linux-kernel - Re: [RFC PATCH] PM / Runtime: runtime: Add sysfs option for forcing runtime suspend

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Date:	Wed, 9 Sep 2015 13:16:34 -0700
From:	Colin Cross <ccross@...gle.com>
To:	"Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:	Alan Stern <stern@...land.harvard.edu>,
	Octavian Purdila <octavian.purdila@...el.com>,
	"Tirdea, Irina" <irina.tirdea@...el.com>,
	"Rafael J. Wysocki" <rafael@...nel.org>,
	Ulf Hansson <ulf.hansson@...aro.org>,
	Oliver Neukum <oneukum@...e.com>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	"linux-input@...r.kernel.org" <linux-input@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Brown, Len" <len.brown@...el.com>, Pavel Machek <pavel@....cz>,
	Dmitry Torokhov <dmitry.torokhov@...il.com>
Subject: Re: [RFC PATCH] PM / Runtime: runtime: Add sysfs option for forcing
 runtime suspend

On Wed, Sep 9, 2015 at 1:35 PM, Rafael J. Wysocki <rjw@...ysocki.net> wrote:
>
> On Wednesday, September 09, 2015 11:20:25 AM Alan Stern wrote:
> > On Wed, 9 Sep 2015, Rafael J. Wysocki wrote:
> >
> > > > The best example and actually the very specific problem we want to
> > > > solve is handling touchscreens on a phone / tablet. When the screen is
> > > > turned off, it is ideal to suspend the touchscreen for two reasons: to
> > > > lower the power consumption as much as possible and to prevent
> > > > interrupts to wake-up the CPU when the user touches the device, and
> > > > thus save even more power as we allow the CPU to stay in deep idle
> > > > states for longer periods.
> > > >
> > > > Note that when the screen is turned-on again, we want to resume the
> > > > touchscreen so that it can send events again.
> > >
> > > In fact, then, what you need seems to be the feature discussed by Alan
> > > and me some time ago allowing remote wakeup do be disabled for runtime
> > > PM from user space as that in combination with autosuspend should
> > > address your use case.
> >
> > That, plus they want the touchscreen to go into runtime suspend
> > whenever the screen is off (was this not the main reason for the
> > patch?).
>
> Right.
>
> > It seems to me that it should be possible to arrange for this to happen
> > simply by making userspace close the touchscreen device when the screen
> > is turned off.  Or am I missing something?
>
> Honestly, I don't know.
>
> Octavian, Irina, any reasons why things can't be done as Alan is suggesting?


Early Android used early suspend, which notified various kernel
drivers that the screen was turning off so they could go into low
power states.  That meant there was no reason to close the touchscreen
fd - the kernel already knew about the screen off event.  We then got
rid of the early suspend hack and moved everything into userspace
using the power HAL and sysfs files.  Getting rid of the touchscreen
sysfs files and closing the touchscreen on screen off was on the
nice-to-have list, but hasn't made it onto anybody's todo list.
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