linux-kernel - Re: [PATCH] PM / Runtime: Only force-resume device if it has been force-suspended

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Message-ID: <1481433.r0tFS4sTcH@avalon>
Date:	Fri, 04 Mar 2016 23:04:46 +0200
From:	Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	Ulf Hansson <ulf.hansson@...aro.org>,
	Laurent Pinchart <laurent.pinchart+renesas@...asonboard.com>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Len Brown <len.brown@...el.com>, Pavel Machek <pavel@....cz>,
	Kevin Hilman <khilman@...nel.org>,
	linux-renesas-soc@...r.kernel.org
Subject: Re: [PATCH] PM / Runtime: Only force-resume device if it has been force-suspended

Hi Ulf and Alan,

Thank you for the review.

On Friday 04 March 2016 10:24:10 Alan Stern wrote:
> On Fri, 4 Mar 2016, Ulf Hansson wrote:
> > On 3 March 2016 at 21:16, Laurent Pinchart wrote:
> >> The pm_runtime_force_suspend() and pm_runtime_force_resume() helpers are
> >> designed to help driver being RPM-centric by offering an easy way to
> >> manager runtime PM state during system suspend and resume. The first
> >> function will force the device into runtime suspend at system suspend
> >> time, while the second one will perform the reverse operation at system
> >> resume time.
> >> 
> >> However, the pm_runtime_force_resume() really forces resume, regarding
> >> of whether the device was running or already suspended before the call
> >> to pm_runtime_force_suspend(). This results in devices being runtime
> >> resumed at system resume time when they shouldn't.
> >> 
> >> Fix this by recording whether the device has been forcefully suspended
> >> in pm_runtime_force_suspend() and condition resume in
> >> pm_runtime_force_resume() to that state.
> >> 
> >> All current users of pm_runtime_force_resume() call the function
> >> uncontionally in their system resume handler (some actually set it as
> >> the resume handler), all after calling pm_runtime_force_suspend() at
> >> system suspend time. The change in behaviour should thus be safe.
> >> 
> >> Signed-off-by: Laurent Pinchart
> >> <laurent.pinchart+renesas@...asonboard.com>
> >> 
> >> @@ -1475,6 +1476,7 @@ int pm_runtime_force_suspend(struct device *dev)
> >>                 goto err;
> >>         
> >>         pm_runtime_set_suspended(dev);
> >> +       dev->power.is_force_suspended = true;
> >>         return 0;
> >>  
> >>  err:
> >>         pm_runtime_enable(dev);
> >> @@ -1510,6 +1515,7 @@ int pm_runtime_force_resume(struct device *dev)
> >>         if (ret)
> >>                 goto out;
> >> 
> >> +       dev->power.is_force_suspended = false;
> >>         pm_runtime_set_active(dev);
> >>         pm_runtime_mark_last_busy(dev);
> >>  out:
>
> Setting a bitflag is not SMP-safe.  When you write to one of the
> runtime-PM bits under dev->power, it is necessary to hold
> dev->power.lock.
> 
> > Overall I have no objections to this change, as I think it's improving
> > the behaviour!
> > 
> > What I was thinking though, but it might be a bit controversial. :-)...
> > Instead of relying on whether we actually forced runtime suspend
> > earlier, why couldn't we instead check the runtime PM usage count of
> > the device?
> > 
> > Only when it's greater than zero, we shall do the forced resume of the
> > device, otherwise just re-enable runtime PM.
> > 
> > This would have the affect of leaving devices in runtime suspend,
> > until they really needs to be used again. It would thus decrease the
> > total system PM resume time.
> > 
> > Do you think this could work?
> 
> If you do this then there would be no need for is_force_suspended.  It
> seems like a good idea to me.

I agree, that's a better idea. Drivers shouldn't call 
pm_runtime_force_resume() if they haven't called pm_runtime_force_suspend(), 
so checking the PM use count should be fine. I'll modify the patch, test it 
and resubmit.

-- 
Regards,

Laurent Pinchart