PM: runtime: document common mistake with pm_runtime_get_sync()

pm_runtime_get_sync(), contradictory to intuition, does not drop the
runtime PM usage counter on errors which lead to several wrong usages in
drivers (missing the put).  pm_runtime_resume_and_get() was added as a
better implementation so document the preference of using it, hoping it
will stop bad patterns.

Suggested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
[ rjw: Documentation change edits ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

diff --git a/Documentation/power/runtime_pm.rst b/Documentation/power/runtime_pm.rst
index 18ae21bf7f925987b70523fab7f06c3809e12513..b48cac5f9048957e585bdf70e52e949fb50dda2f 100644 (file)
--- a/Documentation/power/runtime_pm.rst
+++ b/Documentation/power/runtime_pm.rst
@@ -378,7 +378,11 @@ drivers/base/power/runtime.c and include/linux/pm_runtime.h:
 
   `int pm_runtime_get_sync(struct device *dev);`
     - increment the device's usage counter, run pm_runtime_resume(dev) and
-      return its result
+      return its result;
+      note that it does not drop the device's usage counter on errors, so
+      consider using pm_runtime_resume_and_get() instead of it, especially
+      if its return value is checked by the caller, as this is likely to
+      result in cleaner code.
 
   `int pm_runtime_get_if_in_use(struct device *dev);`
     - return -EINVAL if 'power.disable_depth' is nonzero; otherwise, if the

diff --git a/include/linux/pm_runtime.h b/include/linux/pm_runtime.h
index 6c08a085367bf790fe0eecf500c6b3b5f7ca8c9e..aab8b35e9f8acefbec0ad46845bb1a05cc35884b 100644 (file)
--- a/include/linux/pm_runtime.h
+++ b/include/linux/pm_runtime.h
@@ -380,6 +380,9 @@ static inline int pm_runtime_get(struct device *dev)
  * The possible return values of this function are the same as for
  * pm_runtime_resume() and the runtime PM usage counter of @dev remains
  * incremented in all cases, even if it returns an error code.
+ * Consider using pm_runtime_resume_and_get() instead of it, especially
+ * if its return value is checked by the caller, as this is likely to result
+ * in cleaner code.
  */
 static inline int pm_runtime_get_sync(struct device *dev)
 {