linux-kernel - Re: [PATCH v11 1/4] HID: wacom_sys: Add support for flipping the data values

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Message-ID: <CAKmqyKN0fhJOGZwg6LKag=8fVsANBUNg39Gfr5qdJyUJu1AVrw@mail.gmail.com>
Date:   Wed, 20 Oct 2021 21:34:20 +1000
From:   Alistair Francis <alistair23@...il.com>
To:     Benjamin Tissoires <benjamin.tissoires@...hat.com>
Cc:     Dmitry Torokhov <dmitry.torokhov@...il.com>,
        Ping Cheng <pinglinux@...il.com>,
        Alistair Francis <alistair@...stair23.me>,
        Shawn Guo <shawnguo@...nel.org>,
        Sascha Hauer <s.hauer@...gutronix.de>,
        dl-linux-imx <linux-imx@....com>, Jiri Kosina <jikos@...nel.org>,
        linux-input <linux-input@...r.kernel.org>,
        devicetree <devicetree@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v11 1/4] HID: wacom_sys: Add support for flipping the data values

On Wed, Oct 20, 2021 at 5:40 PM Benjamin Tissoires
<benjamin.tissoires@...hat.com> wrote:
>
> On Wed, Oct 20, 2021 at 4:14 AM Dmitry Torokhov
> <dmitry.torokhov@...il.com> wrote:
> >
> > On Wed, Oct 20, 2021 at 11:44:50AM +1000, Alistair Francis wrote:
> > > On Wed, Oct 20, 2021 at 11:05 AM Dmitry Torokhov
> > > <dmitry.torokhov@...il.com> wrote:
> > > >
> > > > On Wed, Oct 20, 2021 at 09:33:13AM +1000, Alistair Francis wrote:
> > > > > On Tue, Oct 19, 2021 at 11:51 AM Dmitry Torokhov
> > > > > <dmitry.torokhov@...il.com> wrote:
> > > > > >
> > > > > > We already have touchscreen-inverted-x/y defined in
> > > > > > Documentation/devicetree/bindings/input/touchscreen/touchscreen.yaml,
> > > > > > why are they not sufficient?
> > > > >
> > > > > The touchscreen-* properties aren't applied to HID devices though, at
> > > > > least not that I can tell.
> > > >
> > > > No, they are not currently, but that does not mean we need to establish
> > > > a new set of properties (property names) for HID case.
> > >
> > > I can update the names to use the existing touchscreen ones.
> > >
> > > Do you have a hint of where this should be implemented though?
> > >
> > > Right now (without "HID: wacom: Add support for the AG14 Wacom
> > > device") the wacom touchscreen is just registered as a generic HID
> > > device. I don't see any good place in hid-core, hid-input or
> > > hid-generic to invert the input values for this.
> >
> > I think the transformation should happen in
> > hid-multitouch.c::mt_process_slot() using helpers from
> > include/linux/input/touchscreen.h
> >
> > I think the more challenging question is to how pass/attach struct
> > touchscreen_properties * to the hid device (i expect the properties will
> > be attached to i2c-hid device, but maybe we could create a sub-node of
> > it and attach properties there.
> >
>
> Sorry but I don't like that very much. This would mean that we have an
> out of band information that needs to be carried over to
> HID-generic/multitouch and having tests for it is going to be harder.
> I would rather have userspace deal with the rotation if we do not have
> the information from the device itself.

My 2c below

>
> Foreword: I have been given a hammer, so I see nails everywhere.
>
> The past 3 weeks I have been working on implementing some eBPF hooks
> in the HID subsystem. This would IMO be the best solution here: a udev
> hwdb rule sees that there is the not-wacom PID/VID (and maybe the
> platform or parses the OF properties if they are available in the

I'm not sure we have a specific VID/PID to work with here. The VID is
generic AFAIK, not sure about the PID though. Maybe someone from Wacom
could confirm either way.

> sysfs) and adds a couple of functions in the HID stack to rotate the
> screen. The advantage is that we do not need to add a new kernel API

I would say that touchscreen-inverted-x/y isn't a new API, it's
commonly used. To me it makes sense that it's supported for all
touchscreens.

> anymore, the disadvantage is that we need userspace to "fix" the
> kernel behaviour (so at boot, this might be an issue).

That's a pain for me. I'm still stuck with the vendors userspace as I
need their propiritory eInk driver code. It also seems like a hassle
for different distros to handle this (compared to just in the DT).

Alistair

>
> I am not at the point where I can share the code as there is a lot of
> rewriting and my last attempt is resulting in a page fault, but I'd be
> happy to share it more once that hiccup is solved.
>
> Cheers,
> Benjamin
>