Thursday, May 9, 2024

libwacom and Huion/Gaomon devices

TLDR: Thanks to José Exposito, libwacom 2.12 will support all [1] Huion and Gaomon devices when running on a 6.10 kernel.

libwacom, now almost 13 years old, is a C library that provides a bunch of static information about graphics tablets that is not otherwise available by looking at the kernel device. Basically, it's a set of APIs in the form of libwacom_get_num_buttons and so on. This is used by various components to be more precise about initializing devices, even though libwacom itself has no effect on whether the device works. It's only a library for historical reasons [2], if I were to rewrite it today, I'd probably ship libwacom as a set of static json or XML files with a specific schema.

Here are a few examples on how this information is used: libinput uses libwacom to query information about tablet tools.The kernel event node always supports tilt but the individual tool that is currently in proximity may not. libinput can get the tool ID from the kernel, query libwacom and then initialize the tool struct correctly so the compositor and Wayland clients will get the right information. GNOME Settings uses libwacom's information to e.g. detect if a tablet is built-in or an external display (to show you the "Map to Monitor" button or not, if builtin), GNOME's mutter uses the SVGs provided by libwacom to show you an OSD where you can assign keystrokes to the buttons. All these features require that the tablet is supported by libwacom.

Huion and Gamon devices [3] were not well supported by libwacom because they re-use USB ids, i.e. different tablets from seemingly different manufacturers have the same vendor and product ID. This is understandable, the 16-bit product id only allows for 65535 different devices and if you're a company that thinks about more than just the current quarterly earnings you realise that if you release a few devices every year (let's say 5-7), you may run out of product IDs in about 10000 years. Need to think ahead! So between the 140 Huion and Gaomon devices we now have in libwacom I only counted 4 different USB ids. Nine years ago we added name matching too to work around this (i.e. the vid/pid/name combo must match) but, lo and behold, we may run out of unique strings before the heat death of the universe so device names are re-used too! [4] Since we had no other information available to userspace this meant that if you plugged in e.g. a Gaomon M106 and it was detected as S620 and given wrong button numbers, a wrong SVG, etc.

A while ago José got himself a tablet and started contributing to DIGIMEND (and upstreaming a bunch of things). At some point we realised that the kernel actually had the information we needed: the firmware version string from the tablet which conveniently gave us the tablet model too. With this kernel patch scheduled for 6.10 this is now exported as the uniq property (HID_UNIQ in the uevent) and that means it's available to userspace. After a bit of rework in libwacom we can now match on the trifecta of vid/pid/uniq or the quadrella of vid/pid/name/uniq. So hooray, for the first time we can actually detect Huion and Gaomon devices correctly.

The second thing Jose did was to extract all model names from the .deb packages Huion and Gaomon provide and auto-generate all libwacom descriptions for all supported devices. Which meant, in one pull request we added around 130 devices. Nice!

As said above, this requires the future kernel 6.10 but you can apply the patches to your current kernel if you want. If you do have one of the newly added devices, please verify the .tablet file for your device and let us know so we can remove the "this is autogenerated" warnings and fix any issues with the file. Some of the new files may now take precedence over the old hand-added ones so over time we'll likely have to merge them. But meanwhile, for a brief moment in time, things may actually work.

[1] fsvo of all but should be all current and past ones provided they were supported by Huions driver
[2] anecdote: in 2011 Jason Gerecke from Wacom and I sat down to and decided on a generic tablet handling library independent of the xf86-input-wacom driver. libwacom was supposed to be that library but it never turned into more than a static description library, libinput is now what our original libwacom idea was.
[3] and XP Pen and UCLogic but we don't yet have a fix for those at the time of writing
[4] names like "HUION PenTablet Pen"...

