Message-ID: <20110705192759.GA29549@polaris.bitmath.org>
Date:	Tue, 5 Jul 2011 21:27:59 +0200
From:	"Henrik Rydberg" <rydberg@...omail.se>
To:	Daniel Kurtz <djkurtz@...omium.org>
Cc:	dmitry.torokhov@...il.com, chase.douglas@...onical.com,
	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

> > I guess the real question here is: do the following patches really
> > help? Creating additional logic to band-aid yet another special case,
> > which still does not give full MT support, seems to create more
> > problems than it solves. If the code was needed to ensure proper five
> > finger support to userspace, then maybe one could live with
> > it. However, as it stands, keeping the semi-mt behavior also for the
> > slightly better devices may not be such a bad idea, after all.
> >
> > _Iff_ the whole series can be formulated as true protocol B support
> > (no special flags, please), and _iff_ it helps to use software finger
> > tracking for less than four fingers, then please do tell, and we can
> > add that part to the input core to simplify the synaptics
> > implementation a bit.
> 
> Image sensors and profile sensors behave differently.  However, even
> the image sensors do not allow true MT-B, since they only report 2
> fingers.  Hence, it makes sense to add a property to distinguish the 3
> cases.

The lifetime of such a solution should also be considered.

> This implementation addresses the following issues:
> 
>   (1) Improves handling of the 2-finger case.  Image sensors due allow
> true MT-B for two finger case.  This, for example, allows detecting
> whether the click in a click+drag is happening in bottom right or
> bottom left quadrant of the pad.

In principle, this could be done within the semi-mt realm by adding a
binary value to the protocol, distinguishing the diagonals of the
bounding box. It would be ugly too, but at least it would be
compatible with the current semi-mt effort in userspace.

>   (2) Helps improve handling of number-of-fingers transitions.  Most
> of the complexity of the synaptics driver comes with dealing with the
> complicated way that the protocol handles number-of-fingers
> transitions.  Just ignoring them with semi-mt could cause lots of
> jumpy behavior as the transitions are mapped to bounding boxes whose
> coordinates jump around.  Thus, this implementation tries to notify
> userspace of finger transition by 'rolling slots' when necessary.

If the kernel driver cannot create a smooth bounding box, which by all
means is simpler than providing proper finger tracking, then there is
no way this can be done in userspace either.

> What I mean is, if the driver deduces that a given slot may not
> contain the same finger as a previous report, it releases it (sets its
> tracking_id = -1), and reestablishes the slot with a new tracking_id
> when it is more confident that it is now tracking a new finger.

As a general question, one might ask oneself: If the new device
_really_ can track five fingers, then why does it not send enough
information to recover that information? A proper tracking id would
suffice.

>   (3) Properly decode the "AGM_CONTACT" packet type.  4- and 5- finger
> gestures may never be supported, however, I think it is still a good
> idea to detect and parse these packets properly in the kernel driver,
> and leave policy to userspace.  I'm open to alternative suggestions
> for ways to represent this to userspace.  The idea in this patch set
> is to reuse the MT-B plumbing as much as possible, but use the device
> property to mark the fact that the interpretation of the resulting
> slots is somewhat different.

If the data cannot be reliably utilized, I doubt anyone cares.  There
is a lot of hardware out there capable of tracking ten and more
fingers, without the agonizing pain.

> A bare minimum patchset might do:
>    (1) Do true MT-B for two fingers
>    (2) Improved number-of-finger tracking for just the 2-finger case.
> (Keep tracking finger 1 when finger 2 is removed, and vice versa)

In what way is this different from 1)?

>    (3) Properly parse, but don't use, AGM_CONTACT packets

I am tempted to write something about Schrödinger's cat here... ;-)

Thanks,
Henrik
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