linux-kernel - Re: [PATCH v14 20/39] pwm: tegra: Add runtime PM and OPP support

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Date:   Fri, 29 Oct 2021 20:06:43 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Dmitry Osipenko <digetx@...il.com>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Thierry Reding <thierry.reding@...il.com>,
        Jonathan Hunter <jonathanh@...dia.com>,
        Ulf Hansson <ulf.hansson@...aro.org>,
        Viresh Kumar <vireshk@...nel.org>,
        Stephen Boyd <sboyd@...nel.org>,
        Peter De Schrijver <pdeschrijver@...dia.com>,
        Mikko Perttunen <mperttunen@...dia.com>,
        Lee Jones <lee.jones@...aro.org>,
        Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>, Nishanth Menon <nm@...com>,
        Adrian Hunter <adrian.hunter@...el.com>,
        Michael Turquette <mturquette@...libre.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-tegra <linux-tegra@...r.kernel.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Linux PWM List <linux-pwm@...r.kernel.org>,
        linux-mmc <linux-mmc@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        linux-clk <linux-clk@...r.kernel.org>,
        David Heidelberg <david@...t.cz>
Subject: Re: [PATCH v14 20/39] pwm: tegra: Add runtime PM and OPP support

On Fri, Oct 29, 2021 at 6:29 PM Dmitry Osipenko <digetx@...il.com> wrote:
>
> 29.10.2021 18:56, Rafael J. Wysocki пишет:
> > On Fri, Oct 29, 2021 at 5:20 PM Dmitry Osipenko <digetx@...il.com> wrote:
> >>
> >> 26.10.2021 01:40, Dmitry Osipenko пишет:
> >>> +     ret = devm_pm_runtime_enable(&pdev->dev);
> >>> +     if (ret)
> >>> +             return ret;
> >>> +
> >>> +     ret = devm_tegra_core_dev_init_opp_table_common(&pdev->dev);
> >>> +     if (ret)
> >>> +             return ret;
> >>> +
> >>> +     ret = pm_runtime_resume_and_get(&pdev->dev);
> >>> +     if (ret)
> >>> +             return ret;
> >>> +
> >>>       /* Set maximum frequency of the IP */
> >>> -     ret = clk_set_rate(pwm->clk, pwm->soc->max_frequency);
> >>> +     ret = dev_pm_opp_set_rate(pwm->dev, pwm->soc->max_frequency);
> >>>       if (ret < 0) {
> >>>               dev_err(&pdev->dev, "Failed to set max frequency: %d\n", ret);
> >>> -             return ret;
> >>> +             goto put_pm;
> >>>       }
> >>>
> >>>       /*
> >>> @@ -278,7 +294,7 @@ static int tegra_pwm_probe(struct platform_device *pdev)
> >>>       if (IS_ERR(pwm->rst)) {
> >>>               ret = PTR_ERR(pwm->rst);
> >>>               dev_err(&pdev->dev, "Reset control is not found: %d\n", ret);
> >>> -             return ret;
> >>> +             goto put_pm;
> >>>       }
> >>>
> >>>       reset_control_deassert(pwm->rst);
> >>> @@ -291,10 +307,15 @@ static int tegra_pwm_probe(struct platform_device *pdev)
> >>>       if (ret < 0) {
> >>>               dev_err(&pdev->dev, "pwmchip_add() failed: %d\n", ret);
> >>>               reset_control_assert(pwm->rst);
> >>> -             return ret;
> >>> +             goto put_pm;
> >>>       }
> >>>
> >>> +     pm_runtime_put(&pdev->dev);
> >>> +
> >>>       return 0;
> >>> +put_pm:
> >>> +     pm_runtime_put_sync_suspend(&pdev->dev);
> >>> +     return ret;
> >>>  }
> >>>
> >>>  static int tegra_pwm_remove(struct platform_device *pdev)
> >>> @@ -305,20 +326,44 @@ static int tegra_pwm_remove(struct platform_device *pdev)
> >>>
> >>>       reset_control_assert(pc->rst);
> >>>
> >>> +     pm_runtime_force_suspend(&pdev->dev);
> >>
> >> I just noticed that RPM core doesn't reset RPM-enable count of a device
> >> on driver's unbind (pm_runtime_reinit). It was a bad idea to use
> >> devm_pm_runtime_enable() + pm_runtime_force_suspend() here, since RPM is
> >> disabled twice on driver's removal, and thus, RPM will never be enabled
> >> again.
> >>
> >> I'll fix it for PWM and other drivers in this series, in v15.
> >
> > Well, for the record, IMV using pm_runtime_force_suspend() is
> > generally a questionable idea.
> >
>
> Please clarify why it's a questionable idea.

There are a few reasons.

Generally speaking, it makes assumptions that may not be satisfied.

For instance, it assumes that the driver will never have to work with
the ACPI PM domain, because the ACPI PM domain has a separate set of
callbacks for system-wide suspend and resume and they are not the same
as its PM-runtime callbacks, so if the driver is combined with the
ACPI PM domain, running pm_runtime_force_suspend() may not work as
expected.

Next, it assumes that PM-runtime is actually enabled for the device
and the RPM_STATUS of it is valid when it is running.

Further, it assumes that the PM-runtime suspend callback of the driver
will always be suitable for system-wide suspend which may not be the
case if the device can generate wakeup signals and it is not allowed
to wake up the system from sleep by user space.

Next, if the driver has to work with a PM domain (other than the ACPI
one) or bus type that doesn't take the pm_runtime_force_suspend()
explicitly into account, it may end up running the runtime-suspend
callback provided by that entity from within its system-wide suspend
callback which may not work as expected.

I guess I could add a few if I had to.