Date:	Thu, 7 Jul 2011 00:35:21 +0200
From:	"Henrik Rydberg" <rydberg@...omail.se>
To:	Chase Douglas <chase.douglas@...onical.com>
Cc:	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	Daniel Kurtz <djkurtz@...omium.org>, rubini@...l.unipv.it,
	linux-input@...r.kernel.org, linux-kernel@...r.kernel.org,
	derek.foreman@...labora.co.uk, daniel.stone@...labora.co.uk,
	olofj@...omium.org
Subject: Re: [PATCH 09/12] Input: synaptics - add image sensor support

> The evdev protocol is discrete, not continuous, so it's theoretically
> possible that one touch could end at the same time that another begins.
> IIRC, the time resolution of some MT devices makes this completely
> possible, perhaps to the point that it would be trival to make this
> happen in a few seconds of trying.
> 
> I think the MT-B protocol has only ever used tracking_id for signalling
> touch begin and touch end, where the id goes from -1 to something else
> and vice versa. Maybe the protocol could be "extended" by saying that a
> transition from one valid id to another means there is inconsistent
> touch state, but the old touch hasn't necessarily ended and the new
> touch hasn't necessarily started at this point in time.

The in-flight change of tracking id is actually part of the design; it
makes the protocol independent of sample rate. If a particular
tracking id is no longer found in any slot, that touch has ended.

> I'm not sure this is any easier than flagging these as bad devices
> because now we need to watch tracking ID changes on top of touch count
> changes. From someone who has attempted to implement semi-mt in X
> synaptics, adding more complexity here should be avoided at all cost :).

The information available in the proposition suffices to determine
what the device is. Surely the method of transferring that information
will not have any impact on the extra code required.

> > I'll ask this - how much realistically do we care about 3+ finger
> > transitions in context of these particular devices? This is a touchpad
> > so as long as basic 2 finger gestures work (zoom, pinch, 2-finger
> > scroll) with Synaptics X driver we should be fine. I do not want to add
> > all kinds of custom flags to the protocol to deal with this generation
> > of touchpads.
> 
> I've given up on trying to send semi-mt data through the proposed XInput
> 2.1 multitouch protocol. I think the best option is to send all this
> data as valuators of a single touch (a touch and not a traditional
> pointer event due to the multitouch/gesture semantics). Thus, we should
> be focusing on what is possible in the gesture realm since we have
> thrown full multiouch out the window for these devices.
> 
> With these devices we can support one touch drag, two touch pinch,
> rotate, and zoom, and 1-5 touch tap. For these to work, we need to know
> the number of touches at any given time, the locations of the two
> touches when only two touches are active, and some representative
> location for the 1 and 3-5 touch cases.

Right, and we do, so there is no problem there, is there?

> I am sitting here writing possible solutions trying to come up with sane
> ways to handle all this, but every 5 minutes I erase what I came up with
> and start over because you only see the problems once you've analysed
> every scenario. I can't see any way to cater for these devices aside
> from: handle them as single touch because they suck, or something
> similar to what has been described in the past few hours.
> 
> > It sounds to me like latest generation of Synaptocs protocol is a dud
> > and hopefully they will fix it to something more flexible in the next
> > generationof chips...
> 
> We can only hope. In the meantime, it looks like Google is pushing to
> use these devices on reference designs for Chrome OS, and big vendors
> like Dell are perfectly happy to ship Ubuntu with the 100 times worse
> (because we don't know their protocol) ALPS devices. Waiting for sanity
> to win out seems like a lost cause to me :(.

Let us bide our time and see.

Cheers,
Henrik
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