Thursday, April 18, 2024

udev-hid-bpf: quickstart tooling to fix your HID devices with eBPF

For the last few months, Benjamin Tissoires and I have been working on and polishing a little tool called udev-hid-bpf [1]. This is the scaffolding required quickly and easily write, test and eventually fix your HID input devices (mouse, keyboard, etc.) via a BPF program instead of a full-blown custom kernel driver or a semi-full-blown kernel patch. To understand how it works, you need to know two things: HID and BPF [2].

Why BPF for HID?

HID is the Human Interface Device standard and the most common way input devices communicate with the host (HID over USB, HID over Bluetooth, etc.). It has two core components: the "report descriptor" and "reports", both of which are byte arrays. The report descriptor is a fixed burnt-in-ROM byte array that (in rather convoluted terms) tells us what we'll find in the reports. Things like "bits 16 through to 24 is the delta x coordinate" or "bit 5 is the binary button state for button 3 in degrees celcius". The reports themselves are sent at (usually) regular intervals and contain the data in the described format, as the devices perceives reality. If you're interested in more details, see Understanding HID report descriptors.

BPF or more correctly eBPF is a Linux kernel technology to write programs in a subset of C, compile it and load it into the kernel. The magic thing here is that the kernel will verify it, so once loaded, the program is "safe". And because it's safe it can be run in kernel space which means it's fast. eBPF was originally written for network packet filters but as of kernel v6.3 and thanks to Benjamin, we have BPF in the HID subsystem. HID actually lends itself really well to BPF because, well, we have a byte array and to fix our devices we need to do complicated things like "toggle that bit to zero" or "swap those two values".

If we want to fix our devices we usually need to do one of two things: fix the report descriptor to enable/disable/change some of the values the device pretends to support. For example, we can say we support 5 buttons instead of the supposed 8. Or we need to fix the report by e.g. inverting the y value for the device. This can be done in a custom kernel driver but a HID BPF program is quite a lot more convenient.

HID-BPF programs

For illustration purposes, here's the example program to flip the y coordinate. HID BPF programs are usually device specific, we need to know that the e.g. the y coordinate is 16 bits and sits in bytes 3 and 4 (little endian):

int BPF_PROG(hid_y_event, struct hid_bpf_ctx *hctx)
	s16 y;
	__u8 *data = hid_bpf_get_data(hctx, 0 /* offset */, 9 /* size */);

	if (!data)
		return 0; /* EPERM check */

	y = data[3] | (data[4] << 8);
	y = -y;

	data[3] = y & 0xFF;
	data[4] = (y >> 8) & 0xFF;

	return 0;
That's it. HID-BPF is invoked before the kernel handles the HID report/report descriptor so to the kernel the modified report looks as if it came from the device.

As said above, this is device specific because where the coordinates is in the report depends on the device (the report descriptor will tell us). In this example we want to ensure the BPF program is only loaded for our device (vid/pid of 04d9/a09f), and for extra safety we also double-check that the report descriptor matches.

// The bpf.o will only be loaded for devices in this list

int probe(struct hid_bpf_probe_args *ctx)
	* The device exports 3 interfaces.
	* The mouse interface has a report descriptor of length 71.
	* So if report descriptor size is not 71, mark as -EINVAL
	ctx->retval = ctx->rdesc_size != 71;
	if (ctx->retval)
		ctx->retval = -EINVAL;

	return 0;
Obviously the check in probe() can be as complicated as you want.

This is pretty much it, the full working program only has a few extra includes and boilerplate. So it mostly comes down to compiling and running it, and this is where udev-hid-bpf comes in.

udev-hid-bpf as loader

udev-hid-bpf is a tool to make the development and testing of HID BPF programs simple, and collect HID BPF programs. You basically run meson compile and meson install and voila, whatever BPF program applies to your devices will be auto-loaded next time you plug those in. If you just want to test a single bpf.o file you can udev-hid-bpf install /path/to/foo.bpf.o and it will install the required udev rule for it to get loaded whenever the device is plugged in. If you don't know how to compile, you can grab a tarball from our CI and test the pre-compiled bpf.o. Hooray, even simpler.

udev-hid-bpf is written in Rust but you don't need to know Rust, it's just the scaffolding. The BPF programs are all in C. Rust just gives us a relatively easy way to provide a static binary that will work on most tester's machines.

