linux-kernel - Re: [PATCH 09/12] Input: synaptics

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Date:	Wed, 6 Jul 2011 21:31:20 +0200
From:	"Henrik Rydberg" <rydberg@...omail.se>
To:	Dmitry Torokhov <dmitry.torokhov@...il.com>
Cc:	Daniel Kurtz <djkurtz@...omium.org>, 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 believe there is a strong userspace assumption that BTN_TOOL_* has
> > no meaning for real MT devices. Rightfully so, IMO. Hence, I think
> > semi-mt needs to be used here as well.
> 
> I think we need to adjust userspace to pay attention to BTN_TOOL_* for
> MT-B too so that if number of slots advertised does not match
> BTN_TOOL_* capabilities that means that the device does not privide
> tracking data for all contacts.

Well, it is possible, but it is a great deal more complex than just
looking at what the slots contain. At least we should all be able to
agree that MT-B is sufficient for any "proper" MT device.

> Luckily this should be backward-compatible (i.e. older userspace will
> ignore "extended" fingercounts, newer will pay attention to it).

OTOH, letting semi-mt engulf all devices which requires the use of
BTN_TOOL_* for finger count makes it easier to differentiate between
various userspace support levels. "This app supports pure MT-B only",
etc.

> > > This is the best option in my opinion. We will present 2 finger position
> > > data plus extended finger count.
> > 
> > We never did put all the details of the bounding box coordinates in
> > writing, so perhaps this is an opportunity to both fix that and extend
> > usability to the case so described. The only question is whether there
> > are applications out there which now assume min/max instead of contact
> > positions. If anyone knows, please speak up. :-) Otherwise, I am very
> > much for Daniel's case C, with Dmitry's modification.
> > 
> > In short: Use the semi-MT property, and send two suitable fingers
> > along with it.
> 
> Umm... but it is my understanding that 2 fingers will provide real
> tracking data, not bounding box, so why would we set semi-MT?

To indicate that a) the two positions may not represent true fingers
but a bounding box, and b) the contact count is determined by
BTN_TOOL_*.

True, there is no way to distinguish between the real-fingers and
bounding-box cases here (that is why I suggested another binary value
in a previous mail), but without semi-mt, there is no way to know a
priori if special logic is needed for the number of fingers.

> Maybe we have different notions of what semi-MT property conveys? For me
> semi-MT indicates that the device provides 2 coordinates for bounding
> box.  However if semi-MT is not set does not mean that the device
> provides true tracking for all contacts, but only for advertised slots.
> There still may be additional data transmitted.

Yes, it seems we do have different assumptions here. The more reason
to document it further. :-)

To me, it seems we do need a little bit of extra information to
determine this new type of device.

Thanks,
Henrik
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