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

[Ulf Hansson]

Hi Laurent, Doug,

On Tue, 1 Mar 2022 at 17:29, Laurent Pinchart
<laurent.pinchart@xxxxxxxxxxxxxxxx> wrote:
>
> Hi Ulf,
>
> On Tue, Mar 01, 2022 at 12:18:02PM +0100, Ulf Hansson wrote:
> > On Tue, 1 Mar 2022 at 11:49, Laurent Pinchart wrote:
> > > On Tue, Mar 01, 2022 at 11:26:46AM +0100, Ulf Hansson wrote:
> > > > On Wed, 23 Feb 2022 at 17:35, Douglas Anderson 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?
> > >
> > > For the probe path I agree, but are there valid use cases where, at
> > > runtime, a driver would disable runtime PM and re-enable it a bit later
> > > ? If so, we need to ensure this won't disable auto-suspend.
> >
> > I am not sure I fully understand whether there is a problem.
> >
> > Can you perhaps write the sequence of the runtime PM calls that may
> > cause an issue?
>
> Simply
>
>         pm_runtime_disable();
>         /* Do something that requires runtime PM to be disabled */
>         pm_runtime_enable();
>
> at runtime (not at probe or remove time). If probe() has enabled
> auto-suspend, we don't want the above sequence to disable it. What I'm
> not sure is if there are any valid use cases for the above sequence.

The above sequence certainly exists already, for example during system
suspend/resume.

So what happens is that the runtime PM auto-suspend feature gets
temporarily disabled between pm_runtime_disable() and
pm_runtime_enable(). That seems correct, 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().
> > >
> > > My experience with runtime PM is that it's hard to use, at least if you
> > > want to get it right :-) That's especially the case if a driver wants to
> > > support both CONFIG_PM and !CONFIG_PM. Here's an example at probe time:
> > >
> > >         /*
> > >          * We need the driver to work in the event that CONFIG_PM is disabled in
> > >          * the kernel, so power up and verify the chip now. In the event that
> > >          * CONFIG_PM is disabled this will leave the chip on, so that streaming
> > >          * will work.
> > >          */
> > >         ret = ov5693_sensor_powerup(ov5693);
> > >         if (ret)
> > >                 goto err_media_entity_cleanup;
> > >
> > >         ret = ov5693_detect(ov5693);
> > >         if (ret)
> > >                 goto err_powerdown;
> > >
> > >         pm_runtime_set_active(&client->dev);
> > >         pm_runtime_get_noresume(&client->dev);
> > >         pm_runtime_enable(&client->dev);
> > >
> > >         ret = v4l2_async_register_subdev_sensor(&ov5693->sd);
> > >         if (ret) {
> > >                 dev_err(&client->dev, "failed to register V4L2 subdev: %d",
> > >                         ret);
> > >                 goto err_pm_runtime;
> > >         }
> > >
> > >         pm_runtime_set_autosuspend_delay(&client->dev, 1000);
> > >         pm_runtime_use_autosuspend(&client->dev);
> > >         pm_runtime_put_autosuspend(&client->dev);
> > >
> > > And the corresponding code at remove time:
> > >
> > >         /*
> > >          * Disable runtime PM. In case CONFIG_PM is disabled in the kernel,
> > >          * make sure to turn power off manually.
> > >          */
> > >         pm_runtime_disable(&client->dev);
> > >         if (!pm_runtime_status_suspended(&client->dev))
> > >                 ov5693_sensor_powerdown(ov5693);
> > >         pm_runtime_set_suspended(&client->dev);
> > >
> > > And of course there's no documentation that explains all this, so there
> > > are endless variations of patterns originating from cargo-cult
> > > programming.
> > >
> > > I don't know what the right solution is, but we need to move towards an
> > > easier to use API if we want drivers to get it right. Any step in that
> > > direction would be welcome.
> >
> > Yep, I fully agree with you, while it's not an easy task. At least the
> > example above looks fine to me. :-)
>
> It took me several days to figure out how to get it right. Most
> developers don't bother, so we end up with drivers broken in different
> ways :-S

Yes, it's definitely non-trivial.

Power management in general relies on cross-interaction of several
different frameworks, so one really needs a decent overview too,
before adding PM support in a driver.

>
> > Recently I noticed that some drivers are calling
> > pm_runtime_force_suspend() at ->remove(). This works fine in quite
> > many cases, but it wouldn't solve the case when CONFIG_PM is unset.
> >
> > Perhaps we should explore adding a new API, along the lines of
> > pm_runtime_force_suspend(), but make it specific for the ->remove()
> > path, and in some way make it work for when CONFIG_PM is unset too.
>
> I'm all for an improved API for drivers that would make the above
> simpler. And documentation too, Documentation/power/runtime_pm.rst is
> more of a documentation of the runtime PM core than the driver API.
> There are some useful tips for drivers, but they're lost in a sea of
> difficult to understand and/or irrelevant information (and there's also
> a tiny bit of information in Documentation/driver-api/pm/devices.rst).
> We're missing a document targetted at driver authors.

Yes, I agree - the docs can certainly be improved! I will add it to my
TODO list and try to put some time on it, not too far ahead I hope. I
was actually planning for a blog-post/LWN article, maybe I should
spend some time on this instead - or both. :-)

When it comes to the improved API for the ->remove() case, we need to
explore this a bit more. I will think about it.

About $subject patch, if you or Doug insist that you want to move
forward on it, I will not object - even if I think we need something
entirely different, in the long run.

[...]

Kind regards
Uffe