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    SubjectRe: [PATCH v2 2/2] cpufreq: Specify default governor on command line
    On Wed, Jun 24, 2020 at 7:50 AM Viresh Kumar <viresh.kumar@linaro.org> wrote:
    >
    > On 23-06-20, 15:21, Quentin Perret wrote:
    > > Currently, the only way to specify the default CPUfreq governor is via
    > > Kconfig options, which suits users who can build the kernel themselves
    > > perfectly.
    > >
    > > However, for those who use a distro-like kernel (such as Android, with
    > > the Generic Kernel Image project), the only way to use a different
    > > default is to boot to userspace, and to then switch using the sysfs
    > > interface. Being able to specify the default governor on the command
    > > line, like is the case for cpuidle, would enable those users to specify
    > > their governor of choice earlier on, and to simplify slighlty the
    > > userspace boot procedure.
    > >
    > > To support this use-case, add a kernel command line parameter enabling
    > > to specify a default governor for CPUfreq, which takes precedence over
    > > the builtin default.
    > >
    > > This implementation has one notable limitation: the default governor
    > > must be registered before the driver. This is solved for builtin
    > > governors and drivers using appropriate *_initcall() functions. And in
    > > the modular case, this must be reflected as a constraint on the module
    > > loading order.
    > >
    > > Signed-off-by: Quentin Perret <qperret@google.com>
    > > ---
    > > .../admin-guide/kernel-parameters.txt | 5 ++++
    > > Documentation/admin-guide/pm/cpufreq.rst | 6 ++---
    > > drivers/cpufreq/cpufreq.c | 23 +++++++++++++++----
    > > 3 files changed, 26 insertions(+), 8 deletions(-)
    > >
    > > diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
    > > index fb95fad81c79..5fd3c9f187eb 100644
    > > --- a/Documentation/admin-guide/kernel-parameters.txt
    > > +++ b/Documentation/admin-guide/kernel-parameters.txt
    > > @@ -703,6 +703,11 @@
    > > cpufreq.off=1 [CPU_FREQ]
    > > disable the cpufreq sub-system
    > >
    > > + cpufreq.default_governor=
    > > + [CPU_FREQ] Name of the default cpufreq governor to use.
    > > + This governor must be registered in the kernel before
    > > + the cpufreq driver probes.
    > > +
    > > cpu_init_udelay=N
    > > [X86] Delay for N microsec between assert and de-assert
    > > of APIC INIT to start processors. This delay occurs
    > > diff --git a/Documentation/admin-guide/pm/cpufreq.rst b/Documentation/admin-guide/pm/cpufreq.rst
    > > index 0c74a7784964..368e612145d2 100644
    > > --- a/Documentation/admin-guide/pm/cpufreq.rst
    > > +++ b/Documentation/admin-guide/pm/cpufreq.rst
    > > @@ -147,9 +147,9 @@ CPUs in it.
    > >
    > > The next major initialization step for a new policy object is to attach a
    > > scaling governor to it (to begin with, that is the default scaling governor
    > > -determined by the kernel configuration, but it may be changed later
    > > -via ``sysfs``). First, a pointer to the new policy object is passed to the
    > > -governor's ``->init()`` callback which is expected to initialize all of the
    > > +determined by the kernel command line or configuration, but it may be changed
    > > +later via ``sysfs``). First, a pointer to the new policy object is passed to
    > > +the governor's ``->init()`` callback which is expected to initialize all of the
    > > data structures necessary to handle the given policy and, possibly, to add
    > > a governor ``sysfs`` interface to it. Next, the governor is started by
    > > invoking its ``->start()`` callback.
    > > diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
    > > index 0128de3603df..4b1a5c0173cf 100644
    > > --- a/drivers/cpufreq/cpufreq.c
    > > +++ b/drivers/cpufreq/cpufreq.c
    > > @@ -50,6 +50,9 @@ static LIST_HEAD(cpufreq_governor_list);
    > > #define for_each_governor(__governor) \
    > > list_for_each_entry(__governor, &cpufreq_governor_list, governor_list)
    > >
    > > +static char cpufreq_param_governor[CPUFREQ_NAME_LEN];
    > > +static struct cpufreq_governor *default_governor;
    > > +
    > > /**
    > > * The "cpufreq driver" - the arch- or hardware-dependent low
    > > * level driver of CPUFreq support, and its spinlock. This lock
    > > @@ -1055,7 +1058,6 @@ __weak struct cpufreq_governor *cpufreq_default_governor(void)
    > >
    > > static int cpufreq_init_policy(struct cpufreq_policy *policy)
    > > {
    > > - struct cpufreq_governor *def_gov = cpufreq_default_governor();
    > > struct cpufreq_governor *gov = NULL;
    > > unsigned int pol = CPUFREQ_POLICY_UNKNOWN;
    > >
    > > @@ -1065,8 +1067,8 @@ static int cpufreq_init_policy(struct cpufreq_policy *policy)
    > > if (gov) {
    > > pr_debug("Restoring governor %s for cpu %d\n",
    > > policy->governor->name, policy->cpu);
    > > - } else if (def_gov) {
    > > - gov = def_gov;
    > > + } else if (default_governor) {
    > > + gov = default_governor;
    > > } else {
    > > return -ENODATA;
    > > }
    > > @@ -1074,8 +1076,8 @@ 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 (def_gov) {
    > > - pol = cpufreq_parse_policy(def_gov->name);
    > > + } else if (default_governor) {
    > > + pol = cpufreq_parse_policy(default_governor->name);
    > > /*
    > > * In case the default governor is neiter "performance"
    > > * nor "powersave", fall back to the initial policy
    > > @@ -2320,6 +2322,9 @@ int cpufreq_register_governor(struct cpufreq_governor *governor)
    > > list_add(&governor->governor_list, &cpufreq_governor_list);
    > > }
    > >
    > > + if (!strncasecmp(cpufreq_param_governor, governor->name, CPUFREQ_NAME_LEN))
    > > + default_governor = governor;
    > > +
    > > mutex_unlock(&cpufreq_governor_mutex);
    > > return err;
    > > }
    > > @@ -2348,6 +2353,8 @@ void cpufreq_unregister_governor(struct cpufreq_governor *governor)
    > >
    > > mutex_lock(&cpufreq_governor_mutex);
    > > list_del(&governor->governor_list);
    > > + if (governor == default_governor)
    > > + default_governor = cpufreq_default_governor();
    > > mutex_unlock(&cpufreq_governor_mutex);
    > > }
    > > EXPORT_SYMBOL_GPL(cpufreq_unregister_governor);
    > > @@ -2789,7 +2796,13 @@ static int __init cpufreq_core_init(void)
    > > cpufreq_global_kobject = kobject_create_and_add("cpufreq", &cpu_subsys.dev_root->kobj);
    > > BUG_ON(!cpufreq_global_kobject);
    > >
    > > + mutex_lock(&cpufreq_governor_mutex);
    > > + if (!default_governor)
    > > + default_governor = cpufreq_default_governor();
    > > + mutex_unlock(&cpufreq_governor_mutex);
    >
    > I don't think locking is required here at core-initcall level.

    It isn't necessary AFAICS, but it may as well be regarded as
    annotation (kind of instead of having a comment explaining why it need
    not be used).

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