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From:   "Rafael J. Wysocki" <rafael@kernel.org>
Date:   Thu, 21 Apr 2022 20:38:32 +0200
Message-ID: <CAJZ5v0hFstH6goScEhUmZwBQ4gEnt5-tcp=UKgrW8Xb5zdZ_zg@mail.gmail.com>
Subject: Re: [PATCH v3 4/6] cpufreq: amd-pstate: Allow replacing acpi-cpufreq
 when loaded
To:     "Limonciello, Mario" <Mario.Limonciello@amd.com>
Cc:     "Fontenot, Nathan" <Nathan.Fontenot@amd.com>,
        "Huang, Ray" <Ray.Huang@amd.com>,
        "Rafael J . Wysocki" <rafael@kernel.org>,
        Viresh Kumar <viresh.kumar@linaro.org>,
        "open list:AMD PSTATE DRIVER" <linux-pm@vger.kernel.org>,
        "Yuan, Perry" <Perry.Yuan@amd.com>,
        open list <linux-kernel@vger.kernel.org>
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On Thu, Apr 14, 2022 at 7:58 PM Limonciello, Mario
<Mario.Limonciello@amd.com> wrote:
>
> [Public]
>
>
>
> > -----Original Message-----
> > From: Fontenot, Nathan <Nathan.Fontenot@amd.com>
> > Sent: Thursday, April 14, 2022 12:33
> > To: Limonciello, Mario <Mario.Limonciello@amd.com>; Huang, Ray
> > <Ray.Huang@amd.com>; Rafael J . Wysocki <rafael@kernel.org>; Viresh
> > Kumar <viresh.kumar@linaro.org>
> > Cc: open list:AMD PSTATE DRIVER <linux-pm@vger.kernel.org>; Yuan, Perry
> > <Perry.Yuan@amd.com>; open list <linux-kernel@vger.kernel.org>
> > Subject: Re: [PATCH v3 4/6] cpufreq: amd-pstate: Allow replacing acpi-
> > cpufreq when loaded
> >
> > On 4/14/22 11:47, Mario Limonciello wrote:
> > > `amd-pstate` can be compiled as a module.  This however can be a
> > > deficiency because `acpi-cpufreq` will be loaded earlier when compiled
> > > into the kernel meaning `amd-pstate` doesn't get a chance.
> > > `acpi-cpufreq` is also unable to be unloaded in this circumstance.
> > >
> > > To better improve the usability of `amd-pstate` when compiled as a
> > module,
> > > add an optional module parameter that will force it to replace other
> > > cpufreq drivers at startup.
> > >
> > > Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
> > > ---
> > > v2->v3:
> > >  * Rebase on earlier patches
> > >  * Use IS_REACHABLE
> > >  * Only add replace parameter if acpu-cpufreq is enabled
> > >  * Only show info message once
> > > v1->v2:
> > >  * Update to changes from v1.
> > >  * Verify the driver being matched is acpi-cpufreq.
> > >  * Show a message letting users know they can use amd-pstate.
> > >
> > >  drivers/cpufreq/amd-pstate.c | 22 ++++++++++++++++++++--
> > >  1 file changed, 20 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/drivers/cpufreq/amd-pstate.c b/drivers/cpufreq/amd-pstate.c
> > > index d323f3e3888c..8ae65a2072d6 100644
> > > --- a/drivers/cpufreq/amd-pstate.c
> > > +++ b/drivers/cpufreq/amd-pstate.c
> > > @@ -63,6 +63,13 @@ module_param(shared_mem, bool, 0444);
> > >  MODULE_PARM_DESC(shared_mem,
> > >              "enable amd-pstate on processors with shared memory
> > solution (false = disabled (default), true = enabled)");
> > >
> > > +#if defined(CONFIG_X86_ACPI_CPUFREQ) ||
> > defined(CONFIG_X86_ACPI_CPUFREQ_MODULE)
> > > +static bool replace = false;
> > > +module_param(replace, bool, 0444);
> > > +MODULE_PARM_DESC(replace,
> > > +             "replace acpi-cpufreq driver upon init if necessary");
> > > +#endif
> > > +
> > >  static struct cpufreq_driver amd_pstate_driver;
> > >
> > >  /**
> > > @@ -643,6 +650,7 @@ static struct cpufreq_driver amd_pstate_driver = {
> > >
> > >  static int __init amd_pstate_init(void)
> > >  {
> > > +   const char *current_driver;
> > >     int ret;
> > >
> > >     if (boot_cpu_data.x86_vendor != X86_VENDOR_AMD)
> > > @@ -666,9 +674,19 @@ static int __init amd_pstate_init(void)
> > >             return -ENODEV;
> > >     }
> > >
> > > -   /* don't keep reloading if cpufreq_driver exists */
> > > -   if (cpufreq_get_current_driver())
> > > +   current_driver = cpufreq_get_current_driver();
> > > +   if (current_driver) {
> > > +#if IS_REACHABLE(CONFIG_X86_ACPI_CPUFREQ)
> > > +           if (replace && strcmp(current_driver, "acpi-cpufreq") == 0) {
> > > +                   acpi_cpufreq_exit();
> > > +           } else {
> > > +                   pr_info_once("A processor on this system supports
> > amd-pstate, you can enable it with amd_pstate.replace=1\n");
> > > +                   return -EEXIST;
> > > +           }
> > > +#else
> > >             return -EEXIST;
> > > +#endif
> > > +   }
> >
> > A couple of thoughts. First, should this also provide a path to restore the
> > acpi_cpufreq driver
> > if the amd-pstate driver fails during init some time after calling
> > acpi_cpufreq_exit()?
>
> I think that's a reasonable idea; it would involve exporting acpi_cpufreq_init
> as well.
>
> >
> > Which leads me to wonder, should there be a more generic
> > cpufreq_replace_driver() routine that
> > could handle this?
>
> If changing the API for this, my proposal would be that there is a flag used
> in cpufreq_driver->flags to indicate that this driver should replace existing
> drivers when calling cpufreq_register_driver rather than a new routine.
> Then if it fails to register for any reason then the old driver can be restored.
>
> Rafael, what are your thoughts on this?

IMV there need to be two things to make this really work.

First, the currently running driver needs to provide a way to tell it
to go away.  For example, intel_pstate has the "off" mode (in which it
doesn't do anything) for that and similar interfaces can be added to
other drivers as needed.

The reason why is because, for example, intel_pstate cannot go into
the "off" mode when HWP is enabled, because it cannot be disabled and
running acpi_cpufreq in that configuration wouldn't work.  So in
general you need to know that it is OK to unregister the current
driver.

Second, there needs to be a mechanism for registering a driver
"weakly" for future use, so if it cannot be used right away, it will
be added to a list and wait until there's room for it to run.