Re: [PATCH 1/2] HID: multitouch: report MT_TOOL_PALM for non-confident touches

[Dmitry Torokhov]

On Mon, Jun 04, 2018 at 03:18:12PM +0200, Benjamin Tissoires wrote:
> On Fri, Jun 1, 2018 at 8:43 PM, Dmitry Torokhov
> <dmitry.torokhov@xxxxxxxxx> wrote:
> > On Fri, Jun 01, 2018 at 04:16:09PM +0200, Benjamin Tissoires wrote:
> >> On Fri, Aug 11, 2017 at 2:44 AM, Dmitry Torokhov
> >> <dmitry.torokhov@xxxxxxxxx> wrote:
> >> > According to Microsoft specification [1] for Precision Touchpads (and
> >> > Touchscreens) the devices use "confidence" reports to signal accidental
> >> > touches, or contacts that are "too large to be a finger". Instead of
> >> > simply marking contact inactive in this case (which causes issues if
> >> > contact was originally proper and we lost confidence in it later, as
> >> > this results in accidental clicks, drags, etc), let's report such
> >> > contacts as MT_TOOL_PALM and let userspace decide what to do.
> >> > Additionally, let's report contact size for such touches as maximum
> >> > allowed for major/minor, which should help userspace that is not yet
> >> > aware of MT_TOOL_PALM to still perform palm rejection.
> >> >
> >> > An additional complication, is that some firmwares do not report
> >> > non-confident touches as active. To cope with this we delay release of
> >> > such contact (i.e. if contact was active we first report it as still
> >> > active MT+TOOL_PALM and then synthesize the release event in a separate
> >> > frame).
> >>
> >> I am not sure I agree with this part. The spec says that "Once a
> >> device has determined that a contact is unintentional, it should clear
> >> the confidence bit for that contact report and all subsequent
> >> reports."
> >> So in theory the spec says that if a touch has been detected as a
> >> palm, the flow of events should not stop (tested on the PTP of the
> >> Dell XPS 9360).
> >>
> >> However, I interpret a firmware that send (confidence 1, tip switch 1)
> >> and then (confidence 0, tip switch 0) a simple release, and the
> >> confidence bit should not be relayed.
> >
> > This unfortunately leads to false clicks: you start with finger, so
> > confidence is 1, then you transition the same touch to palm (use your
> > thumb and "roll" your hand until heel of it comes into contact with the
> > screen). The firmware reports "no-confidence" and "release" in the same
> > report and userspace seeing release does not pay attention to confidence
> > (i.e. it does exactly "simple release" logic) and this results in UI
> > interpreting this as a click. With splitting no-confidence
> > (MT_TOOL_PALM) and release event into separate frames we help userspace
> > to recognize that the contact should be discarded.
> 
> After further thoughts, I would consider this to be a firmware bug,
> and not how the firmware is supposed to be reporting palm.
> For the precision touchpads, the spec says that the device "should
> clear the confidence bit for that contact report and all subsequent
> reports.". And it is how the Dell device I have here reports palms.
> The firmware is not supposed to cut the event stream.
> 
> There is a test for that:
> https://docs.microsoft.com/en-us/previous-versions/windows/hardware/hck/dn456905%28v%3dvs.85%29
> which tells me that I am right here for PTP.
> 
> The touchscreen spec is blurrier however.

OK, that is great to know.

> 
> >
> >>
> >> Do you have any precise example of reports where you need that feature?
> >
> > It was observed on Pixelbooks which use Wacom digitizers IIRC.
> 
> Pixelbooks + Wacom means that it was likely a touchscreen. I am right
> guessing the device did not went through Microsoft certification
> process?

That would be correct ;) At least the firmware that is shipping with
Pixlebooks hasn't, I do now if anyone else sourced these Wacom parts for
their MSWin devices.

> 
> I am in favor of splitting the patch in 2. One for the generic
> processing of confidence bit, and one for this spurious release. For
> the spurious release, I'm more in favor of explicitly quirking the
> devices in need of such quirk.

Hmm, I am not sure about having specific quirk. It will be hard for
users to accurately diagnose the issue if firmware is broken in this way
so we could add a new quirk for a new device.

> 
> If you agree, I'll rebase your patch on top of my series as rebasing
> my series on top of yours will take more effort.

That would be great.

> 
> I am trying to be cautious in the generic path because I want to merge
> the cleanest multitouch implementation in hid-core/hid-input, and
> leave all the quirks in hid-multitouch for the devices in need.
> 
> Cheers,
> Benjamin
> 
> >
> > Thanks.
> >
> > --
> > Dmitry

Thanks.

-- 
Dmitry