linux-kernel - Re: [PATCH] acpi-cpufreq: Skip initialization if a cpufreq driver exists

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Date:   Mon, 7 Jun 2021 13:02:46 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Takashi Iwai <tiwai@...e.de>, Kyle Meyer <kyle.meyer@....com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux PM <linux-pm@...r.kernel.org>, linux-acpi@...r.kernel.org
Subject: Re: [PATCH] acpi-cpufreq: Skip initialization if a cpufreq driver exists

On Mon, Jun 7, 2021 at 9:26 AM Viresh Kumar <viresh.kumar@...aro.org> wrote:
>
> Hi Rafael,
>
> On Mon, May 24, 2021 at 7:47 PM Rafael J. Wysocki <rafael@...nel.org> wrote:
> > On Sat, May 22, 2021 at 12:19 AM Kyle Meyer <kyle.meyer@....com> wrote:
>
> > > diff --git a/drivers/cpufreq/acpi-cpufreq.c b/drivers/cpufreq/acpi-cpufreq.c
> > > index 7e7450453714..e79a945369d1 100644
> > > --- a/drivers/cpufreq/acpi-cpufreq.c
> > > +++ b/drivers/cpufreq/acpi-cpufreq.c
> > > @@ -1003,7 +1003,7 @@ static int __init acpi_cpufreq_init(void)
> > >
> > >         /* don't keep reloading if cpufreq_driver exists */
> > >         if (cpufreq_get_current_driver())
> > > -               return -EEXIST;
> > > +               return 0;
> > >
> > >         pr_debug("%s\n", __func__);
> > >
> > > --
> >
> > Applied as 5.14 material with some edits in the subject and changelog, thanks!
>
> I am not sure how this is supposed to work. If we return 0 from
> acpi_cpufreq_init(),
> then the driver will never be used, since it's acpi_cpufreq_init()
> will never get
> called again later.

Unless the module is unloaded and loaded again, that is.

> cpufreq drivers don't follow the generic device/driver model where a driver gets
> probed again if a device appears and so this is broken.

It is broken anyway as per the changelog of this patch.

On systems with several hundred logical CPUs this really can be troublesome.

> Please revert this patch.

Well, you can argue that the problem at hand is outside the kernel and
so it's not a kernel's business to address it.

After all, systemd-udevd could learn to avoid attempting to load the
module again if it fails with -EEXIST, but I'm not sure how different
that really would be from what this patch does, in practice.