Re: Communicating tablet mode status to userspace

[Greg KH <greg@kroah.com>]

On Tue, Oct 02, 2012 at 04:36:41PM -0700, Dmitry Torokhov wrote:
> On Tue, Oct 02, 2012 at 04:27:07PM -0600, Daniel Drake wrote:
> > Hi,
> > 
> > For the upcoming OLPC touchscreen laptop we're trying to solve the
> > following problem: we want the on-screen keyboard to only activate
> > itself when the laptop is "folded over" into tablet mode where the
> > keyboard is physically obscured.
> > 
> > We already have an ebook mode switch which detects this, and the
> > driver is upstream. Its an input device that exposes this information
> > via the SW_TABLET_MODE switch.
> > 
> > The on-screen keyboard runs in the user environment, which means that
> > its not root - it can't access /dev/input/event nodes directly. X
> > doesn't treat switches as input devices, so this info is not available
> > through X.
> > 
> > I'm wondering if anyone has suggestions on the best way to solve this
> > problem. The most attractive option I have found would be to do
> > something similar to the accelerometer:
> > http://git.kernel.org/?p=linux/hotplug/udev.git;a=blob;f=src/accelerometer/accelerometer.c
> > 
> > Modify the kernel input driver to generate a uevent (in addition to
> > the evdev event) every time the switch state is changed, then write a
> > udev program to expose this info in a udev property.
> > 
> > The change event would (presumably) trigger libudev's udev_monitor,
> > which would be used by the on-screen keyboard software (maliit) to
> > change its behaviour according to the property value.
> > 
> > Does this sound like a sensible plan, would these udev and kernel
> > changes be acceptable? Or is there a better option that I'm missing?
> 
> Why can't you change event device ownership to the local user?  Then you
> can read and act upon events.

Yes, that's the best way.  Also, you might want to check out the extcon
driver interface, that might provide the api you want for userspace.

greg k-h