Date:	Sun, 14 Jun 2009 12:29:05 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Magnus Damm <magnus.damm@...il.com>
Cc:	Alan Stern <stern@...land.harvard.edu>,
	Oliver Neukum <oliver@...kum.org>,
	Magnus Damm <damm@...l.co.jp>,
	pm list <linux-pm@...ts.linux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	ACPI Devel Maling List <linux-acpi@...r.kernel.org>
Subject: Re: [PATCH] PM: Introduce core framework for run-time PM of I/O devices

On Sunday 14 June 2009, Magnus Damm wrote:
> Hi Rafael,
> 
> On Sun, Jun 14, 2009 at 7:23 AM, Rafael J. Wysocki<rjw@...k.pl> wrote:
> > Below is the current version of my "run-time PM for I/O devices" patch.
> >
> > I've done my best to address the comments received during the recent
> > discussions, but at the same time I've tried to make the patch only contain
> > the most essential things.  For this reason, for example, the sysfs interface
> > is not there and it's going to be added in a separate patch.
> 
> Good decision. Let's do this step by step.
> 
> > Please let me know if you want me to change anything in this patch or to add
> > anything new to it.  [Magnus, I remember you wanted something like
> > ->runtime_wakeup() along with ->runtime_idle(), but I'm not sure it's really
> > necessary.  Please let me know if you have any particular usage scenario for
> > it.]
> 
> I will keep on building my arch specific platform bus code on top of
> the latest version of this patch.
> 
> However, to begin with I'll not make use of the ->runtime_idle()
> callback in the bus code. This because rearranging the existing
> platform devices into a tree will require a lot of rewriting, and I'm
> not convinced it's the right approach. I'd rather focus on getting
> basic functionality in place at this point. So if no one else needs
> ->runtime_idle(), feel free to exclude the ->runtime_idle() part if
> you want to make the patch even leaner to begin with.

I think it's going be useful in general.  If not, we can just drop it.

> Together with the bus specific callbacks I plan to modify device
> drivers to include pm_runtime_suspend() / pm_runtime_resume() calls to
> notify the bus code when they are idle and when they need wakeup,
> similar to my earlier proposal with
> platform_device_idle()/platform_device_wakeup().

That sounds like a good plan.

> > --- linux-2.6.orig/include/linux/pm.h
> > +++ linux-2.6/include/linux/pm.h
> > @@ -182,6 +205,11 @@ struct dev_pm_ops {
> >        int (*thaw_noirq)(struct device *dev);
> >        int (*poweroff_noirq)(struct device *dev);
> >        int (*restore_noirq)(struct device *dev);
> > +#ifdef CONFIG_PM_RUNTIME
> > +       int (*runtime_suspend)(struct device *dev);
> > +       int (*runtime_resume)(struct device *dev);
> > +       void (*runtime_idle)(struct device *dev);
> > +#endif
> 
> Do we really need to wrap these in CONFIG_PM_RUNTIME? The callbacks
> for STR and STD are not wrapped in CONFIG_SUSPEND and
> CONFIG_HIBERNATION, right?
> 
> > --- /dev/null
> > +++ linux-2.6/drivers/base/power/runtime.c
> [snip]
> > +/**
> > + * pm_runtime_suspend - Run a device bus type's runtime_suspend() callback.
> > + * @dev: Device to suspend.
> > + *
> > + * Check if the status of the device is appropriate and run the
> > + * ->runtime_suspend() callback provided by the device's bus type driver.
> > + * Update the run-time PM flags in the device object to reflect the current
> > + * status of the device.
> > + */
> > +int pm_runtime_suspend(struct device *dev)
> > +{
> > +       int error = 0;
> 
> I'm sure you put a lot of thought into this already, but is it really
> the best approach to assume that busses without runtime pm callbacks
> can be suspended? I'd go with an error value by default and only
> return 0 as callback return value.

Hmm, yes.  I think you're right.

> > +/**
> > + * pm_cancel_suspend - Cancel a pending suspend request for given device.
> > + * @dev: Device to cancel the suspend request for.
> > + *
> > + * Should be called under pm_lock_device() and only if we are sure that the
> > + * ->autosuspend() callback hasn't started to yet.
> > + */
> > +static void pm_cancel_suspend(struct device *dev)
> > +{
> > +       dev->power.suspend_aborted = true;
> > +       cancel_delayed_work(&dev->power.runtime_work);
> > +       dev->power.runtime_status = RPM_ACTIVE;
> > +}
> 
> This pm_lock_device() comment seems to come from old code, no?

Correct, I'll fix the comments.

> > +/**
> > + * pm_runtime_resume - Run a device bus type's runtime_resume() callback.
> > + * @dev: Device to resume.
> > + *
> > + * Check if the device is really suspended and run the ->runtime_resume()
> > + * callback provided by the device's bus type driver.  Update the run-time PM
> > + * flags in the device object to reflect the current status of the device.  If
> > + * runtime suspend is in progress while this function is being run, wait for it
> > + * to finish before resuming the device.  If runtime suspend is scheduled, but
> > + * it hasn't started yet, cancel it and we're done.
> > + */
> > +int pm_runtime_resume(struct device *dev)
> > +{
> > +       int error = 0;
> 
> Same here, does non-existing runtime pm callbacks really mean we can resume?

Well, in fact if we get to the callback and it doesn't exist, that will be a
bug.  So, I think it's a good idea to return error code in such a case.

> > +/**
> > + * pm_runtime_disable - Disable run-time power management for given device.
> > + * @dev: Device to handle.
> > + *
> > + * Increase the depth field in the device's dev_pm_info structure, which will
> > + * cause the run-time PM functions above to return without doing anything.
> > + * If there is a run-time PM operation in progress, wait for it to complete.
> > + */
> > +void pm_runtime_disable(struct device *dev)
> > +{
> > +       might_sleep();
> > +
> > +       atomic_inc(&dev->power.depth);
> > +
> > +       if (dev->power.runtime_status & RPM_IN_PROGRESS)
> > +               wait_for_completion(&dev->power.work_done);
> > +}
> > +EXPORT_SYMBOL_GPL(pm_runtime_disable);
> > +
> > +/**
> > + * pm_runtime_enable - Disable run-time power management for given device.
> > + * @dev: Device to handle.
> > + *
> > + * Enable run-time power management for given device by decreasing the depth
> > + * field in its dev_pm_info structure.
> > + */
> > +void pm_runtime_enable(struct device *dev)
> > +{
> > +       if (!atomic_add_unless(&dev->power.depth, -1, 0))
> > +               dev_warn(dev, "PM: Excessive pm_runtime_enable()!\n");
> > +}
> > +EXPORT_SYMBOL_GPL(pm_runtime_enable);
> 
> Any thoughts on performing ->runtime_resume()/->runtime_suspend() in
> enable() and disable()? I guess it's performed too early/late to make
> sense from the driver point of view?

Some thoughts, yes.  As for an implementation, I'd like to wait until at least
one bus type uses the framework.

> Looking good, thanks a lot for your work on this!

Thanks for your comments.

> Any chance we can get this included in -rc1?

Well, in fact I have already pushed all of the changes I wanted in 2.6.31.
Also, I'd like to receive some comments on the $subject patch from the other
people.

That said, the merge window is still open, so if the comments are supportive
and  there's a chance to put the final version into linux-next for a couple of
days before the merge window ends, I may try to push it to Linus.  After all,
the patch is not going to introduce any regressions. ;-)

Best,
Rafael
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