[PATCH V4 3/3] cpufreq: Specify default governor on command line

[Viresh Kumar]

From: Quentin Perret <qperret@xxxxxxxxxx>

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>
[ Viresh: Converted 'default_governor' to a string and parsing it only
	  at initcall level, and several updates to
	  cpufreq_init_policy(). ]
Signed-off-by: Viresh Kumar <viresh.kumar@xxxxxxxxxx>
---
 .../admin-guide/kernel-parameters.txt         |  5 +++
 Documentation/admin-guide/pm/cpufreq.rst      |  6 ++--
 drivers/cpufreq/cpufreq.c                     | 31 +++++++++++++------
 3 files changed, 30 insertions(+), 12 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index fb95fad81c79..8deb5a89328a 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 or
+			policy 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 e9e8200a0211..ad94b1d47ddb 100644
--- a/drivers/cpufreq/cpufreq.c
+++ b/drivers/cpufreq/cpufreq.c
@@ -50,6 +50,8 @@ static LIST_HEAD(cpufreq_governor_list);
 #define for_each_governor(__governor)				\
 	list_for_each_entry(__governor, &cpufreq_governor_list, governor_list)
 
+static char default_governor[CPUFREQ_NAME_LEN];
+
 /**
  * The "cpufreq driver" - the arch- or hardware-dependent low
  * level driver of CPUFreq support, and its spinlock. This lock
@@ -1061,7 +1063,6 @@ __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;
 	int ret;
@@ -1071,21 +1072,27 @@ static int cpufreq_init_policy(struct cpufreq_policy *policy)
 		gov = get_governor(policy->last_governor);
 		if (gov) {
 			pr_debug("Restoring governor %s for cpu %d\n",
-				 policy->governor->name, policy->cpu);
-		} else if (def_gov) {
-			gov = def_gov;
-			__module_get(gov->owner);
+				 gov->name, policy->cpu);
 		} else {
-			return -ENODATA;
+			gov = get_governor(default_governor);
+		}
+
+		if (!gov) {
+			gov = cpufreq_default_governor();
+			if (!gov)
+				return -ENODATA;
+			__module_get(gov->owner);
 		}
+
 	} else {
+
 		/* 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 {
+			pol = cpufreq_parse_policy(default_governor);
 			/*
-			 * In case the default governor is neiter "performance"
+			 * In case the default governor is neither "performance"
 			 * nor "powersave", fall back to the initial policy
 			 * value set by the driver.
 			 */
@@ -2795,13 +2802,19 @@ EXPORT_SYMBOL_GPL(cpufreq_unregister_driver);
 
 static int __init cpufreq_core_init(void)
 {
+	struct cpufreq_governor *gov = cpufreq_default_governor();
+
 	if (cpufreq_disabled())
 		return -ENODEV;
 
 	cpufreq_global_kobject = kobject_create_and_add("cpufreq", &cpu_subsys.dev_root->kobj);
 	BUG_ON(!cpufreq_global_kobject);
 
+	if (!strlen(default_governor))
+		strncpy(default_governor, gov->name, CPUFREQ_NAME_LEN);
+
 	return 0;
 }
 module_param(off, int, 0444);
+module_param_string(default_governor, default_governor, CPUFREQ_NAME_LEN, 0444);
 core_initcall(cpufreq_core_init);
-- 
2.25.0.rc1.19.g042ed3e048af