Wednesday, January 13, 2021

Parsing HID Unit Items

This post explains how to parse the HID Unit Global Item as explained by the HID Specification, page 37. The table there is quite confusing and it took me a while to fully understand it (Benjamin Tissoires was really the one who cracked it). I couldn't find any better explanation online which means either I'm incredibly dense and everyone's figured it out or no-one has posted a better explanation. On the off-chance it's the latter [1], here are the instructions on how to parse this item.

We know a HID Report Descriptor consists of a number of items that describe the content of each HID Report (read: an event from a device). These Items include things like Logical Minimum/Maximum for axis ranges, etc. A HID Unit item specifies the physical unit to apply. For example, a Report Descriptor may specify that X and Y axes are in mm which can be quite useful for all the obvious reasons.

Like most HID items, a HID Unit Item consists of a one-byte item tag and 1, 2 or 4 byte payload. The Unit item in the Report Descriptor itself has the binary value 0110 01nn where the nn is either 1, 2, or 3 indicating 1, 2 or 4 bytes of payload, respectively. That's standard HID.

The payload is divided into nibbles (4-bit units) and goes from LSB to MSB. The lowest-order 4 bits (first byte & 0xf) define the unit System to apply: one of SI Linear, SI Rotation, English Linear or English Rotation (well, or None/Reserved). The rest of the nibbles are in this order: "length", "mass", "time", "temperature", "current", "luminous intensity". In something resembling code this means:

 system = value & 0xf
 length_exponent = (value & 0xf0) >> 4
 mass_exponent   = (value & 0xf00) >> 8
 time_exponent   = (value & 0xf000) >> 12
 ...
 
The System defines which unit is used for length (e.g. SILinear means length is in cm). The actual value of each nibble is the exponent for the unit in use [2]. In something resembling code:
 switch (system)
   case SILinear: 
     print("length is in cm^{length_exponent}"); 
     break;
   case SIRotation: 
     print("length is in rad^{length_exponent}");
     break;
   case EnglishLinear: 
     print("length is in in^{length_exponent}");
     break;
   case EnglishRotation:
     print("length is in deg^{length_exponent}");
     break;
   case None:
   case Reserved"
     print("boo!");
     break;
 

For example, the value 0x321 means "SI Linear" (0x1) so the remaining nibbles represent, in ascending nibble order: Centimeters, Grams, Seconds, Kelvin, Ampere, Candela. The length nibble has a value of 0x2 so it's square cm, the mass nibble has a value of 0x3 so it is cubic grams (well, it's just an example, so...). This means that any report containing this item comes in cm²g³. As a more realistic example: 0xF011 would be cm/s.

If we changed the lowest nibble to English Rotation (0x4), i.e. our value is now 0x324, the units represent: Degrees, Slug, Seconds, F, Ampere, Candela [3]. The length nibble 0x2 means square degrees, the mass nibble is cubic slugs. As a more realistic example, 0xF014 would be degrees/s.

Any nibble with value 0 means the unit isn't in use, so the example from the spec with value 0x00F0D121 is SI linear, units cm² g s⁻³ A⁻¹, which is... Voltage! Of course you knew that and totally didn't have to double-check with wikipedia.

Because bits are expensive and the base units are of course either too big or too small or otherwise not quite right, HID also provides a Unit Exponent item. The Unit Exponent item (a separate item to Unit in the Report Descriptor) then describes the exponent to be applied to the actual value in the report. For example, a Unit Eponent of -3 means 10⁻³ to be applied to the value. If the report descriptor specifies an item of Unit 0x00F0D121 (i.e. V) and Unit Exponent -3, the value of this item is mV (milliVolt), Unit Exponent of 3 would be kV (kiloVolt).

Now, in hindsight all this is pretty obvious and maybe even sensible. It'd have been nice if the spec would've explained it a bit clearer but then I would have nothing to write about, so I guess overall I call it a draw.

[1] This whole adventure was started because there's a touchpad out there that measures touch pressure in radians, so at least one other person out there struggled with the docs...
[2] The nibble value is twos complement (i.e. it's a signed 4-bit integer). Values 0x1-0x7 are exponents 1 to 7, values 0x8-0xf are exponents -8 to -1.
[3] English Linear should've trolled everyone and use Centimetres instead of Centimeters in SI Linear.