The documentation for udev-hid-bpf is here. So if you have a device that needs a hardware quirk or just has an annoying behaviour that you always wanted to fix, well, now's the time. Fixing your device has never been easier! [3].

[1] Yes, the name is meh but you're welcome to come up with a better one and go back in time to suggest it a few months ago.
[2] Because I'm lazy the terms eBPF and BPF will be used interchangeably in this article. Because the difference doesn't really matter in this context, it's all eBPF anyway but nobody has the time to type that extra "e".
[3] Citation needed

Tuesday, March 12, 2024

Enforcing a touchscreen mapping in GNOME

Touchscreens are quite prevalent by now but one of the not-so-hidden secrets is that they're actually two devices: the monitor and the actual touch input device. Surprisingly, users want the touch input device to work on the underlying monitor which means your desktop environment needs to somehow figure out which of the monitors belongs to which touch input device. Often these two devices come from two different vendors, so mutter needs to use ... */me holds torch under face* .... HEURISTICS! :scary face:

Those heuristics are actually quite simple: same vendor/product ID? same dimensions? is one of the monitors a built-in one? [1] But unfortunately in some cases those heuristics don't produce the correct result. In particular external touchscreens seem to be getting more common again and plugging those into a (non-touch) laptop means you usually get that external screen mapped to the internal display.

Luckily mutter does have a configuration to it though it is not exposed in the GNOME Settings (yet). But you, my $age $jedirank, can access this via a commandline interface to at least work around the immediate issue. But first: we need to know the monitor details and you need to know about gsettings relocatable schemas.

Finding the right monitor information is relatively trivial: look at $HOME/.config/monitors.xml and get your monitor's vendor, product and serial from there. e.g. in my case this is:

  <monitors version="2">
          <vendor>DEL</vendor>              <--- this one
          <product>DELL S2722QC</product>   <--- this one
          <serial>59PKLD3</serial>          <--- and this one
Well, so we know the monitor details we want. Note there are two monitors listed here, in this case I want to map the touchscreen to the external Dell monitor. Let's move on to gsettings.

gsettings is of course the configuration storage wrapper GNOME uses (and the CLI tool with the same name). GSettings follow a specific schema, i.e. a description of a schema name and possible keys and values for each key. You can list all those, set them, look up the available values, etc.:

    $ gsettings list-recursively
    ... lots of output ...
    $ gsettings set org.gnome.desktop.peripherals.touchpad click-method 'areas'
    $ gsettings range org.gnome.desktop.peripherals.touchpad click-method
Now, schemas work fine as-is as long as there is only one instance. Where the same schema is used for different devices (like touchscreens) we use a so-called "relocatable schema" and that requires also specifying a path - and this is where it gets tricky. I'm not aware of any functionality to get the specific path for a relocatable schema so often it's down to reading the source. In the case of touchscreens, the path includes the USB vendor and product ID (in lowercase), e.g. in my case the path is:
In your case you can get the touchscreen details from lsusb, libinput record, /proc/bus/input/devices, etc. Once you have it, gsettings takes a schema:path argument like this:
  $ gsettings list-recursively org.gnome.desktop.peripherals.touchscreen:/org/gnome/desktop/peripherals/touchscreens/04f3:2d4a/
  org.gnome.desktop.peripherals.touchscreen output ['', '', '']
Looks like the touchscreen is bound to no monitor. Let's bind it with the data from above:
   $ gsettings set org.gnome.desktop.peripherals.touchscreen:/org/gnome/desktop/peripherals/touchscreens/04f3:2d4a/ output "['DEL', 'DELL S2722QC', '59PKLD3']"
Note the quotes so your shell doesn't misinterpret things.

