Messages in this thread | | | From | "Rafael J. Wysocki" <> | Subject | Re: [PATCH 1/1] PM / Runtime: let rpm_resume fail if rpm disabled and device suspended. | Date | Thu, 19 Jun 2014 15:55:31 +0200 |
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On Thursday, June 19, 2014 04:23:29 PM Allen Yu wrote: > On Thursday, June 19, 2014, Rafael J. Wysocki wrote: > > On Wednesday, June 18, 2014 11:30:51 AM Alan Stern wrote: > > > On Tue, 17 Jun 2014, Rafael J. Wysocki wrote: > > > > > > > On Tuesday, June 17, 2014 10:37:03 PM Rafael J. Wysocki wrote: > > > > > On Tuesday, June 17, 2014 10:26:14 PM Rafael J. Wysocki wrote: > > > > > > On Tuesday, June 17, 2014 10:11:32 AM Alan Stern wrote: > > > > > > > On Mon, 16 Jun 2014, Rafael J. Wysocki wrote: > > > > > > > > > > > > > > > > For reasons having nothing to do with Allen's suggested > > > > > > > > > change, I wonder if we shouldn't replace this line with > > something like: > > > > > > > > > > > > > > > > > > - else if (dev->power.disable_depth == 1 && dev- > > >power.is_suspended > > > > > > > > > + else if (dev->power.disable > 0 && > > > > > > > > > +!dev->power.is_suspended > > > > > > > > > && dev->power.runtime_status == RPM_ACTIVE) > > > > > > > > > retval = 1; > > > > > > > > > > > > > > > > > > It seems that I've been bitten by this several times in the past. > > > > > > > > > When a device is disabled for runtime PM, and more or less > > > > > > > > > permanently stuck in the RPM_ACTIVE state, calls to > > > > > > > > > pm_runtime_resume() or > > > > > > > > > pm_runtime_get_sync() shouldn't fail. > > > > > > > > > > > > > > > > > > For example, suppose some devices of a certain type > > > > > > > > > support runtime power management but others don't. We > > > > > > > > > naturally want to call > > > > > > > > > pm_runtime_disable() for the ones that don't. But we also > > > > > > > > > want the same driver to work for all the devices, which > > > > > > > > > means that > > > > > > > > > pm_runtime_get_sync() should return success -- otherwise > > > > > > > > > the driver will think that something has gone wrong. > > > > > > > > > > > > > > > > > > Rafael, what do you think? > > > > > > > > > > > > > > > > That condition is there specifically to take care of the > > > > > > > > system suspend code path. It means that if runtime PM is > > > > > > > > disabled, but it only has been disabled by the system > > > > > > > > suspend code path, we should treat the device as "active" (ie. > > return 1). That won't work after the proposed change. > > > > > > > > > > > > > > Ah, yes, quite true. Okay, suppose we replace that line with just: > > > > > > > > > > > > > > + else if (dev->power.disable > 0 > > > > > > > > > > > > > > > I guess drivers that want to work with devices where runtime > > > > > > > > PM may be disabled can just check the return value of > > rpm_resume() for -EACCES? > > > > > > > > > > > > > > They could, but it's extra work and it's extremely easy to > > > > > > > forget about. I'd prefer not to do things that way. > > > > > > > > > > > > In that case we need to audit all code that checks the return > > > > > > value of > > > > > > __pm_runtime_resume() to verify that it doesn't depend on the > > > > > > current behavior in any way. It shouldn't, but still. > > > > > > > > > > > > Also we probably should drop the -EACCES return value from > > > > > > rpm_resume() in the same patch, because it specifically only > > > > > > covers the dev->power.disable > 0 case (which BTW is consistent > > > > > > with the suspend side of things, so I'm totally unsure about that being > > the right thing to do to be honest). > > > > > > It's still the correct action with runtime PM is disabled and the > > > device's runtime_status isn't RPM_ACTIVE. > > > > Well, we used to have the notion that runtime_status is not meaningful for > > devices with dev->power.disable_depth greater than 0 (except for the > > special case in the suspend code path where we know why it is greater than > > 0). I think