Friday, December 4, 2020

It's templates all the way down - part 3

In Part 1 I've shown you how to create your own distribution image using the freedesktop.org CI templates. In Part 2, I've shown you how to truly build nested images. In this part, I'll talk about the ci-fairy tool that is part of the same repository of ci-templates.

When you're building a CI pipeline, there are some tasks that most projects need in some way or another. The ci-fairy tool is a grab-bag of solutions for these. Some of those solutions are for a pipeline itself, others are for running locally. So let's go through the various commands available.

Using ci-fairy in a pipeline

It's as simple as including the template in your .gitlab-ci.yml file.

include:
 - 'https://gitlab.freedesktop.org/freedesktop/ci-templates/-/raw/master/templates/ci-fairy.yml'
Of course, if you want to track a specific sha instead of following master, just sub that sha there. freedesktop.org projects can include ci-fairy like this:
include:
 - project: 'freedesktop/ci-templates'
   ref: master
   file: '/templates/ci-fairy.yml'
Once that's done, you have access to a .fdo.ci-fairy job template that you can extends from. This will download an image from quay.io that is capable of git, python, bash and obviously ci-fairy. This image is a fixed one and referenced by a unique sha so even if where we keep working on ci-fairy upstream you should never see regression, updating requires you to explicitly update the sha of the included ci-fairy template. Obviously, if you're using master like above you'll always get the latest.

Due to how the ci-templates work, it's good to set the FDO_UPSTREAM_REPO variable with the upstream project name. This means ci-fairy will be able to find the equivalent origin/master branch, where that's not available in the merge request. Note, this is not your personal fork but the upstream one, e.g. "freedesktop/ci-templates" if you are working on the ci-templates itself.

Checking commit messages

ci-fairy has a command to check commits for a few basic expectations in commit messages. This currently includes things like enforcing a 80 char subject line length, that there is an empty line after the subject line, that no fixup or squash commits are in the history, etc. If you have complex requirements you need to write your own but for most projects this job ensures that there are no obvious errors in the git commit log:

check-commit:
  extends:
    - .fdo.ci-fairy
  script:
    - ci-fairy check-commits --signed-off-by
  except:
    - master@upstream/project
Since you don't ever want this to fail on an already merged commit, exclude this job the master branch of the upstream project - the MRs should've caught this already anyway.

Checking merge requests

To rebase a contributors merge request, the contributor must tick the checkbox to Allow commits from members who can merge to the target branch. The default value is off which is frustrating (gitlab is working on it though) and causes unnecessary delays in processing merge requests. ci-fairy has command to check for this value on an MR and fail - contributors ideally pay attention to the pipeline and fix this accordingly.

check-merge-request:
  extends:
    - .fdo.ci-fairy
  script:
    - ci-fairy check-merge-request --require-allow-collaboration
  allow_failure: true
As a tip: run this job towards the end of the pipeline to give collaborators a chance to file an MR before this job fails.

Using ci-fairy locally

The two examples above are the most useful ones for CI pipelines, but ci-fairy also has some useful local commands. For that you'll have to install it, but that's as simple as

$ pip3 install git+http://gitlab.freedesktop.org/freedesktop/ci-templates
A big focus on ci-fairy for local commands is that it should, usually, be able to work without any specific configuration if you run it in the repository itself.

Linting

Just hacked on the CI config?

$ ci-fairy lint
and done, you get the same error back that the online linter for your project would return.

Pipeline checks

Just pushed to the repo?

$ ci-fairy wait-for-pipeline                        
Pipeline https://gitlab.freedesktop.org/username/project/-/pipelines/238586
status: success   | 7/7 | created:  0 | pending:  0 | running:  0 | failed:  0 | success:  7 ....
The command is self-explanatory, I think.

Summary

There are a few other parts to ci-fairy including templating and even minio handling. I recommend looking at e.g. the libinput CI pipeline which uses much of ci-fairy's functionality. And the online documentation for ci-fairy, who knows, there may be something useful in there for you.

The useful contribution of ci-fairy is primarily that it tries to detect the settings for each project automatically, regardless of whether it's run inside a MR pipeline or just as part of a normal pipeline. So the same commands will work without custom configuration on a per-project basis. And for many things it works without API tokens, so the setup costs are just the pip install.

If you have recurring jobs, let us know, we're always looking to add more useful functionality to this little tool.

Friday, September 4, 2020

No user-specific XKB configuration in X

This is the continuation from these posts: part 1, part 2, part 3 and part 4.

