linux-kernel - Re: [PATCH v2 2/2] cpufreq: Specify default governor on command line

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Date:   Thu, 25 Jun 2020 14:49:53 +0100
From:   Quentin Perret <qperret@...gle.com>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     Viresh Kumar <viresh.kumar@...aro.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Arnd Bergmann <arnd@...db.de>,
        Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        "Cc: Android Kernel" <kernel-team@...roid.com>,
        Todd Kjos <tkjos@...gle.com>, adharmap@...eaurora.org
Subject: Re: [PATCH v2 2/2] cpufreq: Specify default governor on command line

On Thursday 25 Jun 2020 at 15:28:43 (+0200), Rafael J. Wysocki wrote:
> On Thu, Jun 25, 2020 at 1:53 PM Quentin Perret <qperret@...gle.com> wrote:
> >
> > On Thursday 25 Jun 2020 at 13:44:34 (+0200), Rafael J. Wysocki wrote:
> > > On Thu, Jun 25, 2020 at 1:36 PM Viresh Kumar <viresh.kumar@...aro.org> wrote:
> > > > This change is not right IMO. This part handles the set-policy case,
> > > > where there are no governors. Right now this code, for some reasons
> > > > unknown to me, forcefully uses the default governor set to indicate
> > > > the policy, which is not a great idea in my opinion TBH. This doesn't
> > > > and shouldn't care about governor modules and should only be looking
> > > > at strings instead of governor pointer.
> > >
> > > Sounds right.
> > >
> > > > Rafael, I even think we should remove this code completely and just
> > > > rely on what the driver has sent to us. Using the selected governor
> > > > for set policy drivers is very confusing and also we shouldn't be
> > > > forced to compiling any governor for the set-policy case.
> > >
> > > Well, AFAICS the idea was to use the default governor as a kind of
> > > default policy proxy, but I agree that strings should be sufficient
> > > for that.
> >
> > I agree with all the above. I'd much rather not rely on the default
> > governor name to populate the default policy, too, so +1 from me.
> 
> So before this series the default governor was selected at the kernel
> configuration time (pre-build) and was always built-in.  Because it
> could not go away, its name could be used to indicate the default
> policy for the "setpolicy" drivers.
> 
> After this series, however, it cannot be used this way reliably, but
> you can still pass cpufreq_param_governor to cpufreq_parse_policy()
> instead of def_gov->name in cpufreq_init_policy(), can't you?

Good point. I also need to fallback to the default builtin governor if
the command line parameter isn't valid (or non-existent), so perhaps
something like so?

iff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
index dad6b85f4c89..20a2020abf88 100644
--- a/drivers/cpufreq/cpufreq.c
+++ b/drivers/cpufreq/cpufreq.c
@@ -653,6 +653,23 @@ static unsigned int cpufreq_parse_policy(char *str_governor)
        return CPUFREQ_POLICY_UNKNOWN;
 }
 
+static unsigned int cpufreq_default_policy(void)
+{
+       unsigned int pol;
+
+       pol = cpufreq_parse_policy(cpufreq_param_governor);
+       if (pol != CPUFREQ_POLICY_UNKNOWN)
+               return pol;
+
+       if (IS_BUILTIN(CONFIG_CPU_FREQ_DEFAULT_GOV_PERFORMANCE))
+               return CPUFREQ_POLICY_PERFORMANCE;
+
+       if (IS_BUILTIN(CONFIG_CPU_FREQ_DEFAULT_GOV_POWERSAVE))
+               return CPUFREQ_POLICY_POWERSAVE;
+
+       return CPUFREQ_POLICY_UNKNOWN;
+}
+
 /**
  * cpufreq_parse_governor - parse a governor string only for has_target()
  * @str_governor: Governor name.
@@ -1085,8 +1102,8 @@ static int cpufreq_init_policy(struct cpufreq_policy *policy)
                /* Use the default policy if there is no last_policy. */
                if (policy->last_policy) {
                        pol = policy->last_policy;
-               } else if (default_governor) {
-                       pol = cpufreq_parse_policy(default_governor->name);
+               } else {
+                       pol = cpufreq_default_policy();
                        /*
                         * In case the default governor is neiter "performance"
                         * nor "powersave", fall back to the initial policy