Re: Power-managing devices that are not of interest at some point in time

[Bastien Nocera]

On Thu, 2014-07-17 at 01:11 +0200, Rafael J. Wysocki wrote:
> On Tuesday, July 15, 2014 06:32:06 PM Patrik Fimml wrote:
> > (Re-sending with correct mailing list addresses.)
> > 
> > Hi,
> > 
> > When the lid of a laptop is closed, certain devices can no longer
> > provide interesting input or will even produce bogus input, such as:
> > 
> > - input devices: touchscreen, touchpad, keyboard
> > - sensors: ambient light sensor, accelerometer, magnetometer
> > - a video camera mounted on the lid
> > - display backlight
> > 
> > Various workarounds cover some of these cases, and we have some ugly
> > hacks in ChromeOS to make things work. It would be nice if a userspace
> > power management daemon could listen to the lid-close event, and then
> > have a way to temporarily power off these devices, potentially through
> > sysfs.
> > 
> > I've been discussing this with Dmitry and Benson (cc'd), and we've been
> > wondering whether we could come up with a generic solution that could
> > benefit multiple device classes.
> > 
> > There's some overlap with runtime PM here. The action to be taken in
> > such a situation would probably be similar to a runtime suspend. The
> > match is not perfect though, since devices with more than two power
> > states might want to enter different states depending on the situation.
> > 
> > It's somewhat difficult to get the semantics right, since handles to
> > such devices might still be open. It might be easier to implement
> > behavior specific to device classes. On the other hand, it would be nice
> > to have a uniform way of shutting devices down, and not introduce
> > another possible path for a device to enter a power-saving state.
> > 
> > Rafael, can you give us your opinion on this?
> 
> Let me try to understand the scenario in the first place.
> 
> To start with, a number of devices is in use (that is, open, there are
> applications listening/talking to them etc).  Now, an event happens, such
> as a laptop lid close and you want some of those devices, but possibly
> not all of them, to quiesce themselves and go into low-power states.
> 
> Is that correct?

FWIW, I wouldn't build all this complication into the kernel, I would
expect:
- the kernel to make it easy to detect which of those devices are
internal to the machine
- make it easy to detect the state of the whole machine (for some
laptops like the Yoga, there's no SW_TABLET reporting)
- let well-behaved user-space handle the combined information (eg. it's
an internal device + lid is closed = shouldn't be used)

For input devices for example, I'd build this into gnome-settings-daemon
or gnome-shell directly, both of which have knowledge about input
devices used. I'd expect other desktop environments to do similar
things.

Applications can already check the lid status (through UPower), and with
the additional metadata from the kernel, know that the webcam won't be
usable when the lid is closed. Switching to external webcam, a webcam on
the outside of the case, or switch off the video stream would then be
the expected behaviour.

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