Re: [PATCH 3/3] PM: s2idle: Fully block the system from entering s2idle when cpuidle isn't supported

[Kazuki H]

Hi, apologies for the late response.

On Mon, Mar 27, 2023 at 07:43:02PM +0200, Rafael J. Wysocki wrote:
> On Thu, Mar 16, 2023 at 8:49 AM Kazuki H <kazukih0205@xxxxxxxxx> wrote:
> >
> > s2idle isn't supported on platforms that don't support cpuidle as of
> > 31a3409065d1 ("cpuidle / sleep: Do sanity checks in cpuidle_enter_freeze()
> > too").
> 
> This simply isn't correct.
> 
> As of the above commit, s2idle was still supported without cpuidle as
> long as arch_cpu_idle() worked.
Sorry I linked the wrong commit, ef2b22ac540c (cpuidle / sleep: Use
broadcast timer for states that stop local timer) is the correct one.

This adds this block of code

if (cpuidle_not_available(drv, dev))
	goto use_default;

which prevents this code, critical for s2idle, from executing

if (idle_should_freeze()) {
	entered_state = cpuidle_enter_freeze(drv, dev);
	if (entered_state >= 0) {
		local_irq_enable();
		goto exit_idle;
	}
	reflect = false;
	next_state = cpuidle_find_deepest_state(drv, dev);
} else {
	reflect = true;
	/*
	 * Ask the cpuidle framework to choose a convenient idle state.
	 */
	next_state = cpuidle_select(drv, dev);
}
> > There is a check in the cpuidle subsystem which would prevent the
> > system from entering s2idle. However, there is nothing in the suspend
> > framework which prevents this, which can cause the suspend subsystem to
> > think that the machine is entering s2idle while the cpuidle subsystem is
> > not, which can completely break the system.
> >
> > Block the machine from entering s2idle when cpuidle isn't supported in
> > the suspend subsystem as well.
> 
> First, please explain why the cpuidle_not_available() check in
> cpuidle_idle_call() is not sufficient to avoid the breakage.
cpuidle_idle_call() doesn't notify the PM subsystem that the host
doesn't support s2idle. We need a seperate check here, otherwise the PM
subsystem will happily continue entering s2idle without the other
subsystem's knowledge.
> 
> > Link: https://lore.kernel.org/all/20230204152747.drte4uitljzngdt6@kazuki-mac
> > Fixes: 31a3409065d1 ("cpuidle / sleep: Do sanity checks in cpuidle_enter_freeze() too")
> > Signed-off-by: Kazuki H <kazukih0205@xxxxxxxxx>
> > ---
> >  kernel/power/main.c    | 12 +++++++++---
> >  kernel/power/suspend.c |  5 +++++
> >  2 files changed, 14 insertions(+), 3 deletions(-)
> >
> > diff --git a/kernel/power/main.c b/kernel/power/main.c
> > index 31ec4a9b9d70..b14765447989 100644
> > --- a/kernel/power/main.c
> > +++ b/kernel/power/main.c
> > @@ -133,6 +133,8 @@ static ssize_t mem_sleep_show(struct kobject *kobj, struct kobj_attribute *attr,
> >         for (i = PM_SUSPEND_MIN; i < PM_SUSPEND_MAX; i++) {
> >                 if (i >= PM_SUSPEND_MEM && cxl_mem_active())
> >                         continue;
> > +               if (i == PM_SUSPEND_TO_IDLE && cpuidle_not_available())
> > +                       continue;
> 
> Not really.
> 
> >                 if (mem_sleep_states[i]) {
> >                         const char *label = mem_sleep_states[i];
> >
> > @@ -185,11 +187,15 @@ static ssize_t mem_sleep_store(struct kobject *kobj, struct kobj_attribute *attr
> >         }
> >
> >         state = decode_suspend_state(buf, n);
> > -       if (state < PM_SUSPEND_MAX && state > PM_SUSPEND_ON)
> > +       if (state == PM_SUSPEND_TO_IDLE && cpuidle_not_available())
> 
> And same here.
> 
> > +               goto einval;
> > +       if (state < PM_SUSPEND_MAX && state > PM_SUSPEND_ON) {
> >                 mem_sleep_current = state;
> > -       else
> > -               error = -EINVAL;
> > +               goto out;
> > +       }
> >
> > + einval:
> > +       error = -EINVAL;
> >   out:
> >         pm_autosleep_unlock();
> >         return error ? error : n;
> > diff --git a/kernel/power/suspend.c b/kernel/power/suspend.c
> > index 3f436282547c..55ddf525aaaf 100644
> > --- a/kernel/power/suspend.c
> > +++ b/kernel/power/suspend.c
> > @@ -556,6 +556,11 @@ static int enter_state(suspend_state_t state)
> >
> >         trace_suspend_resume(TPS("suspend_enter"), state, true);
> >         if (state == PM_SUSPEND_TO_IDLE) {
> > +               if (cpuidle_not_available()) {
> > +                       pr_warn("s2idle is unsupported when cpuidle is unavailable");
> > +                       return -EINVAL;
> > +               }
> > +
> >  #ifdef CONFIG_PM_DEBUG
> >                 if (pm_test_level != TEST_NONE && pm_test_level <= TEST_CPUS) {
> >                         pr_warn("Unsupported test mode for suspend to idle, please choose none/freezer/devices/platform.\n");
> > --
> 
> Again, I need to understand what the problem is in the first place.
Thanks,
Kazuki