[RFC][PATCH 2/2] cpuidle: Allow idle states to be disabled by default

["Rafael J. Wysocki" <rjw@rjwysocki.net>]

From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

In certain situations it may be useful to prevent some idle states
from being used by default while allowing user space to enable them
later on.

For this purpose, introduce a new state flag, CPUIDLE_FLAG_UNCERTAIN,
to mark idle states that should be disabled by default, make the
core set CPUIDLE_STATE_DISABLED_BY_USER for those states at the
initialization time and add a new state attribute in sysfs,
"disable_default", to provide user space with the default value
of the "disable" attribute (1 if CPUIDLE_FLAG_UNCERTAIN is set for
the given state, 0 otherwise) to allow it to restore the initial
configuration of idle states easily if need be.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
---
 Documentation/ABI/testing/sysfs-devices-system-cpu |    6 ++++++
 Documentation/admin-guide/pm/cpuidle.rst           |    3 +++
 drivers/cpuidle/cpuidle.c                          |    3 +++
 drivers/cpuidle/sysfs.c                            |    9 +++++++++
 include/linux/cpuidle.h                            |    1 +
 5 files changed, 22 insertions(+)

Index: linux-pm/drivers/cpuidle/sysfs.c
===================================================================
--- linux-pm.orig/drivers/cpuidle/sysfs.c
+++ linux-pm/drivers/cpuidle/sysfs.c
@@ -327,6 +327,13 @@ static ssize_t store_state_disable(struc
 	return size;
 }
 
+static ssize_t show_state_disable_default(struct cpuidle_state *state,
+					  struct cpuidle_state_usage *state_usage,
+					  char *buf)
+{
+	return sprintf(buf, "%u\n", !!(state->flags & CPUIDLE_FLAG_UNCERTAIN));
+}
+
 define_one_state_ro(name, show_state_name);
 define_one_state_ro(desc, show_state_desc);
 define_one_state_ro(latency, show_state_exit_latency);
@@ -337,6 +344,7 @@ define_one_state_ro(time, show_state_tim
 define_one_state_rw(disable, show_state_disable, store_state_disable);
 define_one_state_ro(above, show_state_above);
 define_one_state_ro(below, show_state_below);
+define_one_state_ro(disable_default, show_state_disable_default);
 
 static struct attribute *cpuidle_state_default_attrs[] = {
 	&attr_name.attr,
@@ -347,6 +355,7 @@ static struct attribute *cpuidle_state_d
 	&attr_usage.attr,
 	&attr_time.attr,
 	&attr_disable.attr,
+	&attr_disable_default.attr,
 	&attr_above.attr,
 	&attr_below.attr,
 	NULL
Index: linux-pm/include/linux/cpuidle.h
===================================================================
--- linux-pm.orig/include/linux/cpuidle.h
+++ linux-pm/include/linux/cpuidle.h
@@ -77,6 +77,7 @@ struct cpuidle_state {
 #define CPUIDLE_FLAG_COUPLED	BIT(1) /* state applies to multiple cpus */
 #define CPUIDLE_FLAG_TIMER_STOP BIT(2) /* timer is stopped on this state */
 #define CPUIDLE_FLAG_UNUSABLE	BIT(3) /* avoid using this state */
+#define CPUIDLE_FLAG_UNCERTAIN	BIT(4) /* disable this state by default */
 
 struct cpuidle_device_kobj;
 struct cpuidle_state_kobj;
Index: linux-pm/drivers/cpuidle/cpuidle.c
===================================================================
--- linux-pm.orig/drivers/cpuidle/cpuidle.c
+++ linux-pm/drivers/cpuidle/cpuidle.c
@@ -572,6 +572,9 @@ static int __cpuidle_register_device(str
 	for (i = 0; i < drv->state_count; i++) {
 		if (drv->states[i].flags & CPUIDLE_FLAG_UNUSABLE)
 			dev->states_usage[i].disable |= CPUIDLE_STATE_DISABLED_BY_DRIVER;
+
+		if (drv->states[i].flags & CPUIDLE_FLAG_UNCERTAIN)
+			dev->states_usage[i].disable |= CPUIDLE_STATE_DISABLED_BY_USER;
 	}
 
 	per_cpu(cpuidle_devices, dev->cpu) = dev;
Index: linux-pm/Documentation/ABI/testing/sysfs-devices-system-cpu
===================================================================
--- linux-pm.orig/Documentation/ABI/testing/sysfs-devices-system-cpu
+++ linux-pm/Documentation/ABI/testing/sysfs-devices-system-cpu
@@ -196,6 +196,12 @@ Description:
 		does not reflect it. Likewise, if one enables a deep state but a
 		lighter state still is disabled, then this has no effect.
 
+What:		/sys/devices/system/cpu/cpuX/cpuidle/stateN/disable_dafault
+Date:		November 2019
+KernelVersion:	v5.4
+Contact:	Linux power management list <linux-pm@vger.kernel.org>
+Description:
+		(RO) The default value of the disable state attribute (bool).
 
 What:		/sys/devices/system/cpu/cpuX/cpuidle/stateN/residency
 Date:		March 2014
Index: linux-pm/Documentation/admin-guide/pm/cpuidle.rst
===================================================================
--- linux-pm.orig/Documentation/admin-guide/pm/cpuidle.rst
+++ linux-pm/Documentation/admin-guide/pm/cpuidle.rst
@@ -506,6 +506,9 @@ object corresponding to it, as follows:
 ``disable``
 	Whether or not this idle state is disabled.
 
+``disable_default``
+	The default value of the ``disable`` attribute.
+
 ``latency``
 	Exit latency of the idle state in microseconds.