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

From: Benjamin Tissoires <benjamin.tissoires@redhat.com>
To: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Cc: Alistair Francis <alistair23@gmail.com>,
	Ping Cheng <pinglinux@gmail.com>,
	Alistair Francis <alistair@alistair23.me>,
	Shawn Guo <shawnguo@kernel.org>,
	Sascha Hauer <s.hauer@pengutronix.de>,
	dl-linux-imx <linux-imx@nxp.com>, Jiri Kosina <jikos@kernel.org>,
	linux-input <linux-input@vger.kernel.org>,
	devicetree <devicetree@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	linux-arm-kernel <linux-arm-kernel@lists.infradead.org>
Subject: Re: [PATCH v11 1/4] HID: wacom_sys: Add support for flipping the data values
Date: Wed, 20 Oct 2021 09:40:22 +0200	[thread overview]
Message-ID: <CAO-hwJKSxVFAiAriWU0No7sFxzo9XB1-T9LFeF5Zn27B8erFPA@mail.gmail.com> (raw)
In-Reply-To: <YW97lwsMrLHetJGy@google.com>

On Wed, Oct 20, 2021 at 4:14 AM Dmitry Torokhov
<dmitry.torokhov@gmail.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@gmail.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@gmail.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.

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

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