linux-kernel - Re: [PATCH] cpuidle: Consolidate disabled state checks

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Message-ID: <CAJvTdKn9wuoXkKecZxCJHPZAG7XK_BqAZT5=7k9Mi4zo4SBL0g@mail.gmail.com>
Date:   Sun, 17 Nov 2019 23:45:53 -0500
From:   Len Brown <lenb@...nel.org>
To:     "Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:     Linux PM <linux-pm@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Doug Smythies <dsmythies@...us.net>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] cpuidle: Consolidate disabled state checks

On Mon, Nov 4, 2019 at 6:16 AM Rafael J. Wysocki <rjw@...ysocki.net> wrote:
>
> From: Rafael J. Wysocki <rafael.j.wysocki@...el.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.

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.

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?

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.

However, the ARM drivers, at least, seem to want to reserve the right
to set and clear the drv->state.disabled,
and to have them continue to have that right, we have to continue
checking that field at run-time.
And giving drivers the opportunity to do that disabling driver-wide,
instead of per-cpu (usage) wide,
seems to be something we may want to keep.

-Len

-- 
Len Brown, Intel Open Source Technology Center