linux-kernel - Re: [PATCH 2/2] PM: sleep: Fix runtime PM based cpuidle support

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Message-ID: <CAJZ5v0i5qQ7bXoG3cK2LESibKRb3sahmaC-M2_mgwqSSS6Sz1Q@mail.gmail.com>
Date:   Fri, 22 Oct 2021 14:02:26 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Ulf Hansson <ulf.hansson@...aro.org>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Maulik Shah <mkshah@...eaurora.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Len Brown <len.brown@...el.com>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] PM: sleep: Fix runtime PM based cpuidle support

On Fri, Oct 22, 2021 at 12:18 PM Ulf Hansson <ulf.hansson@...aro.org> wrote:
>
> On Thu, 21 Oct 2021 at 21:56, Ulf Hansson <ulf.hansson@...aro.org> wrote:
> >
> > On Thu, 21 Oct 2021 at 21:02, Rafael J. Wysocki <rafael@...nel.org> wrote:
> > >
> > > On Thu, Oct 21, 2021 at 8:12 PM Ulf Hansson <ulf.hansson@...aro.org> wrote:
> > > >
> > > > On Thu, 21 Oct 2021 at 18:33, Rafael J. Wysocki <rafael@...nel.org> wrote:
> > > > >
> > > > > On Thu, Oct 21, 2021 at 6:17 PM Ulf Hansson <ulf.hansson@...aro.org> wrote:
> > >
> > > [cut]
> > >
> > > > > So in theory you could check the pm_runtime_put_sync_suspend() return
> > > > > value and fall back to something like WFI if that's an error code.
> > > >
> > > > I have already tried that, but it simply got too complicated. The main
> > > > issue was that runtime PM could become disabled for the device in the
> > > > middle of executing the ->enter() callback.
> > >
> > > So IIUC the problem is that you cannot resume after suspending in that case.
> > >
> > > IOW, you need to guarantee that if the suspend is successful, the
> > > resume also will take place, but if the suspend fails, you basically
> > > don't care.
> >
> > Exactly.
> >
> > >
> > > > For example, if pm_runtime_get_sync() fails, I still need to make sure
> > > > the reference counting in genpd becomes correct - and I can't do that
> > > > using dev_pm_genpd_resume(). That's because it's not designed to be
> > > > called in this "unknown" suspend phase, but should be called after the
> > > > noirq phase and be properly balanced with dev_pm_genpd_suspend().
> > > >
> > > > In other words, the error path didn't work out for me.
> > >
> > > It should be sufficient to call wake_up_all_idle_cpus() in the suspend
> > > path before dpm_suspend_late(), because system suspend acquires a
> > > PM-runtime reference on every device.  IOW, it won't let any devices
> > > runtime-suspend, so if your power domain devices are resumed in that
> > > path, they will never suspend again in it and the
> > > pm_runtime_put_sync_suspend() in __psci_enter_domain_idle_state()
> > > becomes a reference counter management call which works regardless of
> > > whether or not PM runtime is disabled.
> >
> > That sounds like a great idea, this should work too! Then the question
> > is, how to make that call to wake_up_all_idle_cpus() to become
> > optional - or only invoked for the cpuidle drivers that need it.

It need not be optional.

For suspend-to-idle it doesn't matter, because all CPUs will be woken
up from idle shortly anyway.

For other suspend variants this doesn't matter, because all secondary
CPUs will be taken offline shortly and the primary CPU will call into
the platform "sleep" handler.

> >
> > In any case, I will try this out, thanks for the suggestion!
>
> I now recall that I have already tried this, but unfortunately it doesn't work.
>
> The problem is that the dev->power.syscore flag is set for the device,
> which makes device_prepare() to bail out early and skip calling
> pm_runtime_get_noresume().

This needs to be fixed, then.