On Thu, Jan 31, 2019 at 01:10:01PM +0100, Rafael J. Wysocki wrote:
On Thu, Jan 31, 2019 at 12:59 PM Takashi Iwai tiwai@suse.de wrote:
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@suse.de 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:
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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).
This basically is about setup, because after that point all should just work in both cases.
Personally, I would do
if (IS_ENABLED(CONFIG_PM)) { do setup based on pm-runtime } else { do manual setup }
Right now, we have a code like below, pushing the initialization in an async work and let the probe returning quickly.
hda_tegra_probe() { ....So why don't you do
if (!IS_ENABLED(CONFIG_PM)) { do manual clock setup }
here?
I think that's exactly what Jon and Sameer were proposing, although the discussion started primarily because of the way it was done.
So basically the idea was to do:
pm_runtime_enable() if (!pm_runtime_enabled()) /* basically !IS_ENABLED(CONFIG_PM) */ hda_runtime_resume()
So we're not calling pm_runtime_resume() but rather the driver's implementation of it. This is to avoid duplicating the code, which under some circumstances can be fairly long. Duplicating is also error prone because both instances may not always be in sync.
My understanding is that Takashi had reservations about using this kind of construct because, well, frankly, it looks a little weird. We'd also likely want to have a similar construct again in the ->remove() callback to make sure we properly power off the device when it is no longer needed. I'm just wondering if perhaps there should be a mechanism in the core to take care of this, because this is basically something that we'd need to do for every single driver.
For example, if !CONFIG_PM couldn't the pm_runtime_enable() function be modified to do the above? This would be somewhat tricky because drivers usually use SET_RUNTIME_PM_OPS to populate the struct dev_pm_ops and that would result in an empty structure if !CONFIG_PM, but we could probably work around that by adding a __SET_RUNTIME_PM_OPS that would never be compiled out for this kind of case. Or such drivers could even manually set .runtime_suspend and .runtime_resume to make sure they're always populated.
Another way out of this would be to make sure we never run into the case where runtime PM is disabled. If we always "select PM" on Tegra, then PM should always be available. But is it guaranteed that runtime PM for the devices is functional in that case? From a cursory look at the code it would seem that way.
Thierry