In the posts linked above, I describe how it's possible to have custom keyboard layouts in $HOME or /etc/xkb that will get picked up by libxkbcommon. This only works for the Wayland stack, the X stack doesn't use libxkbcommon. In this post I'll explain why it's unlikely this will ever happen in X.

As described in the previous posts, users configure with rules, models, layouts, variants and options (RMLVO). What XKB uses internally though are keycodes, compat, geometry, symbols types (KcCGST) [1].

There are, effectively, two KcCGST keymap compilers: libxkbcommon and xkbcomp. libxkbcommon can go from RMLVO to a full keymap, xkbcomp relies on other tools (e.g. setxkbmap) which in turn use a utility library called libxkbfile to can parse rules files. The X server has a copy of the libxkbfile code. It doesn't use libxkbfile itself but it relies on the header files provided by it for some structs.

Wayland's keyboard configuration works like this:

  • the compositor decides on the RMLVO keybard layout, through an out-of-band channel (e.g. gsettings, weston.ini, etc.)
  • the compositor invokes libxkbcommon to generate a KcCGST keymap and passes that full keymap to the client
  • the client compiles that keymap with libxkbcommon and feeds any key events into libxkbcommon's state tracker to get the right keysyms
The advantage we have here is that only the full keymap is passed between entities. Changing how that keymap is generated does not affect the client. This, coincidentally [2], is also how Xwayland gets the keymap passed to it and why Xwayland works with user-specific layouts.

X works differently. Notably, KcCGST can come in two forms, the partial form specifying names only and the full keymap. The partial form looks like this:

$ setxkbmap -print -layout fr -variant azerty -option ctrl:nocaps
xkb_keymap {
 xkb_keycodes  { include "evdev+aliases(azerty)" };
 xkb_types     { include "complete" };
 xkb_compat    { include "complete" };
 xkb_symbols   { include "pc+fr(azerty)+inet(evdev)+ctrl(nocaps)" };
 xkb_geometry  { include "pc(pc105)" };
};
This defines the component names but not the actual keymap, punting that to the next part in the stack. This will turn out to be the achilles heel. Keymap handling in the server has two distinct aproaches:
  • During keyboard device init, the input driver passes RMLVO to the server, based on defaults or xorg.conf options
  • The server has its own rules file parser and creates the KcCGST component names (as above)
  • The server forks off xkbcomp and passes the component names to stdin
  • xkbcomp generates a keymap based on the components and writes it out as XKM file format
  • the server reads in the XKM format and updates its internal structs
This has been the approach for decades. To give you an indication of how fast-moving this part of the server is: XKM caching was the latest feature added... in 2009.

Driver initialisation is nice, but barely used these days. You set your keyboard layout in e.g. GNOME or KDE and that will apply it in the running session. Or run setxkbmap, for those with a higher affinity to neckbeards. setxkbmap works like this:

  • setkxkbmap parses the rules file to convert RMLVO to KcCGST component names
  • setkxkbmap calls XkbGetKeyboardByName and hands those component names to the server
  • The server forks off xkbcomp and passes the component names to stdin
  • xkbcomp generates a keymap based on the components and writes it out as XKM file format
  • the server reads in the XKM format and updates its internal structs
Notably, the RMLVO to KcCGST conversion is done on the client side, not the server side. And the only way to send a keymap to the server is that XkbGetKeyboardByName request - which only takes KcCGST, you can't even pass it a full keymap. This is also a long-standing potential issue with XKB: if your client tools uses different XKB data files than the server, you don't get the keymap you expected.

Other parts of the stack do basically the same as setxkbmap which is just a thin wrapper around libxkbfile anyway.

Now, you can use xkbcomp on the client side to generate a keymap, but you can't hand it as-is to the server. xkbcomp can do this (using libxkbfile) by updating the XKB state one-by-one (XkbSetMap, XkbSetCompatMap, XkbSetNames, etc.). But at this point you're at the stage where you ask the server to knowingly compile a wrong keymap before updating the parts of it.

So, realistically, the only way to get user-specific XKB layouts into the X server would require updating libxkbfile to provide the same behavior as libxkbcommon, update the server to actually use libxkbfile instead of its own copy, and updating xkbcomp to support the changes in part 2, part 3. All while ensuring no regressions in code that's decades old, barely maintained, has no tests, and, let's be honest, not particularly pretty to look at. User-specific XKB layouts are somewhat a niche case to begin with, so I don't expect anyone to ever volunteer and do this work [3], much less find the resources to review and merge that code. The X server is unlikely to see another real release and this is definitely not something you want to sneak in in a minor update.

