Re: [PATCH v2] ALSA: hda/tegra: enable clock during probe

[Takashi Iwai]

On Thu, 31 Jan 2019 12:46:54 +0100,
Rafael J. Wysocki wrote:
> 
> On Thu, Jan 31, 2019 at 12:21 PM Takashi Iwai <tiwai@xxxxxxx> wrote:
> >
> > On Thu, 31 Jan 2019 12:05:30 +0100,
> > Thierry Reding wrote:
> > >
> > > On Wed, Jan 30, 2019 at 05:40:42PM +0100, Takashi Iwai wrote:
> 
> [cut]
> 
> > > > If I understand correctly the code, the pm domain is already activated
> > > > at calling driver's probe callback.
> > >
> > > As far as I can tell, the domain will also be powered off again after
> > > probe finished, unless the device grabs a runtime PM reference. This is
> > > what happens via the dev->pm_domain->sync() call after successful probe
> > > of a driver.
> >
> > Ah, a good point.  This can be a problem with a probe work like this
> > case.
> >
> > > It seems to me like it's not a very well defined case what to do when a
> > > device needs to be powered up but runtime PM is not enabled.
> > >
> > > Adding Rafael and linux-pm, maybe they can provide some guidance on what
> > > to do in these situations.
> > >
> > > To summarize, what we're debating here is how to handle powering up a
> > > device if the pm_runtime infrastructure doesn't take care of it. Jon's
> > > proposal here was, and we use this elsewhere, to do something like this:
> > >
> > >       pm_runtime_enable(dev);
> > >       if (!pm_runtime_enabled(dev)) {
> > >               err = foo_runtime_resume(dev);
> > >               if (err < 0)
> > >                       goto fail;
> > >       }
> > >
> > > So basically when runtime PM is not available, we explicitly "resume"
> > > the device to power it up.
> > >
> > > It seems to me like that's a fairly common problem, so I'm wondering if
> > > there's something that the runtime PM core could do to help with this.
> > > Or perhaps there's already a way to achieve this that we're all
> > > overlooking?
> > >
> > > Rafael, any suggestions?
> >
> > If any, a common helper would be appreciated, indeed.
> 
> I'm not sure that I understand the problem correctly, so let me
> restate it the way I understand it.
> 
> What we're talking about is a driver ->probe() callback.  Runtime PM
> is disabled initially and the device is off.  It needs to be powered
> up, but the way to do that depends on some configuration of the board
> etc., so ideally
> 
> pm_runtime_enable(dev);
> ret = pm_runtime_resume(dev);
> 
> should just work, but the question is what to do if runtime PM doesn't
> work as expected.  That is, CONFIG_PM_RUNTIME is unset?  Or something
> else?

Yes, the question is how to write the code for both with and without
CONFIG_PM (or CONFIG_PM_RUNTIME).

Right now, we have a code like below, pushing the initialization in an
async work and let the probe returning quickly.

	hda_tegra_probe() {
		....
		pm_runtime_enable();
		schedule_work();
		return;
	}

	hda_tegra_probe_work() {
		pm_runtime_get_sync();
		....
		pm_runtime_put_sync();
	}

Then it truned outhis code lacks of the clock initialization when
runtime PM isn't enabled.  Normally it's done via runtime resume

	hda_tegra_runtime_resume() {
		hda_tegra_enable_clocks();
		....
	}

And now the question is what is the standard idiom in such a case.

IMO, calling pm_runtime_resume() inside the probe function looks
weird, and my preference was to initialize the clocks explicitly, then
enable runtime PM.  But if using pm_runtime_resume() in the proc
should be seen as a standard procedure, I'm fine with that.


thanks,

Takashi