Re: Question on expiring HRtimer in-kernel

[Linus Walleij]

Sorry for top posting, the helpful autocomplete in gmail think Rohm Europe
is better to address than linux-pm@vger...

On Tue, Mar 1, 2022 at 1:19 PM Linus Walleij <linus.walleij@xxxxxxxxxx> wrote:
>
> I have a problem with a premature expiring HRtimer.
>
> The HRtimer hrtimer_set_expires_range() is used in two places in
> the upstream kernel:
> kernel/futex/core.c
> drivers/power/supply/ab8500_chargalg.c
>
> Now I am testing the code in the latter, and it has seen some
> bitrot since merged in 2012. Maybe it was correct at one point.
> The timer is started like this:
>
>     hrtimer_init(&di->safety_timer, CLOCK_REALTIME, HRTIMER_MODE_ABS);
> (...)
>     hrtimer_set_expires_range(&di->safety_timer,
>         ktime_set(timer_expiration * ONE_HOUR_IN_SECONDS, 0),
>         ktime_set(FIVE_MINUTES_IN_SECONDS, 0));
>     hrtimer_start_expires(&di->safety_timer, HRTIMER_MODE_REL);
>
> What the author wanted to achieve is a very definitive callback in one
> hour relative to now +/- 5 min, and that is one hour later in the
> physical world,
> as this deals with battery charging.
>
> However sometimes this fires almost immediately rather than in an hour.
>
> My first thought is to pass HRTIMER_MODE_REL also to init as
> hrtimer_set_expires_range() could make things happen immediately
> if we have ABS set, but this is all just intuitive.
>
> Any hints? Better ways to create a definitive event in one hour?
>
> Yours,
> Linus Walleij