Tuesday, February 15, 2022

The xf86-input-wacom driver hits 1.0

After roughly 20 years and counting up to 0.40 in release numbers, I've decided to call the next version of the xf86-input-wacom driver the 1.0 release. [1] This cycle has seen a bulk of development (>180 patches) which is roughly as much as the last 12 releases together. None of these patches actually added user-visible features, so let's talk about technical dept and what turned out to be an interesting way of reducing it.

The wacom driver's git history goes back to 2002 and the current batch of maintainers (Ping, Jason and I) have all been working on it for one to two decades. It used to be a Wacom-only driver but with the improvements made to the kernel over the years the driver should work with most tablets that have a kernel driver, albeit some of the more quirky niche features will be more limited (but your non-Wacom devices probably don't have those features anyway).

The one constant was always: the driver was extremely difficult to test, something common to all X input drivers. Development is a cycle of restarting the X server a billion times, testing is mostly plugging hardware in and moving things around in the hope that you can spot the bugs. On a driver that doesn't move much, this isn't necessarily a problem. Until a bug comes along, that requires some core rework of the event handling - in the kernel, libinput and, yes, the wacom driver.

After years of libinput development, I wasn't really in the mood for the whole "plug every tablet in and test it, for every commit". In a rather caffeine-driven development cycle [2], the driver was separated into two logical entities: the core driver and the "frontend". The default frontend is the X11 one which is now a relatively thin layer around the core driver parts, primarily to translate events into the X Server's API. So, not unlike libinput + xf86-input-libinput in terms of architecture. In ascii-art:

                     +--------------------+  |   big giant 
  /dev/input/event0->|  core driver | x11 |->|    X server
                     +--------------------+  |    process

Now, that logical separation means we can have another frontend which I implemented as a relatively light GObject wrapper and is now a library creatively called libgwacom:

                     +-----------------------+  |
  /dev/input/event0->|  core driver | gwacom |--| tools or test suites
                     +-----------------------+  |

This isn't a public library or API and it's very much focused on the needs of the X driver so there are some peculiarities in there. What it allows us though is a new wacom-record tool that can hook onto event nodes and print the events as they come out of the driver. So instead of having to restart X and move and click things, you get this:
$ ./builddir/wacom-record
  version: 0.99.2
  git: xf86-input-wacom-0.99.2-17-g404dfd5a
    path: /dev/input/event6
    name: "Wacom Intuos Pro M Pen"
    - source: 0
      event: new-device
      name: "Wacom Intuos Pro M Pen"
      type: stylus
        keys: true
        is-absolute: true
        is-direct-touch: false
        ntouches: 0
        naxes: 6
          - {type: x           , range: [    0, 44800], resolution: 200000}
          - {type: y           , range: [    0, 29600], resolution: 200000}
          - {type: pressure    , range: [    0, 65536], resolution:     0}
          - {type: tilt_x      , range: [  -64,    63], resolution:    57}
          - {type: tilt_y      , range: [  -64,    63], resolution:    57}
          - {type: wheel       , range: [ -900,   899], resolution:     0}
    - source: 0
      mode: absolute
      event: motion
      mask: [ "x", "y", "pressure", "tilt-x", "tilt-y", "wheel" ]
      axes: { x: 28066, y: 17643, pressure:    0, tilt: [ -4, 56], rotation:   0, throttle:   0, wheel: -108, rings: [  0,   0] 
This is YAML which means we can process the output for comparison or just to search for things.

A tool to quickly analyse data makes for faster development iterations but it's still a far cry from reliable regression testing (and writing a test suite is a daunting task at best). But one nice thing about GObject is that it's accessible from other languages, including Python. So our test suite can be in Python, using pytest and all its capabilities, plus all the advantages Python has over C. Most of driver testing comes down to: create a uinput device, set up the driver with some options, push events through that device and verify they come out of the driver in the right sequence and format. I don't need C for that. So there's pull request sitting out there doing exactly that - adding a pytest test suite for a 20-year old X driver written in C. That this is a) possible and b) a lot less work than expected got me quite unreasonably excited. If you do have to maintain an old C library, maybe consider whether's possible doing the same because there's nothing like the warm fuzzy feeling a green tick on a CI pipeline gives you.

[1] As scholars of version numbers know, they make as much sense as your stereotypical uncle's facebook opinion, so why not.
[2] The Colombian GDP probably went up a bit