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

[Viresh Kumar]

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@xxxxxxxxxx>
> ---
>  .../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. Apart
from that:

Acked-by: Viresh Kumar <viresh.kumar@xxxxxxxxxx>

> +
>  	return 0;
>  }
>  module_param(off, int, 0444);
> +module_param_string(default_governor, cpufreq_param_governor, CPUFREQ_NAME_LEN, 0444);
>  core_initcall(cpufreq_core_init);
> -- 
> 2.27.0.111.gc72c7da667-goog

-- 
viresh