The other option would be to extend XKB-the-protocol with a request to take a full keymap so the server. Given the inertia involved and that the server won't see more full releases, this is not going to happen.

So as a summary: if you want custom keymaps on your machine, switch to Wayland (and/or fix any remaining issues preventing you from doing so) instead of hoping this will ever work on X. xmodmap will remain your only solution for X.

[1] Geometry is so pointless that libxkbcommon doesn't even implement this. It is a complex format to allow rendering a picture of your keyboard but it'd be a per-model thing and with evdev everyone is using the same model, so ...
[2] totally not coincidental btw
[3] libxkbcommon has been around for a decade now and no-one has volunteered to do this in the years since, so...

Tuesday, September 1, 2020

User-specific XKB configuration - putting it all together

This is the continuation from these posts: part 1, part 2, part 3

This is the part where it all comes together, with (BYO) fireworks and confetti, champagne and hoorays. Or rather, this is where I'll describe how to actually set everything up. It's a bit complicated because while libxkbcommon does the parsing legwork now, we haven't actually changed other APIs and the file formats which are still 1990s-style nerd cool and requires significant experience in CS [1] to understand what goes where.

The below relies on software using libxkbcommon and libxkbregistry. At the time of writing, libxkbcommon is used by all mainstream Wayland compositors but not by the X server. libxkbregistry is not yet used because I'm typing this before we had a release for it. But at least now I have a link to point people to.

libxkbcommon has a xkbcli-scaffold-new-layout tool that The xkblayout tool creates the template files as shown below. At the time of writing, this tool must be run from the git repo build directory, it is not installed.

I'll explain here how to add the us(banana) variant and the custom:foo option, and I will optimise for simplicity and brevity.

Directory structure

First, create the following directory layout:

$ tree $XDG_CONFIG_HOME/xkb
/home/user/.config/xkb
├── compat
├── keycodes
├── rules
│   ├── evdev
│   └── evdev.xml
├── symbols
│   ├── custom
│   └── us
└── types
If $XDG_CONFIG_HOME is unset, fall back to $HOME/.config.

Rules files

Create the rules file and add an entry to map our custom:foo option to a section in the symbols/custom file.

$ cat $XDG_CONFIG_HOME/xkb/rules/evdev
! option = symbols
  custom:foo = +custom(foo)

// Include the system 'evdev' file
! include %S/evdev
Note that no entry is needed for the variant, that is handled by wildcards in the system rules file. If you only want a variant and no options, you technically don't need this rules file.

Second, create the xml file used by libxkbregistry to display your new entries in the configuration GUIs:

$ cat $XDG_CONFIG_HOME/xkb/rules/evdev.xml
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE xkbConfigRegistry SYSTEM "xkb.dtd">
<xkbConfigRegistry version="1.1">
  <layoutList>
    <layout>
      <configItem>
        <name>us</name>
      </configItem>
      <variantList>
        <variant>
          <configItem>
            <name>banana</name>
            <shortDescription>banana</shortDescription>
            <description>US(Banana)</description>
          </configItem>
        </variant>
      </variantList>
    </layout>
  </layoutList>
  <optionList>
    <group allowMultipleSelection="true">
      <configItem>
        <name>custom</name>
        <description>custom options</description>
      </configItem>
      <option>
        <configItem>
          <name>custom:foo</name>
          <description>This option does something great</description>
        </configItem>
      </option>
    </group>
  </optionList>
</xkbConfigRegistry>
Our variant needs to be added as a layoutList/layout/variantList/variant, the option to the optionList/group/option. libxkbregistry will combine this with the system-wide evdev.xml file in /usr/share/X11/xkb/rules/evdev.xml.

Overriding and adding symbols

Now to the actual mapping. Add a section to each of the symbols files that matches the variant or option name:

$ cat $XDG_CONFIG_HOME/xkb/symbols/us
partial alphanumeric_keys modifier_keys
xkb_symbols "banana" {
    name[Group1]= "Banana (us)";

    include "us(basic)"

    key <CAPS> { [ Escape ] };
};
with this, the us(banana) layout will be a US keyboard layout but with the CapsLock key mapped to Escape. What about our option? Mostly the same, let's map the tilde key to nothing:
$ cat $XDG_CONFIG_HOME/xkb/symbols/custom
partial alphanumeric_keys modifier_keys
xkb_symbols "foo" {
    key <TLDE> { [ VoidSymbol ] };
};
A note here: NoSymbol means "don't overwrite it" whereas VoidSymbol is "map to nothing".

