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

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Date:   Fri, 22 Oct 2021 14:56:23 +0200
From:   Ulf Hansson <ulf.hansson@...aro.org>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     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, 22 Oct 2021 at 14:02, Rafael J. Wysocki <rafael@...nel.org> wrote:
>
> 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.

So bumping the usage count even if the dev->power.syscore is set,
should be fine? (And of course dropping it in the complete phase).

I can work with that, let me try!

Kind regards
Uffe