linux-kernel - Re: [PATCH v2 08/11] HID: hid-multitouch: fix Win 8 protocol

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Message-ID: <20121104205459.GA777@polaris.bitmath.org>
Date:	Sun, 4 Nov 2012 21:54:59 +0100
From:	"Henrik Rydberg" <rydberg@...omail.se>
To:	Benjamin Tissoires <benjamin.tissoires@...il.com>
Cc:	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	Jiri Kosina <jkosina@...e.cz>,
	Stephane Chatty <chatty@...c.fr>,
	linux-input <linux-input@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 08/11] HID: hid-multitouch: fix Win 8 protocol

Hi Benjamin,

> The goal of this patch is to implement in a reliable way Win 8
> multitouch protocol (to avoid quirking many devices). Thanks to the
> precision they made in the specification, I think it is feasible:
> they add the dynamic part that were missing in Win 7 spec:
> """
> When sending data in hybrid or parallel mode, a contact that is
> delivered in one report must be delivered in all subsequent reports
> until it is lifted off the screen. If time is needed to adequate
> determine if the contact was lifted off the surface, the device must
> report the last known position of the contact and then deliver the
> “UP” state of the contact in a subsequent report. Devices should not
> send a report without the information for that contact while trying to
> determine its current state.
> """

The text seems to say that devices are not required to send touch
state information in a separate frame, but if the device needs time to
determine the touch state, the touch properties should stay the same
during that time.

> Thus, the quirk ALWAYS_VALID fits very well with win 8 devices (the
> device has to send the touch until it is lifted and out of range, and
> the device must send the 'up' state).

One could simply add another quirk which fits the win8 case exactly
instead. No need to change the existing one.

> The problem lies that some devices may reuse contact id 0 within the
> frame for the end of the report (my Win8 device doesn't has this kind
> of problem):
> 
> With the following hid usages:
> I -> contact Id
> T -> tip switch
> X, Y -> X, Y
> S -> scan time
> C -> contact count
> 
> a friendly device would report:
> 
> I:1 T:1 X:125 X:130 Y:750 Y:755 I:0 T:0 X:0 X:0 Y:0 Y:0 S:0 C:01
> I:1 T:1 X:130 X:135 Y:750 Y:755 I:0 T:0 X:0 X:0 Y:0 Y:0 S:1 C:01
> I:1 T:1 X:135 X:140 Y:750 Y:755 I:0 T:0 X:0 X:0 Y:0 Y:0 S:2 C:01
> I:1 T:1 X:140 X:145 Y:750 Y:755 I:0 T:0 X:0 X:0 Y:0 Y:0 S:3 C:01
> I:1 T:0 X:140 X:145 Y:750 Y:755 I:0 T:0 X:0 X:0 Y:0 Y:0 S:4 C:01
> 
> *but*, I've already seen win 7 devices, that do send:
> 
> I:0 T:1 X:125 X:130 Y:750 Y:755 I:0 T:0 X:0 X:0 Y:0 Y:0 S:0 C:01
> I:0 T:1 X:130 X:135 Y:750 Y:755 I:0 T:0 X:0 X:0 Y:0 Y:0 S:1 C:01
> I:0 T:1 X:135 X:140 Y:750 Y:755 I:0 T:0 X:0 X:0 Y:0 Y:0 S:2 C:01
> I:0 T:1 X:140 X:145 Y:750 Y:755 I:0 T:0 X:0 X:0 Y:0 Y:0 S:3 C:01
> I:0 T:0 X:140 X:145 Y:750 Y:755 I:0 T:0 X:0 X:0 Y:0 Y:0 S:4 C:01

I see, more brain-damaged usage. :-) Still, there seems to be a
simpler way to distinguish this case: if there are more than one touch
with the same id in the frame, use the one with T=1.

> The difference lies in the first bit, contact id is 0.
> 
> So, the quirk always valid is not sufficient because the second touch
> in the frame will override the values of the first (the valid one).
> 
> As Microsoft says that "the device must report the last known position
> of the contact and then deliver the “UP” state of the contact", so we
> can safely discard the second touch because X and Y do not match the
> current state of the valid touch.
> 
> Then, as exposed in the "How to Design and Test Multitouch Hardware
> Solutions for Windows 8" document here:
> http://msdn.microsoft.com/en-us/library/windows/hardware/hh872968.aspx
> when the device attempt the certification, if the "up" is not valid,
> the error "Last move location different" raises, which, I hope will
> prevent the device to get the certification.

I think it would be too fragile to rely on this assumption. Hopefully
the suggestion above will work equally well.

Thanks,
Henrik
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