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

[Peter Hutterer]

On Mon, Jun 04, 2018 at 02:19:44PM -0700, Dmitry Torokhov wrote:
> On Mon, Jun 04, 2018 at 10:42:31PM +0200, Benjamin Tissoires wrote:
> > On Mon, Jun 4, 2018 at 7:33 PM, Dmitry Torokhov
> > <dmitry.torokhov@xxxxxxxxx> wrote:
> > > 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.
> > 
> > One thing we can do is keep the quirked mechanism as default in
> > hid-multitouch, but remove it in hid-core. If people need the quirk,
> > they can just use hid-multitouch instead (talking about the long run
> > here).
> 
> Hmm, I am confused. My patch did not touch hid-core or hid-input, only
> hid-multitouch... So we are already doing what you are proposing?..
> 
> > 
> > However, I really believe this might only be required for a handful of
> > devices, and probably only touchscreens. So I would be tempted to not
> > make it default and see how many bug reports we have.
> 
> Up to you but it is hard to detect for users. If just sometimes there
> are stray clicks...

fwiw, from my POV, if you give me MT_TOOL_PALM in the same frame as the
ABS_MT_TRACKING_ID -1 I can work that into libinput to do the right thing.
Not 100% whether that already works anyway but probably not. I'd prefer it
being fixed in the kernel though, less work for me :)

Cheers,
   Peter