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From:   Benjamin Tissoires <benjamin.tissoires@redhat.com>
Date:   Tue, 9 Nov 2021 18:02:17 +0100
Message-ID: <CAO-hwJL0keXdmRMTU6HHMBmaCROrR2jhoT_Z68dqa2xwbFOOpg@mail.gmail.com>
Subject: Re: [RFC 0/8] HID: add support for USI style pens
To:     Tero Kristo <tero.kristo@linux.intel.com>
Cc:     Jiri Kosina <jikos@kernel.org>,
        Mika Westerberg <mika.westerberg@linux.intel.com>,
        "open list:HID CORE LAYER" <linux-input@vger.kernel.org>,
        lkml <linux-kernel@vger.kernel.org>,
        Dmitry Torokhov <dmitry.torokhov@gmail.com>,
        Peter Hutterer <peter.hutterer@who-t.net>
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Hi Tero,

On Mon, Nov 8, 2021 at 8:44 AM Tero Kristo <tero.kristo@linux.intel.com> wrote:
>
> Hi Benjamin,
>
> Thanks for your feedback! Couple of quick replies below.
>
> On 05/11/2021 20:22, Benjamin Tissoires wrote:
> > Hi Tero,
> >
> > [just a quick note, I am supposed to be on holiday this week]
> >
> > On Thu, Nov 4, 2021 at 2:38 PM Tero Kristo <tero.kristo@linux.intel.com> wrote:
> >> Hi,
> >>
> >> This series is an RFC for USI (Universal Stylus Interface) style pen
> >> support. This is based on documentation from USB org describing the HID
> >> usage tables for digitizers (page 0x0D) and experimentation with actual
> >> USI capable controllers.
> >>
> >> This series introduces the USI support with a new HID driver, which
> >> applies the controller specific quirks. The most problematic part of the
> >> USI support is handling of the pen parameters (color, line width, line
> >> style), which are not immediately available from the controller from pen
> >> down event, but must be cached and queried separately from the controller.
> >> In addition to that, when a get-feature report is sent to the
> >> controller, there is a delay before the proper value is reported out; it
> >> is not part of the feature report coming back immediately.
> >> Most of the code in the driver is to handle this (otherwise we could
> >> just use hid-generic.)
> >>
> >> This also boils down to the reason why this series is an RFC, I would like
> >> to receive some feedback which option to pick for programming of the new
> >> values for the programmable pen parameters; whether to parse the input
> >> events so userspace can directly write the new values to the input event
> >> file handle, or whether to use IOCTL. Patches #7 / #8 are sort of optional
> >> choices towards this, but are there to show that both approaches can be
> >> done. Direct write to evdev causes some confusion on the driver level
> >> though, thus patch #7 is there to avoid some of that introducing new
> >> input events for writing the parameters. IOCTL might be the cleanest
> >> approach and I am slightly leaning towards that myself (see patch #8,
> >> this would need to be squashed and cleaned up a bit though but would
> >> effectively get rid of some code from patch #6 and completely rid patch #7.)
> > This series unfortunately raised quite a few red flags for me, and I
> > am glad this is just an RFC.
> > Let me enumerate them first and discuss a little bit more about those:
> >   1. USI is supposed to be generic, so why is there a new driver for it
> > instead of being handled by hid-input.c?
> >   2. new MSC_EVENTS are created without Dmitry or Peter being CC-ed
> >   3. new ioctls???
> >   4. direct write to evdev to write parameters
> >   5. patch 1/8 doesn't compile without 5/8
> >   6. no tests :)
> >
> > 1. new driver
> > After quickly reading the RFC, I think the main issue there is that we
> > are now having a transducer index which is incompatible with the way
> > input and evedev works nowadays. Yay, we have a new hid-multitouch for
> > pen :(
> >
> > Wacom has been dealing with that situation for years by tweaking the
> > protocol and by just emitting a different serial number (roughly). I
> > think the safest approach would be to keep the existing protocol
> > running so that our user space can handle it properly.
> >
> > I'd need to read the rest of the code more carefully, but if we could
> > have a basic generic handling (without the fancy features like
> > changing the pen style/color) I'd be happier.
>
> The USI pen support is partially compatible with existing input
> framework, e.g. co-ordinates + touch events work out of box with
> no-modification to the kernel whatsoever, just by using hid-generic
> driver. What is missing completely is the new features;
> color/width/style. It would be possible to move all these to the
> hid-input driver obviously, I don't think there is anything to prevent
> that. And, I could split up the series so that in the initial patches we
> would only support reporting current color/width/style parameters, and
> add the programmability as a subsequent patch if that would be better.
> It would also be possible to move parts of the code to the input
> subsystem from HID (some initial patches from our side were done this
> way, but I don't think this is necessary.)

I think I'd like to see the common behavior in hid-input.c, yes. This
is mostly because other drivers will be able to use that for free
(hid-multitouch for devices that are both touch and pen capable for
instance).

We can deal with the programmability from userspace later.

As for merging part of the code from HID to input, I'd like to see
what you are talking about.

