Messages in this thread | | | From | Ulf Hansson <> | Date | Wed, 1 Dec 2021 16:22:26 +0100 | Subject | Re: [PATCH] PM: runtime: Allow rpm_resume() to succeed when runtime PM is disabled |
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On Wed, 1 Dec 2021 at 14:49, Rafael J. Wysocki <rafael@kernel.org> wrote: > > On Wed, Dec 1, 2021 at 10:02 AM Ulf Hansson <ulf.hansson@linaro.org> wrote: > > > > On Tue, 30 Nov 2021 at 18:26, Rafael J. Wysocki <rafael@kernel.org> wrote: > > > > > > On Tue, Nov 30, 2021 at 5:41 PM Ulf Hansson <ulf.hansson@linaro.org> wrote: > > > > > > > > On Tue, 30 Nov 2021 at 14:02, Rafael J. Wysocki <rafael@kernel.org> wrote: > > > > > > > > > > On Tue, Nov 30, 2021 at 12:58 PM Ulf Hansson <ulf.hansson@linaro.org> wrote: > > > > > > > > > > > > [...] > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > Am I thinking correctly that this is mostly about working around the > > > > > > > > > > > > limitations of pm_runtime_force_suspend()? > > > > > > > > > > > > > > > > > > > > > > No, this isn't related at all. > > > > > > > > > > > > > > > > > > > > > > The cpuidle-psci driver doesn't have PM callbacks, thus using > > > > > > > > > > > pm_runtime_force_suspend() would not work here. > > > > > > > > > > > > > > > > > > > > Just wanted to send a ping on this to see if we can come to a > > > > > > > > > > conclusion. Or maybe we did? :-) > > > > > > > > > > > > > > > > > > > > I think in the end, what slightly bothers me, is that the behavior is > > > > > > > > > > a bit inconsistent. Although, maybe it's the best we can do. > > > > > > > > > > > > > > > > > > I've been thinking about this and it looks like we can do better, but > > > > > > > > > instead of talking about this I'd rather send a patch. > > > > > > > > > > > > > > > > Alright. > > > > > > > > > > > > > > > > I was thinking along the lines of make similar changes for > > > > > > > > rpm_idle|suspend(). That would make the behaviour even more > > > > > > > > consistent, I think. > > > > > > > > > > > > > > > > Perhaps that's what you have in mind? :-) > > > > > > > > > > > > > > Well, not exactly. > > > > > > > > > > > > > > The idea is to add another counter (called restrain_depth in the patch) > > > > > > > to prevent rpm_resume() from running the callback when that is potentially > > > > > > > problematic. With that, it is possible to actually distinguish devices > > > > > > > with PM-runtime enabled and it allows the PM-runtime status to be checked > > > > > > > when it is still known to be meaningful. > > > > > > > > > > > > Hmm, I don't quite understand the benefit of introducing a new flag > > > > > > for this. rpm_resume() already checks the disable_depth to understand > > > > > > when it's safe to invoke the callback. Maybe there is a reason why > > > > > > that isn't sufficient? > > > > > > > > > > The problem is that disable_depth > 0 may very well mean that runtime > > > > > PM has not been enabled at all for the given device which IMO is a > > > > > problem. > > > > > > > > > > As it stands, it is necessary to make assumptions, like disable_depth > > > > > == 1 meaning that runtime PM is really enabled, but the PM core has > > > > > disabled it temporarily, which is somewhat questionable. > > > > > > > > > > Another problem with disabling is that it causes rpm_resume() to fail > > > > > even if the status is RPM_ACTIVE and it has to do that exactly because > > > > > it cannot know why runtime PM has been disabled. If it has never been > > > > > enabled, rpm_resume() must fail, but if it has been disabled > > > > > temporarily, rpm_resume() may return 1 when the status is RPM_ACTIVE. > > > > > > > > > > The new count allows the "enabled in general, but temporarily disabled > > > > > at the moment" to be handled cleanly. > > > > > > > > My overall comment is that I fail to understand why we need to > > > > distinguish between these two cases. To me, it shouldn't really > > > > matter, *why* runtime PM is (or have been) disabled for the device. > > > > > > It matters if you want to trust the status, because "disabled" means > > > "the status doesn't matter". > > > > Well, that doesn't really match how the runtime PM interface is being > > used today. > > Well, I clearly disagree.
Alright, then we can agree to disagree. :-)
> > > For example, we have a whole bunch of helper functions, allowing us to > > update and check the runtime PM state of the device, even when the > > disable_depth > 0. Some functions, like pm_runtime_set_active() for > > example, even take parents and device-links into account. > > That's true, but that's for a purpose. > > If runtime PM becomes enabled after using pm_runtime_set_active(), the > status should better be consistent with the settings of the parent > etc. > > > > > > > If you want the status to stay meaningful, but prevent callbacks from > > > running, you need something else. > > > > > > > The important point is that the default state for a device is > > > > RPM_SUSPENDED and someone has moved into RPM_ACTIVE, for whatever > > > > reason. That should be sufficient to allow rpm_resume() to return '1' > > > > when disable_depth > 0, shouldn't it? > > > > > > No, because there is no rule by which the status of devices with > > > PM-runtime disabled must be RPM_SUSPENDED. > > > > That's not what I was trying to say. > > > > The initial/default runtime PM state for a device is RPM_SUSPENDED, > > which is being set in pm_runtime_init(). Although, I agree that it > > can't be trusted that this state actually reflects the state of the > > HW, it's still a valid state for the device from a runtime PM point of > > view. > > No, it is not. It's just the default. > > > However, and more importantly, if the state has moved to RPM_ACTIVE, > > someone must have deliberately moved the device into that state. > > Sure, but it cannot be regarded as an indication on whether or not > runtime PM is supported and has ever been enabled for the given > device. > > Again, there is no rule regarding the status value for devices with > runtime PM disabled, either way.
If I understand correctly, that means you think the pm_runtime_status_suspended() should really be converted to an internal runtime PM interface, not being exported to users outside. Right?
When it comes to the pm_runtime_active() interface, I noticed that it's actually completely broken, as it returns "true" when runtime PM has been disabled for the device - no matter whether the state is RPM_ACTIVE or not. It's quite heavily used, so I guess the behaviour is what the callers expect?
> > > For this reason, I believe it seems reasonable to trust it, both from HW > > point of view, but definitely also from a runtime PM point of view. If > > not, then what should we do? > > Trust it only when runtime PM is enabled, ie. dev->power.disable_depth == 0. > > That's exactly the reason why pm_runtime_suspended() returns false if > runtime PM is disabled for the target device and if > pm_runtime_suspended() is called during system-wide suspend and > resume, it is not clear how to interpret its return value.
To me, I don't think there is a problem interpreting pm_runtime_suspended()'s return value. As you said, it tells whether runtime PM has been enabled and whether the state is RPM_SUSPENDED.
For those that want to know the state when runtime PM has been disabled, they can use pm_runtime_status_suspended().
What's the issue?
> > If it returns true outside the system suspend-resume code path, that > means "runtime PM has been enabled and the device has been > runtime-suspended" and I want it to mean exactly the same thing during > system-wide suspend and resume, so people don't need to wonder about > the context in which the code is running.
Well, even if this seems nice, I wonder how useful this would be in the end.
Perhaps you have some concrete examples where this could improve things?
Kind regards Uffe
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