linux-kernel - [PATCH] PM: runtime: document common mistake with pm_runtime_get

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Date:   Thu, 22 Apr 2021 18:46:06 +0200
From:   Krzysztof Kozlowski <krzysztof.kozlowski@...onical.com>
To:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Len Brown <len.brown@...el.com>, Pavel Machek <pavel@....cz>,
        linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     Krzysztof Kozlowski <krzysztof.kozlowski@...onical.com>,
        Marek Szyprowski <m.szyprowski@...sung.com>
Subject: [PATCH] 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@...sung.com>
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@...onical.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