PM: runtime: docs: Update pm_runtime_allow/forbid() documentation

Drop confusing descriptions of pm_runtime_allow() and pm_runtime_forbid()
from Documentation/power/runtime_pm.rst and update the kerneldoc comments
of these functions to better explain their purpose.

Link: https://lore.kernel.org/linux-pm/08976178-298f-79d9-1d63-cff5a4e56cc3@linux.intel.com/
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Brian Norris <briannorris@chromium.org>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://patch.msgid.link/12780841.O9o76ZdvQC@rafael.j.wysocki

Author: rafaeljw

Documentation/power/runtime_pm.rst

@@ -480,16 +480,6 @@ drivers/base/power/runtime.c and include/linux/pm_runtime.h:
   `bool pm_runtime_status_suspended(struct device *dev);`
     - return true if the device's runtime PM status is 'suspended'
 
-  `void pm_runtime_allow(struct device *dev);`
-    - set the power.runtime_auto flag for the device and decrease its usage
-      counter (used by the /sys/devices/.../power/control interface to
-      effectively allow the device to be power managed at run time)
-
-  `void pm_runtime_forbid(struct device *dev);`
-    - unset the power.runtime_auto flag for the device and increase its usage
-      counter (used by the /sys/devices/.../power/control interface to
-      effectively prevent the device from being power managed at run time)
-
   `void pm_runtime_no_callbacks(struct device *dev);`
     - set the power.no_callbacks flag for the device and remove the runtime
       PM attributes from /sys/devices/.../power (or prevent them from being

drivers/base/power/runtime.c

@@ -1664,9 +1664,12 @@ EXPORT_SYMBOL_GPL(devm_pm_runtime_get_noresume);
  * pm_runtime_forbid - Block runtime PM of a device.
  * @dev: Device to handle.
  *
- * Increase the device's usage count and clear its power.runtime_auto flag,
- * so that it cannot be suspended at run time until pm_runtime_allow() is called
- * for it.
+ * Resume @dev if already suspended and block runtime suspend of @dev in such
+ * a way that it can be unblocked via the /sys/devices/.../power/control
+ * interface, or otherwise by calling pm_runtime_allow().
+ *
+ * Calling this function many times in a row has the same effect as calling it
+ * once.
  */
 void pm_runtime_forbid(struct device *dev)
 {
@@ -1687,7 +1690,13 @@ EXPORT_SYMBOL_GPL(pm_runtime_forbid);
  * pm_runtime_allow - Unblock runtime PM of a device.
  * @dev: Device to handle.
  *
- * Decrease the device's usage count and set its power.runtime_auto flag.
+ * Unblock runtime suspend of @dev after it has been blocked by
+ * pm_runtime_forbid() (for instance, if it has been blocked via the
+ * /sys/devices/.../power/control interface), check if @dev can be
+ * suspended and suspend it in that case.
+ *
+ * Calling this function many times in a row has the same effect as calling it
+ * once.
  */
 void pm_runtime_allow(struct device *dev)
 {