Re: [PATCH v2 0/3] Fix NXP STM clockevent bugs causing lockups

[Sebastian Andrzej Siewior]

On 2026-11-02 22:16:19 [+0100], Enric Balletbo i Serra wrote:
> This series fixes three bugs in the NXP STM clocksource driver that can
> cause soft lockups and missed timer events, particularly visible on
> PREEMPT_RT kernels running on S32G platforms.
> 
> The NXP STM generates an interrupt only on exact match between the
> counter and comparator registers. If the counter passes the programmed
> value, the match is missed and the hardware must count through all 2^32
> ticks (~32.2s at 133 MHz) before firing, causing soft lockups.
> 
> Patch 1 fixes max_delta: on 64-bit platforms, ULONG_MAX exceeds the
> hardware's 32-bit compare register width, making it possible to program
> an unreachable compare value.

This looks fine

> Patch 2 fixes the overshoot detection in set_next_event: the existing
> check only detected counter wraps, not the normal case where the counter
> advances past the programmed value during the read-modify-write window.
> Replace it with a signed comparison following the pattern used by
> timer-imx-gpt.

okay…

> Patch 3 increases min_delta from 2us to 20us: under bus contention on
> PREEMPT_RT, 2us is not enough margin and the counter frequently
> overshoots the programmed value, triggering repeated -ETIME retries.

This all sounds like the hpet trouble on x86. I'm not sure what heavy
busy contention means and why the device is enabled/ disable during
setting. I would assume that the programmed value hits the HW after the
wrap so you wait until the counter wraps again for the equal match.
Worst case would be you read back the expected value but the HW sees it
later (after the wrap).
hpet_clkevt_set_next_event() does in principle the same check as you in
#2 except it compares against the "min" value instead <=0.
Would this make sense?

Sebastian