Re: [RFC][PATCH 2/3] PM / sleep: Mechanism to avoid resuming runtime-suspended devices unnecessarily

Alan Stern
2014-05-14T14:53:17Z

On Tue, 13 May 2014, Rafael J. Wysocki wrote:

> > It would be surprising if ->prepare() needed to make any difficult
> > checks. This would imply that the device could have multiple
> > runtime-suspend states, some of which are appropriate for system
> > suspend while others aren't. Not impossible, but I wouldn't expect it
> > to come up often.
>
> That is the case for every device with ACPI power management in principle. :-)
>
> Please see patch [3/3] for details.

I don't understand enough about the ACPI subsystem to follow the
details of that patch.

> OK, I've updated the $subject patch in the meantime and the result is appended
> Former patch [1/3] is not necessary any more now and patch [3/3] is still valid.
>
> Rafael
>
> ---
> From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> Subject: PM / sleep: Mechanism to avoid resuming runtime-suspended devices unnecessarily
>
> Currently, some subsystems (e.g. PCI and the ACPI PM domain) have to
> resume all runtime-suspended devices during system suspend, mostly
> because those devices may need to be reprogrammed due to different
> wakeup settings for system sleep and for runtime PM.

...

> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

This is looking quite good. I have one suggestion for a small
improvement...

> @@ -1332,6 +1338,16 @@ static int __device_suspend(struct devic
> if (dev->power.syscore)
> goto Complete;
>
> + if (dev->power.direct_complete) {
> + pm_runtime_disable(dev);
> + if (dev->power.disable_depth == 1
> + && pm_runtime_status_suspended(dev))
> + goto Complete;
> +
> + dev->power.direct_complete = false;
> + pm_runtime_enable(dev);
> + }

Do we want to allow ->prepare() to return > 0 if the device isn't
runtime suspended? If we do then non-suspended devices may be a common
case. We should then avoid the extra overhead of disable + enable.
So I would write:

if (dev->power.direct_complete) {
if (pm_runtime_status_suspended(dev)) {
pm_runtime_disable(dev);
if (dev->power.disable_depth == 1
&& pm_runtime_status_suspended(dev))
goto Complete;
pm_runtime_enable(dev);
}
dev->power.direct_complete = false;
}

Also, now that we have finally settled on the appropriate API, there
needs to ba a patch updating the PM documentation.

Alan Stern