Notes

You may notice that the variant and option sections are almost identical. XKB doesn't care about variants vs options, it only cares about components to combine. So the sections do what we expect of them: variants include enough other components to make them a full keyboard layout, options merely define a few keys so they can be combined with layouts(variants). Due to how the lookups work, you could load the option template as layout custom(foo).

For the actual documentation of keyboard configuration, you'll need to google around, there are quite a few posts on how to map keys. All that has changed is where from and how things are loaded but not the actual data formats.

If you wanted to install this as system-wide custom rules, replace $XDG_CONFIG_HOME with /etc.

The above is a replacement for xmodmap. It does not require a script to be run manually to apply the config, the existing XKB integration will take care of it. It will work in Wayland (but as said above not in X, at least not for now).

A final word

Now, I fully agree that this is cumbersome, clunky and feels outdated. This is easy to fix, all that is needed is for someone to develop a better file format, make sure it's backwards compatible with the full spec of the XKB parser (the above is a fraction of what it can do), that you can generate the old files from the new format to reduce maintenance, and then maintain backwards compatibility with the current format for the next ten or so years. Should be a good Google Decade of Code beginner project.

[1] Cursing and Swearing

Monday, August 31, 2020

User-specific XKB configuration - part 3

This is the continuation from these posts: part 1, part 2

Let's talk about everyone's favourite [1] keyboard configuration system again: XKB. If you recall the goal is to make it simple for users to configure their own custom layouts. Now, as described earlier, XKB-the-implementation doesn't actually have a concept of a "layout" as such, it has "components" and something converts your layout desires into the combination of components. RMLVO (rules, model, layout, variant, options) is what you specify and gets converted to KcCGST (keycodes, compat, geometry, symbols, types). This is a one-way conversion, the resulting keymaps no longer has references to the RMLVO arguments. Today's post is about that conversion, and we're only talking about libxkbcommon as XKB parser because anything else is no longer maintained.

The peculiar thing about XKB data files (provided by xkeyboard-config [3]) is that the filename is part of the API. You say layout "us" variant "dvorak", the rules file translates this to symbols 'us(dvorak)' and the parser will understand this as "load file 'symbols/us' and find the dvorak section in that file". [4] The default "us" keyboard layout will require these components:

xkb_keymap {
 xkb_keycodes  { include "evdev+aliases(qwerty)" };
 xkb_types     { include "complete" };
 xkb_compat    { include "complete" };
 xkb_symbols   { include "pc+us+inet(evdev)" };
 xkb_geometry  { include "pc(pc105)" };
};
So the symbols are really: file symbols/pc, add symbols/us and then the section named 'evdev' from symbols/inet [5]. Types are loaded from types/complete, etc. The lookup paths for libxkbcommon are $XDG_CONFIG_HOME/xkb, /etc/xkb, and /usr/share/X11/xkb, in that order.

Most of the symbols sections aren't actually full configurations. The 'us' default section only sets the alphanumeric rows, everything else comes from the 'pc' default section (hence: include "pc+us+inet(evdev)"). And most variant sections just include the default one, usually called 'basic'. For example, this is the 'euro' variant of the 'us' layout which merely combines a few other sections:

partial alphanumeric_keys
xkb_symbols "euro" {

    include "us(basic)"
    name[Group1]= "English (US, euro on 5)";

    include "eurosign(5)"

    include "level3(ralt_switch)"
};
Including things works as you'd expect: include "foo(bar)" loads section 'bar' from file 'foo' and this works for 'symbols/', 'compat/', etc., it'll just load the file in the same subdirectory. So yay, the directory is kinda also part of the API.

Alright, now you understand how KcCGST files are loaded, much to your despair.

For user-specific configuration, we could already load a 'custom' layout from the user's home directory. But it'd be nice if we could just add a variant to an existing layout. Like "us(banana)", because potassium is important or somesuch. This wasn't possible because the filename is part of the API. So our banana variant had to be in $XDG_CONFIG_HOME/xkb/symbols/us and once we found that "us" file, we could no longer include the system one.

