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

[Benjamin Tissoires]

On Mon, Jun 4, 2018 at 11:19 PM, Dmitry Torokhov
<dmitry.torokhov@xxxxxxxxx> 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?..

It's the long run solution. I am trying in my last series to
streamline hid-multitouch so I can merge it in hid-core. I am planning
on having a few revision with hid-multitouch as an external module,
and then switch the win 8 (touchscreens and PTPs) devices to use a
generic processing in hid-input.c. So that's why I'd rather keep the
quirked devices separately and iron out the generic ones. It's easier
to override hid-multiouch for users than it is to override hid.ko.

>
>>
>> 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...

Users might not directly target the bugs toward me, but will likely
complain to Peter first :)
We can also add some automatic sanity check in libinput to relay the
information that there is a FW/kernel bug for such devices.

Cheers,
Benjamin

>
> Thanks.
>
> --
> Dmitry