[PATCH] PM: runtime: document common mistake with pm_runtime_get_sync()

Krzysztof Kozlowski
2021-04-22T16:46:30Z

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>
---
Documentation/power/runtime_pm.rst | 4 +++-
include/linux/pm_runtime.h | 3 +++
2 files changed, 6 insertions(+), 1 deletion(-)

diff --git a/Documentation/power/runtime_pm.rst b/Documentation/power/runtime_pm.rst
index 18ae21bf7f92..478f08942811 100644
--- a/Documentation/power/runtime_pm.rst
+++ b/Documentation/power/runtime_pm.rst
@@ -378,7 +378,9 @@ 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;
+ be aware that it does not drop the device's usage counter on errors so
+ usage of pm_runtime_resume_and_get(dev) usually results 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 6c08a085367b..0dfd23d2cfc3 100644
--- 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.
+ * Lack of decrementing the runtime PM usage counter on errors is a common
+ * mistake, so consider using pm_runtime_resume_and_get() instead for a cleaner
+ * code.
*/
static inline int pm_runtime_get_sync(struct device *dev)
{
--
2.25.1