linux-kernel - Re: [PATCH v7 3/3] Input: add driver for the Hycon HY46XX touchpanel series

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Message-ID: <YHfaP4lc1ZDWeBqb@jelly>
Date:   Thu, 15 Apr 2021 16:16:31 +1000
From:   Peter Hutterer <peter.hutterer@...-t.net>
To:     Dmitry Torokhov <dmitry.torokhov@...il.com>
Cc:     Giulio Benetti <giulio.benetti@...ettiengineering.com>,
        Rob Herring <robh@...nel.org>,
        Jonathan Neuschäfer <j.neuschaefer@....net>,
        Henrik Rydberg <rydberg@...math.org>,
        linux-input@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v7 3/3] Input: add driver for the Hycon HY46XX touchpanel
 series

On Wed, Apr 14, 2021 at 10:26:13AM -0700, Dmitry Torokhov wrote:
> Hi Giulio, Peter,
> 
> On Wed, Apr 14, 2021 at 01:22:55PM +0200, Giulio Benetti wrote:
> > Hi Peter, Dmitry,
> > 
> > On 4/14/21 8:46 AM, Peter Hutterer wrote:
> > > On Tue, Apr 13, 2021 at 10:44:07PM -0700, Dmitry Torokhov wrote:
> > > > Hi Giulio,
> > > > 
> > > > On Tue, Apr 13, 2021 at 04:44:46PM +0200, Giulio Benetti wrote:
> > > > > +
> > > > > +	input_mt_report_pointer_emulation(tsdata->input, true);
> > > > 
> > > > For touchscreens it does not make much sense to report BTN_DOUBLETAP,
> > > > BTN_TRIPLETAP, etc, events (they are really for touchpads), so I changed
> > > > this to
> > > > 
> > > > 	input_mt_report_pointer_emulation(tsdata->input, false);
> > > > 
> > > > to only report ABS_X, ABS_Y, and BTN_TOUCH, and applied.
> > > 
> > > Can you expand on this please, just to make sure I'm not misinterpreting
> > > those codes? Those bits are just for how many fingers are down (but without
> > > position), dropping those bits means you restrict the device to a pure
> > > single-touch screen. Or am I missing something here?
> 
> They are indeed represent number of fingers on the surface. I think I
> over-simplified this a bit by saying these events are only for
> touchpads, as there is allowance for BTN_TOOL_*TAP in
> Documentation/input/multi-touch-protocol.rst for MT devices that may
> report more contacts than what they can distinctly track, and it is
> not restricted to touchpads (but I believe in reality only used by a
> couple of "semi-MT" touchpad drivers).

fwiw, almost all touchpads on ps2 use that functionality - they can track 2
touchpoints but *detect* up to 5. There's significant insanity in libinput
to deal with that because it is so common :)

semi-mt is orthogonal to that, it's the an inability to track two
touchpoints correctly (only get top-left and bottom-right, but it's
guesswork which finger is in which corner).

> What I meant to say that for ST fallback of MT-capable devices we
> typically provide BTN_TOOL_*TAP for devices declared as INPUT_MT_POINTER
> (which is touchpads) and for INPUT_MT_DIRECT and others we only provide
> ABS_X, ABS_Y, ABS_PRESSURE and BTN_TOUCH (see input_mt_sync_frame()),
> and I think this driver should follow the suit.

ah, right. that makes sense, thanks for the clarification.

Cheers,
   Peter