsors.

Signed-off-by: Angus Gratton <angus@redyak.com.au>
@dpgeorge dpgeorge force-pushed the feature/rp2_wfe_tickless branch from f5e3250 to 8c432ea Compare November 3, 2023 07:31
@dpgeorge dpgeorge merged commit 8c432ea into micropython:master Nov 3, 2023
@projectgus projectgus mentioned this pull request Nov 9, 2023
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@projectgus projectgus mentioned this pull request Nov 30, 2023
Merged
@projectgus projectgus deleted the feature/rp2_wfe_tickless branch November 1, 2024 05:12
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@projectgus @dpgeorge
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