And that's it. Now I have my internal touchscreen mapped to my external monitor which makes no sense at all but shows that you can map a touchscreen to any screen if you want to.

[1] Probably the one that most commonly takes effect since it's the vast vast majority of devices

Monday, January 29, 2024

New 🚯 emoji-based spamfighting abilities

This is a follow-up from our Spam-label approach, but this time with MOAR EMOJIS because that's what the world is turning into.

Since March 2023 projects could apply the "Spam" label on any new issue and have a magic bot come in and purge the user account plus all issues they've filed, see the earlier post for details. This works quite well and gives every project member the ability to quickly purge spam. Alas, pesky spammers are using other approaches to trick google into indexing their pork [1] (because at this point I think all this crap is just SEO spam anyway). Such as commenting on issues and merge requests. We can't apply labels to comments, so we found a way to work around that: emojis!

In GitLab you can add "reactions" to issue/merge request/snippet comments and in recent GitLab versions you can register for a webhook to be notified when that happens. So what we've added to the instance is support for the :do_not_litter: (🚯) emoji [2] - if you set that on an comment the author of said comment will be blocked and the comment content will be removed. After some safety checks of course, so you can't just go around blocking everyone by shotgunning emojis into gitlab. Unlike the "Spam" label this does not currently work recursively so it's best to report the user so admins can purge them properly - ideally before setting the emoji so the abuse report contains the actual spam comment instead of the redacted one. Also note that there is a 30 second grace period to quickly undo the emoji if you happen to set it accidentally.

Note that for purging issues, the "Spam" label is still required, the emojis only work for comments.

Happy cleanup!

[1] or pork-ish
[2] Benjamin wanted to use :poop: but there's a chance that may get used for expressing disagreement with the comment in question

Thursday, December 14, 2023

Xorg being removed. What does this mean?

You may have seen the news that Red Hat Enterprise Linux 10 plans to remove Xorg. But Xwayland will stay around, and given the name overloading and them sharing a git repository there's some confusion over what is Xorg. So here's a very simple "picture". This is the xserver git repository:

$ tree -d -L 2 xserver
├── composite
├── config
├── damageext
├── dbe
├── dix
├── doc
│   └── dtrace
├── dri3
├── exa
├── fb
├── glamor
├── glx
├── hw
│   ├── kdrive
│   ├── vfb
│   ├── xfree86              <- this one is Xorg
│   ├── xnest
│   ├── xquartz
│   ├── xwayland
│   └── xwin
├── include
├── m4
├── man
├── mi
├── miext
│   ├── damage
│   ├── rootless
│   ├── shadow
│   └── sync
├── os
├── present
├── pseudoramiX
├── randr
├── record
├── render
├── test
│   ├── bigreq
│   ├── bugs
│   ├── damage
│   ├── scripts
│   ├── sync
│   ├── xi1
│   └── xi2
├── Xext
├── xfixes
├── Xi
└── xkb
The git repo produces several X servers, including the one designed to run on bare metal: Xorg (in hw/xfree86 for historical reasons). The other hw directories are the other X servers including Xwayland. All the other directories are core X server functionality that's shared between all X servers [1]. Removing Xorg from a distro but keeping Xwayland means building with --disable-xfree86 -enable-xwayland [1]. That's simply it (plus the resulting distro packaging work of course).

Removing Xorg means you need something else that runs on bare metal and that is your favourite Wayland compositor. Xwayland then talks to that while presenting an X11-compatible socket to existing X11 applications.

Of course all this means that the X server repo will continue to see patches and many of those will also affect Xorg. For those who are running git master anyway. Don't get your hopes up for more Xorg releases beyond the security update background noise [2].

Xwayland on the other hand is actively maintained and will continue to see releases. But those releases are a sequence [1] of

$ git new-branch xwayland-23.x.y
$ git rm hw/{kdrive/vfb/xfree86/xnest,xquartz,xwin}
$ git tag xwayland-23.x.y
In other words, an Xwayland release is the xserver git master branch with all X servers but Xwayland removed. That's how Xwayland can see new updates and releases without Xorg ever seeing those (except on git master of course). And that's how your installed Xwayland has code from 2023 while your installed Xorg is still stuck on the branch created and barely updated after 2021.

