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

Alan Stern
2014-05-13T14:49:33Z

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

> From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
>
> 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.
>
> For some devices, though, it's OK to remain in runtime suspend
> throughout a complete system suspend/resume cycle (if the device was in
> runtime suspend at the start of the cycle). We would like to do this
> whenever possible, to avoid the overhead of extra power-up and power-down
> events.
>
> However, problems may arise because the device's descendants may require
> it to be at full power at various points during the cycle. Therefore the
> most straightforward way to do this safely is if the device and all its
> descendants can remain runtime suspended until the complete stage of
> system resume.
>
> To this end, introduce a new device PM flag, power.direct_complete
> and modify the PM core to use that flag as follows.
>
> If the ->prepare() callback of a device returns a positive number,
> the PM core will regard that as an indication that it may leave the
> device runtime-suspended. It will then check if the system power
> transition in progress is a suspend (and not hibernation in particular)
> and if the device is, indeed, runtime-suspended. In that case, the PM
> core will set the device's power.direct_complete flag. Otherwise it
> will clear power.direct_complete for the device and it also will later
> clear it for the device's parent (if there's one).
>
> Next, the PM core will not invoke the ->suspend() ->suspend_late(),
> ->suspend_irq(), ->resume_irq(), ->resume_early(), or ->resume()
> callbacks for all devices having power.direct_complete set. It
> will invoke their ->complete() callbacks, however, and those
> callbacks are then responsible for resuming the devices as
> appropriate, if necessary.

Perhaps you should mention here (and maybe even as a comment in the
code) that ->complete() callbacks may want to call pm_request_resume()
if dev->power.direct_resume is set, but they shouldn't call
pm_runtime_resume().

> Changelog partly based on an Alan Stern's description of the idea
> (http://marc.info/?l=linux-pm&m=139940466625569&w=2).
>
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

...

> @@ -1518,17 +1527,19 @@ static int device_prepare(struct device
> callback = dev->driver->pm->prepare;
> }
>
> - if (callback) {
> - error = callback(dev);
> - suspend_report_result(callback, error);
> - }
> + if (callback)
> + ret = callback(dev);
>
> device_unlock(dev);
>
> - if (error)
> + if (ret < 0) {
> + suspend_report_result(callback, ret);
> pm_runtime_put(dev);
> -
> - return error;
> + return ret;
> + }
> + dev->power.direct_complete = ret > 0 && state.event == PM_EVENT_SUSPEND
> + && pm_runtime_suspended(dev);

Shouldn't the flag be set under the spinlock?

Alan Stern