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From:   "Rafael J. Wysocki" <rafael@kernel.org>
Date:   Mon, 18 Nov 2019 12:26:57 +0100
Message-ID: <CAJZ5v0jsQG37VF3-tiSndE0pXX9jEfgucm0UyvpM0bsyoOcpuA@mail.gmail.com>
Subject: Re: [PATCH] cpuidle: Consolidate disabled state checks
To:     "Rafael J. Wysocki" <rafael@kernel.org>
Cc:     Len Brown <lenb@kernel.org>,
        "Rafael J. Wysocki" <rjw@rjwysocki.net>,
        Linux PM <linux-pm@vger.kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Daniel Lezcano <daniel.lezcano@linaro.org>,
        Doug Smythies <dsmythies@telus.net>,
        LKML <linux-kernel@vger.kernel.org>
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On Mon, Nov 18, 2019 at 10:22 AM Rafael J. Wysocki <rafael@kernel.org> wrote:
>
> On Mon, Nov 18, 2019 at 5:46 AM Len Brown <lenb@kernel.org> wrote:
> >
> > On Mon, Nov 4, 2019 at 6:16 AM Rafael J. Wysocki <rjw@rjwysocki.net> wrote:
> > >
> > > From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> > >
> > > There are two reasons why CPU idle states may be disabled: either
> > > because the driver has disabled them or because they have been
> > > disabled by user space via sysfs.
> > >
> > > In the former case, the state's "disabled" flag is set once during
> > > the initialization of the driver and it is never cleared later (it
> > > is read-only effectively).
> >
> > for x86 (intel_idle and acpi_idle), no states with disabled=1 are  registered
> > with cpuidle.  Instead, intel_idle (currently) skips them in the loop
> > that registers states.
> > (and acpi_idle never touches the disabled field)
> >
> > And so for x86, governors checking for drv->states[i].disabled is a NOP,
> > and the condition described by CPUIDLE_STATE_DISABLED_BY_DRIVER
> > does not (yet) exist.
>
> OK
>
> > Looking at the ARM code, it seems that cpuidle-imx6q.c and cpuidle-tegra20.c
> > reach into the cpuidle states at run time and toggle the
> > drv->states[i].disabled.
>
> I might have overlooked that, let me check.
>
> > It seems that this patch takes the initial value of
> > drv->states->disabled, and sets the (per cpu)
> > usage.disable=..BY_DRIVER,
> > but that subsequent run-time toggles in drv->states[i]disabled by
> > these drivers would be missed,
> > because you're removed the run-time checking of drv->states->disabled?
>
> If it is updated at run time, then yes, the updates will be missed, so
> thanks for pointing that out.
>
> > Finally, I'd like to change intel_idle so that it *can* register a
> > state that is disabled, by default.
> > If I change the driver to NOT skip registering disabled states, and
> > the cpuidle copy has cpuidle_state.disabled=1,
> > then the state is indeed, unused at run-time.  But as you said,
> > it is effectively read-only, and is not indicated in sysfs, and can
> > not be changed via sysfs.
> >
> > One way to do this is to do what you do here and initialize
> > usage.disabled to drv->state.disabled. (not distinguishing between
> > DRIVER and USER)
> > That way the user could later over-ride what a driver set, by clearing
> > the disabled attribute.

I'd rather get rid of the "disabled" field from struct cpuidle_state
entirely and introduce a new state flag to indicate the "disabled by
default" status.

I also would expose that new flag in a new sysfs attribute of idle
states, say "disable_default".

Then, the DISABLED_BY_DRIVER bit would be reserved for driver quirks
(as per https://patchwork.kernel.org/patch/11249519/) and the
DISABLED_BY_USER one could be used for all of the other purposes.

Cheers,
Rafael