Enabling multitouch in input-evdev

Benjamin Tissoires tissoire at cena.fr
Tue Jan 19 10:24:10 PST 2010

Peter Hutterer wrote:
> On Mon, Jan 18, 2010 at 01:02:54PM +0100, Benjamin Tissoires wrote:
>> Sorry, when this freezes occurs I only had backtracks from the
>> driver (a xmalloc) and not the other threads (I forgot to check
>> them). I add this to my todo list.
> Right, as Julien said you can't malloc in evdev. Any path that's hit during
> ReadInput cannot malloc anything.
That's exactly what happened. Thanks both of you.
>>> The evdev driver is first and foremost a generic mouse driver. Which means
>>> that devices that provide x and y are the most important to work, regardless
>>> of relative or absolute. The need for x/y is also implied by the core
>>> protocol. However, there's no reason that you couldn't fake up x/y
>>> and simply leave it as a mute axis. IMO the kernel should not report axes
>>> that don't exist.
>>> The axis mapping evdev is a convention not a rule, so you could just
>>> reshuffle the axis checking to cater for the multitouch situation.
>>> That goes for the rest of the driver too, if something seems wrong to cater
>>> for a new situation then the driver can be rewritten to suit this.
>> As I had to rewrite the patch to keep all the events in one device,
>> I will have to play with axes... and tackle this issue.
> right, you could have an 0/1 axis for this type of things, that's about the
> best approach. there's also the chance of using a button but that's more
> likely to be misinterpreted.
> the correct approach is that you only send those axes with a value, thus
> explicitly specifying the on/off. This is possible with XI2 but the input
> driver APIs aren't in place for that (no need so far).
You mean that in XI2, we (the driver) can choose which axes are send at 
each event ?
>> One last question I forgot to ask:
>> Do we need to dynamically change the number of touch that can be detected ?
>> I.e. do we need to have only 10 touches (as in Windows I think) or
>> do we let the choice to the user (the toolkit) if the device can
>> handle more ?
> same again, you can change the number of axes in XI2, it's just that the
> input driver APIs are missing that feature so far.
great ! I'll have to play with it.

Currently, I'm waiting a little before patching evdev with the "one-blob 
with all touches", because I think that Carsten was right when he told 
that a touch may not be just an x and a y. It can also have many 
valuators. It would be a mess to send 10*(nb_valuators+1) at each event. 
(+1 due to the state of the touch: idle, start, etc...)



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