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From:   Benjamin Tissoires <benjamin.tissoires@redhat.com>
Date:   Mon, 4 Jun 2018 22:42:31 +0200
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Subject: Re: [PATCH 1/2] HID: multitouch: report MT_TOOL_PALM for
 non-confident touches
To:     Dmitry Torokhov <dmitry.torokhov@gmail.com>
Cc:     Jiri Kosina <jikos@kernel.org>,
        Henrik Rydberg <rydberg@bitmath.org>,
        Jason Gerecke <killertofu@gmail.com>,
        Dennis Kempin <denniskempin@google.com>,
        Andrew de los Reyes <adlr@google.com>,
        "open list:HID CORE LAYER" <linux-input@vger.kernel.org>,
        lkml <linux-kernel@vger.kernel.org>
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On Mon, Jun 4, 2018 at 7:33 PM, Dmitry Torokhov
<dmitry.torokhov@gmail.com> 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@gmail.com> 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@gmail.com> 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).

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.

>
>>
>> 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.

\o/

Cheers,
Benjamin

>
>>
>> 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