So as of two days ago, libxkbcommon now extends the parser to have merged KcCGST files, or in other words: it'll load the symbols/us file in the lookup path order until it finds the section needed. With that, you can now copy this into your $XDG_CONFIG_HOME/xkb/symbols/us file and have it work as variant:

partial alphanumeric_keys
xkb_symbols "banana" {

    include "us(basic)"
    name[Group1]= "English (Banana)";

    // let's assume there are some keymappings here
};
And voila, you now have a banana variant that can combine with the system-level "us" layout.

And because there must be millions [6] of admins out there that maintain custom XKB layouts for a set of machines, the aforementioned /etc/xkb lookup path was also recently added to libxkbcommon. So we truly now have the typical triplet of lookup paths:

  • vendor-provided ones in /usr/share/X11/xkb,
  • host-specific ones in /etc/xkb, and
  • user-specific ones in $XDG_CONFIG_HOME/xkb [7].
Good times, good times.

[1] Much in the same way everyone's favourite Model T colour was black
[2] This all follows the UNIX philosophy, there are of course multiple tools involved and none of them know what the other one is doing
[3] And I don't think Sergey gets enough credit for maintaining that pile of language oddities
[4] Note that the names don't have to match, we could map layout 'us' to the symbols in 'banana' but life's difficult enough as it is
[5] I say "add" when it's sort of a merge-and-overwrite and, yes, of course there are multiple ways to combine layouts, thanks for asking
[6] Actual number may be less
[7] Notice how "X11" is missing in the latter two? If that's not proof that we want to get rid of X, I don't know what is!

Tuesday, August 25, 2020

libei - a library to support emulated input

Let's talk about eggs. X has always supported XSendEvent() which allows anyone to send any event to any client [1]. However, this event had a magic bit to make it detectable, so clients detect and subsequently ignore it. Spoofing input that just gets ignored is of course not productive, so in the year 13 BG [2] the XTest extension was conceived. XTest has a few requests that allow you to trigger a keyboard event (press and release, imagine the possibilities), buttons and pointer motion. The name may seem odd until someone explains to you that it was primarily written to support automated testing of X servers. But no-one has the time to explain that.

Having a separate extension worked around the issue of detectability and thus any client could spoof input events. Security concerns were addressed with "well, just ifdef out that extension then" which worked great until other applications started using it for input emulation. Since around ~2008 XTest events are emulated through special XTest devices in the server but that is solely to make the implementation less insane. Technically this means that XTest events are detectable again, except that no-one bothers to actually do that. Having said that, these devices only make it possible to detect an XTest event, but not which client sent that event. And, due to how the device hierarchy works, it's really hard to filter out those events anyway.

Now it's 2020 and we still have an X server that basically allows access to anything and any client to spoof input. This level of security is industry standard for IoT devices but we are trying to be more restrictive than that on your desktop, lest the stuff you type actually matters. The accepted replacement for X is of course Wayland, but Wayland-the-protocol does not provide an extension to emulate input. This makes sense, emulating input is not exactly a display server job [3] but it does leaves us with a bunch of things no longer working.

libei

So let's talk about libei. This new library for Emulated Input consists of two components: libei, the client library to be used in applications, and libeis, the part to be used by an Emulated Input Server, to be used in the compositors. libei provides the API to connect to an EIS implementation and send events. libeis provides the API to receive those events and pass them on to the compositor. Let's see how this looks like in the stack:

    +--------------------+             +------------------+
    | Wayland compositor |---wayland---| Wayland client B |
    +--------------------+\            +------------------+
    | libinput | libeis  | \_wayland______
    +----------+---------+                \
        |          |           +-------+------------------+
 /dev/input/       +---brei----| libei | Wayland client A |
                               +-------+------------------+
     
"brei" is the communication "Bridge for EI" and is a private protocol between libei and libeis.

Emulated input is not particularly difficult, mostly because when you're emulating input, you know exactly what you are trying to do. There is no interpretation of bad hardware data like libinput has to do, it's all straightforward. Hence the communication between libei and libeis is relatively trivial, it's all button, keyboard, pointer and touch events. Nothing fancy here and I won't go into details because that part will work as you expect it to. The selling point of libei is elsewhere, primarily in separation, distinction and control.

libei is a separate library to libinput or libwayland or Xlib or whatever else may have. By definition, it is thus a separate input stream in both the compositor and the client. This means the compositor can do things like display a warning message while emulated input is active. Or it can re-route the input automatically, e.g. assign a separate wl_seat to the emulated input devices so they can be independently focused, etc. Having said that, the libeis API is very similar to libinput's API so integration into compositors is quite trivial because most of the hooks to process incoming events are already in place.

libei distinguishes between different clients, so the compositor is always aware of who is currently emulating input. You have synergy, xdotool and a test suite running at the same time? The compositor is aware of which client is sending which events and can thus handle those accordingly.