>
> >
> > 2. MSC_* events
> > there is an issue with those: they are not cached like the ABS_* ones.
> > Meaning that each report will wake up userspace for something which
> > basically doesn't change.
> > I know ABS_* is saturated, but I'd like to have reviews from others on
> > what could be done here instead of just using MSC_* as a new ABS_*
> In my tests, it seems like MSC_* from the USI pens with these patches
> only get reported to userspace if something changes. Otherwise they do
> not get through at all. I have a small quirk in the driver to address
> this for a case where a new userspace handle is opened while pen is
> already in use; it does not get the params reported at all unless I
> flush the current entries out (by reporting a bogus value followed
> immediately by the real value.) Anyways, ABS events would be fine for
> the parameters also I believe if this is desirable.

That is by design. When a userspace program opens a device node, it
has to query its current state by running a few ioctls.
We have libevdev for that that simplifies everything for the userspace.
libevdev does a lot more than just that, and every userspace program
should use it to not have to deal with SYN_DROPPED and other
subtleties in the protocol.

> >
> > 3. ioctls
> > this is problematic to me. Any new kernel ABI is problematic to me,
> > and I'd much rather not add any new ones.
> I agree I am torn between the ioctl / direct evdev write. Both have
> their bad sides to them, thus I provided sort of support for both. :)
> >
> > My new set of mind is because of the recent work I have been
> > conducting regarding eBPF.
> > Basically I managed to have eBPF programs handling the device
> > configuration and event processing in a local branch.
> > I should be able to push a WIP next week, but basically this should
> > allow me to not have to deal with new kernel APIs besides the generic
> > eBPF one.
> > We can imagine a generic hid-input.c processing for those tablets, and
> > have a new userspace component that loads an eBPF program with its own
> > userspace API which is capable of the fancy features.
> >
> > For instance, my current playground is setting the haptic feedback of
> > the Surface Dial depending on the resolution I set on it.
> >
> > Furthermore, ioctls on a new cdev means that the classic userspace
> > libraries will not have access to it without some heavy tuning in the
> > systemd space (libinput only has read/write access to
> > /dev/input/event*).
>
> So, you are saying it would be possible to use this to support the USI
> pen parameters also somehow? I can take a look at this once available.

Yeah, basically once you load the ebpf program in the kernel, you have
a shared memory with userspace that you can use to create your own
API.
You could even ditch entirely the input subsystem and do the
processing in the eBPF program and pull the data from that shared
memory.

The way I see is the following:
- hid-tools (or maybe an other repo, not sure there) is responsible
for holding the list of bpf programs that we need to maintain (it's
the new upstream for those kind of quirks)
- we have one or more userspace program to load those eBPF program as
an intrinsic in udev
- those userspace program could be a one shot (attach a bpf program,
pin it and return)
- but userspace program could also present any RPC mechanism (dbus,
unix pipe, etc)

The advantage here is that we can control the API from the userspace:
if we do not use it anymore, we can just ditch the eBPF program (or
not load it). We can also rely on classic lib versioning or dbus
versioning.

I just pushed a branch "hid-bpf-2" at
https://gitlab.freedesktop.org/bentiss/hid/-/commits/hid-bpf-2 with
the first examples. The interesting commits are between the tip and
the `build` branch.

>
> >
> > 4. direct write to evdev
> >
> > We enabled that once for LEDs, and it's a pain to maintain. Maybe we
> > can make a use case for it but given that you don't seem very
> > enthusiastic about it too, I wonder if this is not a dead end.
> Well, we need to be able to program the pen parameters somehow from
> userspace, I am open to any suggestions how this could/should be done.
> >
> > 5. patch ordering doesn't compile
> > I guess this is just a rebase hiccup. Not an issue for an RFC
> Yeah sorry about that. Will fix all those for next rev.
> >
> > 6. tests
> > For these kinds of new classes of devices, I'd like to have tests in
> > the https://gitlab.freedesktop.org/libevdev/hid-tools repository.
> > There is already an initial MR for tablet support (!115 in this
> > project), and we should extend it with more tests.
> Ok I can take a look at these, thanks for the pointer. I am quite new to
> the input side of things and have been using whatever I have been able
> to craft myself, or found via googling.
> >
> > I'd happily help with those tests if you could share the report
> > descriptors and some device dumps made with the hid-recorder tool from
> > that repository.
>
> Yeah, I can share these in your preferred format once I figure out the
> test tools. I have been using some custom tools to parse things myself
> so far (mostly kernel traces for both HID + I2C subsystem where my USI
> controller is connected.)

Maybe just add a new test in
https://gitlab.freedesktop.org/libevdev/hid-tools/-/blob/master/tests/test_tablet.py
like https://gitlab.freedesktop.org/libevdev/hid-tools/-/commit/7fa34c2c86e1380eb9c233422567b24a0fd6541f

To extract the report descriptor, use `sudo hid-recorder` at the root
of this repo, and copy the line starting with `R:` or use your
favorite tool :).

Cheers,
Benjamin

>
> -Tero
>
> >
> >> The driver has been tested with chromebooks that contain either Goodix
> >> or Elan manufactured USI capable touchscreen controllers in them.
> >>
> >> Any feedback appreciated!
> > I'll try to have a deeper look next week (though it seems a few bits
> > stacked up during my week off, sigh).
> >
> > Cheers,
> > Benjamin
> >
> >> -Tero
> >>
> >>
>