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

From: Neil Brown
Date: Wed Aug 20 2008 - 20:10:53 EST


On Monday August 18, dmitry.torokhov@xxxxxxxxx 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

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