linux-kernel - AW: AW: [PATCH 1/4] clocksource/drivers/timer-rtl-otto: work around dying timers

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Message-ID: <020f01dc227f$03e40b60$0bac2220$@gmx.de>
Date: Wed, 10 Sep 2025 20:16:22 +0200
From: <markus.stockhausen@....de>
To: "'Daniel Lezcano'" <daniel.lezcano@...aro.org>,
	<tglx@...utronix.de>
Cc: <howels@...thatwemight.be>,
	<bjorn@...k.no>,
	<linux-kernel@...r.kernel.org>
Subject: AW: AW: [PATCH 1/4] clocksource/drivers/timer-rtl-otto: work around dying timers

> Von: Daniel Lezcano <daniel.lezcano@...aro.org> 
> Gesendet: Mittwoch, 10. September 2025 18:39
>
> > What I tried:
> > 
> > 1. Read out the current (remaining) timer value: In the error cases
> > this can give any value between 1 (=320ns) and 15 (=4800ns).
> > 
> > 2. Check if IRQ flag is already set and IRQ might trigger next. This was
> > never the case.
>
> It would have been interesting to check if we are in the time bug range 
> to wait with a delay (5us), check the IRQ flag as the current timer 
> should have expired, then set the counter and recheck the IRQ flag.

It's been 2 months that I dived deep into this case. Finding a 
reproducer, adding lightweight logging and try&error a solution 
was really hard. In the end I was happy to have a fix that was 
intensively tested.

For some notes see
https://github.com/openwrt/openwrt/pull/19468#issuecomment-3095570297

From what I remember:

- I started on a multithreading SoC and went over to a single
core SoC to reduce side effects during analysis. 

- The timer never died when it was reprogrammed from
an interrupt of a just finished timer. The reason was always
a reprogramming from outside the interrupt->reprogram
call sequence.

- Reprogramming always worked fine. A timer with <5us left, was 
restarted with a timer >5us. The new timer started to count.
No interrupt flag seemed to be magically toggled during this 
process. There was no active IRQ notification directly after the
reprogramming. That was how I expected it.

- But in rare cases the new timer did not trigger the subsequent
interrupt. I was totally confused that the future interrupt of 
a newly started timer did not work.

Graphically:

- timer run ---+-------------------->|
               | issue stop & start 
               | timer run ------------------>|
                                              | no IRQ here

Conclusion was for me: If we "kill" a running timer and restart 
it and it will not fire an interrupt after the newly set time, 
then something must be somehow broken. The ending timer and 
the stop/start sequence (that consists of two register writes) 
have some interference. Whatever it might be.

Markus