Finally, libei provides control over the emulated input. This works on multiple levels. A libei client has to request device capabilities (keyboard, touch, pointer) and the compositor can restrict to a subset of those (e.g. "no keyboard for you"). Second, the compositor can suspend/resume a device at any time. And finally, since the input events go through the compositor anyway, it can discard events it doesn't like. For example, even where the compositor allowed keyboards and the device is not suspended, it may still filter out Escape key presses. Or rewrite those to be Caps Lock because we all like a bit of fun, don't we?

The above isn't technically difficult either, libei itself is not overly complex. The interaction between an EI client and an EIS server is usually the following:

  • client connects to server and says hello
  • server disconnects the client, or accepts it
  • client requests one or more devices with a set of capabilities
  • server rejects those devices or allows those devices and/or limits their capabilities
  • server resumes the device (because they are suspended by default)
  • client sends events
  • client disconnects, server disconnects the client, server suspends the device, etc.
Again, nothing earth-shattering here. There is one draw-back though: the server must approve of a client and its devices, so a client is not able to connect, send events and exit. It must wait until the server has approved the devices before it can send events. This means tools like xdotool need to be handled in a special way, more on that below.

Flatpaks and portals

With libei we still have the usual difficulties: a client may claim it's synergy when really it's bad-hacker-tool. There's not that much we can do outside a sandbox, but once we are guarded by a portal, things look different:

    +--------------------+
    | Wayland compositor |_
    +--------------------+  \
    | libinput | libeis  |   \_wayland______
    +----------+---------+                  \
        |     [eis-0.socket]                 \
 /dev/input/     /   \\       +-------+------------------+
                |      ======>| libei | Wayland client A |
                |      after    +-------+------------------+
         initial|     handover   /
      connection|               / initial request
                |              /  dbus[org.freedesktop.portal.EmulatedInput]
        +--------------------+
        | xdg-desktop-portal |
        +--------------------+
The above shows the interaction when a client is run inside a sandbox with access to the org.freedesktop.portal.Desktop bus. The libei client connects to the portal (it won't see the EIS server otherwise), the portal hands it back a file descriptor to communicate on and from then on it's like a normal EI session. What the portal implementation can do though is write several Restrictions for EIS on the server side of the file descriptor using libreis. Usually, the portal would write the app-id of the client (thus guaranteeing a reliable name) and maybe things like "don't allow keyboard capabilities for any device". Once the fd is handed to the libei client, the restrictions cannot be loosened anymore so a client cannot overwrite its own name.

So the full interaction here becomes:

  • Client connects to org.freedesktop.portal.EmulatedInput
  • Portal implementation verifies that the client is allowed to emulate input
  • Portal implementation obtains a socket to the EIS server
  • Portal implementation sends the app id and any other restrictions to the EIS server
  • Portal implementation returns the socket to the client
  • Client creates devices, sends events, etc.
For a client to connect to the portal instead of the EIS server directly is currently a one line change.

Note that the portal implementation is still in its very early stages and there are changes likely to happen. The general principle is unlikely to change though.

Xwayland

Turns out we still have a lot of X clients around so somehow we want to be able to use those. Under Wayland, those clients connect to Xwayland which translates X requests to Wayland requests. X clients will use XTest to emulate input which currently goes to where the dodos went. But we can add libei support to Xwayland and connect XTest, at which point the diagram looks like this:

    +--------------------+             +------------------+
    | Wayland compositor |---wayland---| Wayland client B |
    +--------------------+\            +------------------+
    | libinput | libeis  | \_wayland______
    +----------+---------+                \
        |          |           +-------+------------------+
 /dev/input/       +---brei----| libei |     XWayland     |
                               +-------+------------------+
                                                |
                                                | XTEST
                                                |
                                         +-----------+
                                         |  X client |
                                         +-----------+
XWayland is basically just another Wayland client but it knows about XTest requests, and it knows about the X clients that request those. So it can create a separate libei context for each X client, with pointer and keyboard devices that match the XTest abilities. The end result of that is you can have a xdotool libei context, a synergy libei context, etc. And of course, where XWayland runs in a sandbox it could use the libei portal backend. At which point we have a sandboxed X client using a portal to emulate input in the Wayland compositor. Which is pretty close to what we want.