I hope this helps a bit with the confusion of the seemingly mixed messages sent when you see headlines like "Xorg is unmaintained", "X server patches to fix blah", "Xorg is abandoned", "new Xwayland release.

[1] not 100% accurate but close enough
[2] historically an Xorg release included all other X servers (Xquartz, Xwin, Xvfb, ...) too so this applies to those servers too unless they adopt the Xwayland release model

Friday, November 10, 2023

PSA: For Xorg GNOME sessions, use the xf86-input-wacom driver for your tablets

TLDR: see the title of this blog post, it's really that trivial.

Now that GodotWayland has been coming for ages and all new development focuses on a pile of software that steams significantly less, we're seeing cracks appear in the old Xorg support. Not intentionally, but there's only so much time that can be spent on testing and things that are more niche fall through. One of these was a bug I just had the pleasure of debugging and was triggered by GNOME on Xorg user using the xf86-input-libinput driver for tablet devices.

On the surface of it, this should be fine because libinput (and thus xf86-input-libinput) handles tablets just fine. But libinput is the new kid on the block. The old kid on said block is the xf86-input-wacom driver, older than libinput by slightly over a decade. And oh man, history has baked things into the driver that are worse than raisins in apple strudel [1].

The xf86-input-libinput driver was written as a wrapper around libinput and makes use of fancy things that (from libinput's POV) have always been around: things like input device hotplugging. Fancy, I know. For tablet devices the driver creates an X device for each new tool as it comes into proximity first. Future events from that tool will go through that device. A second tool, be it a new pen or the eraser on the original pen, will create a second X device and events from that tool will go through that X device. Configuration on any device will thus only affect that particular pen. Almost like the whole thing makes sense.

The wacom driver of course doesn't do this. It pre-creates X devices for some possible types of tools (pen, eraser, and cursor [2] but not airbrush or artpen). When a tool goes into proximity the events are sent through the respective device, i.e. all pens go through the pen tool, all erasers through the eraser tool. To actually track pens there is the "Wacom Serial IDs" property that contains the current tool's serial number. If you want to track multiple tools you need to query the property on proximity in [4]. At the time this was within a reasonable error margin of a good idea.

Of course and because MOAR CONFIGURATION! will save us all from the great filter you can specify the "ToolSerials" xorg.conf option as e.g. "airbrush;12345;artpen" and get some extra X devices pre-created, in this case a airbrush and artpen X device and an X device just for the tool with the serial number 12345. All other tools multiplex through the default devices. Again, at the time this was a great improvement. [5]

Anyway, where was I? Oh, right. The above should serve as a good approximation of a reason why the xf86-input-libinput driver does not try to be fullly compatible to the xf86-input-wacom driver. In everyday use these things barely matter [6] but for the desktop environment which needs to configure these devices all these differences mean multiple code paths. Those paths need to be tested but they aren't, so things fall through the cracks.

So quite a while ago, we made the decision that until Xorg goes dodo, the xf86-input-wacom driver is the tablet driver to use in GNOME. So if you're using a GNOME on Xorg session [7], do make sure the xf86-input-wacom driver is installed. It will make both of us happier and that's a good aim to strive for.