it was useful. :-) > > So what's the exact state of device if dev->power.is_suspended flag is set and runtime_status is RPM_ACTIVE? Is it a state like "suspended but still can be accessed"? > > I'm just afraid the existing code would cause a device hang if we allow it to be accessed even though it's suspended (at this point RPM_ACTIVE could be meaningless). I don't understand the original motivation of these code. If it's a valid case, most likely it should be handled in the specific device driver instead of the PM core. > > > > > > > > Perhaps it'd be better to rework __pm_runtime_resume() to convert > > > > > the -EACCES return value from rpm_resume() into 0 if RPM_GET_PUT is > > set? > > > > > > > > Or do something like this? > > > > > > > > --- > > > > drivers/base/power/runtime.c | 3 ++- > > > > 1 file changed, 2 insertions(+), 1 deletion(-) > > > > > > > > Index: linux-pm/drivers/base/power/runtime.c > > > > > > ========================================================== > > ========= > > > > --- linux-pm.orig/drivers/base/power/runtime.c > > > > +++ linux-pm/drivers/base/power/runtime.c > > > > @@ -608,7 +608,8 @@ static int rpm_resume(struct device *dev > > > > repeat: > > > > if (dev->power.runtime_error) > > > > retval = -EINVAL; > > > > - else if (dev->power.disable_depth == 1 && dev- > > >power.is_suspended > > > > + else if (((dev->power.disable_depth > 0 && (rpmflags & > > RPM_GET_PUT)) > > > > + || (dev->power.disable_depth == 1 && dev- > > >power.is_suspended)) > > > > && dev->power.runtime_status == RPM_ACTIVE) > > > > retval = 1; > > > > else if (dev->power.disable_depth > 0) > > > > > > So pm_runtime_resume() and pm_request_resume() would still fail, but > > > pm_runtime_get() and pm_runtime_get_sync() would work? I'm not sure > > > about the reason for this distinction. > > > > The meaning of pm_runtime_get()/pm_runtime_get_sync() is "prevent the > > device from being suspended from now on and resume it if necessary" while > > "runtime PM disabled and runtime_status == RPM_ACTIVE" may be > > interpreted as "not necessary to resume", so it is reasonable to special case > > this particular situation for these particular routines IMHO. > > As Rafael mentioned above that runtime_sataus is not meaningful if runtime PM > is disabled, so shouldn't we avoid using the runtime_staus here and instead use > dev->power.is_suspended only to decide the return value?
No, we shouldn't.
This is a special case. If dev->power.disable_depth == 1 and dev->power.is_suspended is set at the same time, we know for a fact that runtime PM was only disabled by the system suspend code path and it was enabled otherwise, so dev->power.runtime_status equal to RPM_ACTIVE is actually meaningful in that particular case.
> @@ -608,11 +608,13 @@ static int rpm_resume(struct device *dev, int rpmflags) > repeat: > if (dev->power.runtime_error) > retval = -EINVAL; > - else if (dev->power.disable_depth == 1 && dev->power.is_suspended > - && dev->power.runtime_status == RPM_ACTIVE) > - retval = 1; > - else if (dev->power.disable_depth > 0) > - retval = -EACCES; > + else if (dev->power.disable_depth > 0) { > + if (!dev->power.is_suspended) > + retval = 1; > + else > + retval = -EACCES; > + } > + > if (retval) > goto out; > > However, this requires us to make sure device is in full functional state if > it's not suspended before disabling runtime PM, just like the case runtime PM > is not configured at all.
If runtime PM is not configured at all, the device has to be in full functional state (from the PM perspective) outside of the system suspend-resume sequence.
The only problematic case I can see is when runtime PM is disabled, runtime_status is RPM_ACTIVE, but the device is actually suspended for some reason. I'd say that avoiding it is the caller's problem.
> And also requires device suspend routine to check dev->power.usage_count before > suspending device.
Why? And which routine exactly are you talking about?
Rafael
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