linux-kernel - Re: [PATCH v2] PM: runtime: document common mistake with pm_runtime_get

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Date:   Mon, 31 May 2021 14:18:13 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Krzysztof Kozlowski <krzysztof.kozlowski@...onical.com>
Cc:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Len Brown <len.brown@...el.com>, Pavel Machek <pavel@....cz>,
        Linux PM <linux-pm@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Marek Szyprowski <m.szyprowski@...sung.com>
Subject: Re: [PATCH v2] PM: runtime: document common mistake with pm_runtime_get_sync()

On Wed, May 26, 2021 at 6:23 PM Krzysztof Kozlowski
<krzysztof.kozlowski@...onical.com> wrote:
>
> 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>
>
> ---
>
> Changes since v1:
> 1. Reword after comments from Rafael.
> ---
>  Documentation/power/runtime_pm.rst | 6 +++++-
>  include/linux/pm_runtime.h         | 3 +++
>  2 files changed, 8 insertions(+), 1 deletion(-)
>
> diff --git a/Documentation/power/runtime_pm.rst b/Documentation/power/runtime_pm.rst
> index 18ae21bf7f92..33619bf83f69 100644
> --- 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;
> +      be aware that it does not drop the device's usage counter on errors so
> +      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 6c08a085367b..aab8b35e9f8a 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.
> + * 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)
>  {
> --

Applied as 5.14 material (with some cosmetic edits), thanks!