[1] It's just a joke. Put the pitchforks down already.
[2] The cursor is the mouse-like thing Wacom sells. Which is called cursor [3] because the English language has a limited vocabulary and we need to re-use words as much as possible lest we run out of them.
[3] It's also called puck. Because [2].
[4] And by "query" I mean "wait for the XI2 event notifying you of a property change". Because of lolz the driver cannot update the property on proximity in but needs to schedule that as idle func so the property update for the serial always arrives at some unspecified time after the proximity in but hopefully before more motion events happen. Or not, and that's how hope dies.
[5] Think about this next time someone says they long for some unspecified good old days.
[6] Except the strip axis which on the wacom driver is actually a bit happily moving left/right as your finger moves up/down on the touch strip and any X client needs to know this. libinput normalizes this to...well, a normal value but now the X client needs to know which driver is running so, oh deary deary.
[7] e.g because your'e stockholmed into it by your graphics hardware

Monday, July 3, 2023 now has a bugbot for automatic issue/merge request processing

As of today, provides easy hooks to invoke the gitlab-triage tool for your project. gitlab-triage allows for the automation of recurring tasks, for example something like

If the label FOO is set, close the issue and add a comment containing ".... blah ..."
Many project have recurring tasks like this, e.g. the wayland project gets a lot of issues that are compositor (not protocol) issues. Being able to just set a label and have things happen is much more convenient than having to type out the same explanations over and over again.

The goal for us was to provide automated handling for these with as little friction as possible. And of course each project must be able to decide what actions should be taken. Usually gitlab-triage is run as part of project-specific scheduled pipelines but since we already have webhook-based spam-fighting tools we figured we could make this even easier.

So, bugbot was born. Any project registered with bugbot can use labels prefixed with "bugbot::" to have gitlab-triage invoked against the project's policies file. These labels thus serve as mini-commands for bugbot, though each project decides what happens for any particular label. bugbot effectively works like this:

sleep 30
for label in {issue|merge_request}.current_labels:
  if label.startswith("bugbot::"):
     run-gitlab-triage --as-user @bugbot --use-file .triage-policies.yml
And this is triggered on every issue/merge request update for any registered project which means that all you need to do is set the label and you're done. The things of note here:
  • bugbot delays by 30 seconds, giving you time to unset an accidentally applied label before it takes effect
  • bugbot doesn't care about the label beyond the (hard-coded) "bugbot::" prefix
  • bugbot always runs your project's triage policies, from your main or master branch (whichever succeeds first)
  • The actions are performed as the bugbot user, not your user
The full documentation of what you can do in a policies file is available at the gitlab-triage documentation but let's look at a simple example that shouldn't even need explanation:
      - name: convert bugbot label to other label
            - "bugbot::foo"
            - "foo"
            - "bugbot::foo"
          comment: |
            Nice label you have there. Would be a shame 
            if someone removed it
          status: "close"
And the effect of this file can be seen in this issue here.

Registering a project

Bugbot is part of the damspam project and registering a project can be done with a single command. Note: this can only be done by someone with the Maintainer role or above.

Create a personal access token with API access and save the token value as $XDG_CONFIG_HOME/bugbot/user.token Then run the following commands with your project's full path (e.g. mesa/mesa, pipewire/wireplumber, xorg/lib/libX11):

$ pip install git+
$ bugbot request-webhook foo/bar
After this you may remove the token file and the package
$ pip uninstall damspam
$ rm $XDG_CONFIG_HOME/bugbot/user.token
The bugbot command will file an issue in the freedesktop/fdo-bots repository. This issue will be automatically processed and should be done by the time you finish the above commands, see this issue for an example. Note: the issue processing requires a git push to an internal repo - if you script this for multiple repos please put a sleep(30) in to avoid conflicts.

Adding triage policies

Once registered, the .triage-policies.yml file must be added to the root directory of your project. What bugbot commands you want to respond to (and the actions to take) is up to you, though there are two things of note: you should always remove the bugbot label you are reacting to to avoid duplicate processing and gitlab-triage does not create new labels. So any label in your actions must be manually created in the project first. Beyond that - the sky's your limit.

Remember you can test your policies file with

 $ gitlab-triage --dry-run --token $GITLAB_TOKEN \
   --source-id foo/bar  --resource-reference 1234

As usual, many thanks to Benjamin Tissoires for reviews and the magic of integrating this into infrastructure.