Re: [PATCH] PM: runtime: Have devm_pm_runtime_enable() handle pm_runtime_dont_use_autosuspend()

[Ulf Hansson <ulf.hansson@linaro.org>]

On Wed, 23 Feb 2022 at 17:35, Douglas Anderson <dianders@chromium.org> wrote:
>
> The PM Runtime docs say:
>   Drivers in ->remove() callback should undo the runtime PM changes done
>   in ->probe(). Usually this means calling pm_runtime_disable(),
>   pm_runtime_dont_use_autosuspend() etc.
>
> From grepping code, it's clear that many people aren't aware of the
> need to call pm_runtime_dont_use_autosuspend().

Well, I admit it's good practice that they should take care of this.

However, it doesn't really matter to keep the autosuspend turned on
when runtime PM becomes disabled, I think. When the driver gets probed
again, it will most likely call pm_runtime_use_autosuspend() again,
which should work fine, right?

>
> When brainstorming solutions, one idea that came up was to leverage
> the new-ish devm_pm_runtime_enable() function. The idea here is that:
> * When the devm action is called we know that the driver is being
>   removed. It's the perfect time to undo the use_autosuspend.
> * The code of pm_runtime_dont_use_autosuspend() already handles the
>   case of being called when autosuspend wasn't enabled.

Hmm, I am hesitating to extend devm_pm_runtime_enable(), as it
currently makes it look too simple to turn off things at ->remove()
for runtime PM. While in fact it's more complicated.

A bigger problem, for example, is that a driver calls
pm_runtime_put_sync() during ->remove(), relying on that it actually
ends up calling its ->runtime_suspend() callback to turn off various
specific resources for the device. And in fact there are no guarantees
that will happen - and when it doesn't, the next time the driver's
->probe() runs, things are likely to be really screwed up.

To cover this case, one could use the below code in the ->remove() callback:

...
pm_runtime_get_sync();

"turn off resources for the devices - like calling
clk_disable_unprepare(), for example"

pm_runtime_disable();
pm_runtime_put_noidle();
...

In this example, it would be too late to call pm_runtime_disable()
through the pm_runtime_disable_action().

Kind regards
Uffe

>
> Suggested-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
> Signed-off-by: Douglas Anderson <dianders@chromium.org>
> ---
>
>  drivers/base/power/runtime.c | 5 +++++
>  include/linux/pm_runtime.h   | 4 ++++
>  2 files changed, 9 insertions(+)
>
> diff --git a/drivers/base/power/runtime.c b/drivers/base/power/runtime.c
> index 2f3cce17219b..d4059e6ffeae 100644
> --- a/drivers/base/power/runtime.c
> +++ b/drivers/base/power/runtime.c
> @@ -1476,11 +1476,16 @@ EXPORT_SYMBOL_GPL(pm_runtime_enable);
>
>  static void pm_runtime_disable_action(void *data)
>  {
> +       pm_runtime_dont_use_autosuspend(data);
>         pm_runtime_disable(data);
>  }
>
>  /**
>   * devm_pm_runtime_enable - devres-enabled version of pm_runtime_enable.
> + *
> + * NOTE: this will also handle calling pm_runtime_dont_use_autosuspend() for
> + * you at driver exit time if needed.
> + *
>   * @dev: Device to handle.
>   */
>  int devm_pm_runtime_enable(struct device *dev)
> diff --git a/include/linux/pm_runtime.h b/include/linux/pm_runtime.h
> index 9f09601c465a..2bff6a10095d 100644
> --- a/include/linux/pm_runtime.h
> +++ b/include/linux/pm_runtime.h
> @@ -567,6 +567,10 @@ static inline void pm_runtime_disable(struct device *dev)
>   * Allow the runtime PM autosuspend mechanism to be used for @dev whenever
>   * requested (or "autosuspend" will be handled as direct runtime-suspend for
>   * it).
> + *
> + * NOTE: It's important to undo this with pm_runtime_dont_use_autosuspend()
> + * at driver exit time unless your driver initially enabled pm_runtime
> + * with devm_pm_runtime_enable() (which handles it for you).
>   */
>  static inline void pm_runtime_use_autosuspend(struct device *dev)
>  {
> --
> 2.35.1.473.g83b2b277ed-goog
>