Re: [PATCH/RFC] Resolve 2 year old issue with different demands on EVIOCGRAB

[Neil Brown <neilb@suse.de>]

On Monday August 18, dmitry.torokhov@gmail.com wrote:
> On Mon, Aug 18, 2008 at 10:51:33AM +1000, Neil Brown wrote:
> >  But the X server still needs to know a little bit about /dev/tty to
> >  make sure that control-C doesn't get delivered the wrong way.  That's
> >  awkward.
> 
> Does it need to do anything besides switching VC into the raw mode?

Probably not, no.

> 
> >  It also negates much of the power of the input layer (easy hot-plug).
> >  I don't much like that approach.
> > 
> 
> I think this is the only sensible approach though. X needs to have
> native hotplug capabilities anyway because of all these new mice that
> have bazillion of buttons on them that PS/2 emulation simply can not
> support. And once you have hotplug support in X and don't rely on
> myultiplexors anymore you can remove bunch of things, like grabbing
> devices in one fashion or another and you can even keep the devices
> open while switching to the text mode - no need to close and reopen
> them all the time.

So your position is that for anything non-trivial, the clever stuff in
the input layer like multiplexing and simulating PS2 mice is not
sufficiently powerful and not worth fixing, and it should all be left
to userspace.  Correct?
I must confess a certain sympathy for that position, but it does make
solving my current problem harder - which dampens my enthusiasm :-)

So:  The X server need to support hotplug of input devices. 
Apparently there is code out there (for nearly 2 years) but it doesn't
seem to be in a release yet.
Quoting http://wiki.x.org/wiki/XInputHotplug

   The X.org server supports hotplugging input devices since November
   2006
   http://lists.freedesktop.org/archives/xorg/2006-October/019007.html
   (X11R7.2 will NOT have hotplug support yet).  

So both the kernel and the X server could provide the hotplug support
required, but neither are really quite usable at the moment for very
different reasons.  Sad.
Options...
 First: problem description.
   Device with a touchscreen wants:
    - touch screen events to be delivered as absolute events to X
         server.
    - Other programs to be able to monitor the touch screen to
      e.g. detect activity independently of whether an X server is in
      use or direct fbdev access is happening (e.g. Qtopia).
    - hot-plugged mice (e.g. bluetooth) should be recognised by
      X server.

 We could:
  1/ Wait for X11 XInputHotplug to be released.
  2/ Hack a little input driver for X which somehow finds 
     mouse devices and interprets them.
  3/ Use kernel's input multiplexing and:
   3a/ change EVIOCGRAB to not exclude other evdev devices
   3b/ change EVIOCGRAB to optionally not exclude other evdev devices.
   3c/ somehow convince mousedev never to listen on the touchscreen.

 Did I miss anything?

 3a is what the openmoko kernel (and presumably others) do today.  But
 it breaks backwards compatibility .
 3b is what I suggested but you don't like.
 1 does not provide a solution in a reasonable time frame
 2 is likely to be very messy and error prone

 So I'm now wondering about 3c.
 While I can understand the value of pretending that a touch-pad looks
 like a mouse, a touch-screen is a very different thing.  I don't
 think it would ever make sense for a touch screen to generate
 relative events.
 So how might we make our touchscreen appear uninteresting to
 mousedev?

 We could get it to return some other 'key' rather one of
     BTN_TOUCH BTN_TOOL_FINGER BTN_LEFT

 Maybe BTN_STYLUS.  Given that the touchpad is designed for finger
 usage it is a bit of a lie.  But it would work.

 This is really sidestepping the issue.  We really want to be able to
 say "This is a device where relative events are completely
 meaningless".  I suspect that isn't going to happen though.

 tslib doesn't seem to care much about what key is sent.  It only
 looks for BTN_TOUCH, but all it sees is when the value goes to zero
 as a "finger is removed" event.  It equally gets that from
 ABS_PRESSURE going to zero.

 So if I just arranged for the touchscreen to deliver BTN_STYLUS
 instead of BTN_TOUCH it would hide it from all the mousedev devices
 (which I think is the correct thing) and mean that the Xserver
 doesn't need to use EVIOCGRAB, so we don't need to break it in the
 kernel.

OK, I think I can go forward with that.  Thanks for listening.

NeilBrown