Where to go from here?

So, at this point the libei repo is still sitting in my personal gitlab namespace. Olivier Fourdan has done most of the Xwayland integration work and there's a basic Weston implementation. The portal work (tracked here) is the one needing the most attention right now, followed by the implementations in the real compositors. I think I have tentative agreement from the GNOME and KDE developers that this all this is a good idea. Given that the goal of libei is to provide a single approach to emulate input that works on all(-ish) compositors [4], that's a good start.

Meanwhile, if you want to try it, grab libei from git, build it, and run the eis-demo-server and ei-demo-client tools. And for portal support, run the eis-fake-portal tool, just so you don't need to mess with the real one. At the moment, those demo tools will have a client connecting a keyboard and pointer and sending motion, button and 'qwerty' keyboard events every few seconds. The latter with client and/or server-set keymaps because that's possible too.

Eggs

What does all this have to do with eggs? "Ei", "Eis", "Brei", and "Reis" are, respectively, the German words for "egg", "ice" or "ice cream", "mush" (think: porridge) and "rice". There you go, now you can go on holidays to a German speaking country and sustain yourself on a nutritionally imbalanced diet.

[1] The whole "any to any" is a big thing in X and just shows that in the olden days you could apparently trust, well, apparently anyone
[2] "before git", i.e. too bothersome to track down so let's go with the Copyright notice in the protocol specification
[3] XTest the protocol extension exists presumably because we had a hammer and a protocol extension thus looked like a nail
[4] Let's not assume that every compositor having their own different approach to emulation is a good idea

Monday, July 6, 2020

User-specific XKB configuration - part 2

This is the continuation from this post.

Several moons have bypassed us [1] in the time since the first post, and Things Have Happened! If you recall (and of course you did because you just re-read the article I so conveniently linked above), libxkbcommon supports an include directive for the rules files and it will load a rules file from $XDG_CONFIG_HOME/xkb/rules/ which is the framework for custom per-user keyboard layouts. Alas, those files are just sitting there, useful but undiscoverable.

To give you a very approximate analogy, the KcCGST format I described last time are the ingredients to a meal (pasta, mince, tomato). The rules file is the machine-readable instruction set to assemble your meal but it relies a lot on wildcards. Feed it "spaghetti, variant bolognese" and the actual keymap ends up being the various components put together: "pasta(spaghetti)+sauce(tomato)+mince". But for this to work you need to know that spag bol is available in the first place, i.e you need the menu. This applies to keyboard layouts too - the keyboard configuration panel needs to present a list so the users can clickedy click-click on whatever layout they think is best for them.

This menu of possible layouts is provided by the xkeyboard-config project but for historical reasons [2], it is stored as an XML file named after the ruleset: usually /usr/share/X11/xkb/rules/evdev.xml [3]. Configuration utilities parse that file directly which is a bit of an issue when your actual keymap compiler starts supporting other include paths. Your fancy new layout won't show up because everything insists on loading the system layout menu only. This bit is part 2, i.e. this post here.

If there's one thing that the world doesn't have enough of yet, it's low-level C libraries. So I hereby present to you: libxkbregistry. This library has now been merged into the libxkbcommon repository and provides a simple C interface to list all available models, layouts and options for a given ruleset. It sits in the same repository as libxkbcommon - long term this will allow us to better synchronise any changes to XKB handling or data formats as we can guarantee that the behaviour of both components is the same.

Speaking of data formats, we haven't actually changed any of those which means they're about as easy to understand as your local COVID19 restrictions. In the previous post I outlined the example for the KcCGST and rules file, what you need now with libxkbregistry is an XKB-compatible XML file named after your ruleset. Something like this:

$ cat $HOME/.config/xkb/rules/evdev.xml
<?xml version="1.0" encoding="UTF-8"?>                                           
<!DOCTYPE xkbConfigRegistry SYSTEM "xkb.dtd">                                    
<xkbConfigRegistry version="1.1">                                                
  <layoutList>                                                                   
    <layout>                                                                     
      <configItem>                                                               
        <name>us</name>                                                      
      </configItem>                                                              
      <variantList>                                                              
        <variant>                                                                
          <configItem>                                                           
            <name>banana</name>                                                  
            <shortDescription>banana</shortDescription>                              
            <description>US (Banana)</description>                           
          </variant>                                                             
      </variantList>                                                             
  </layoutList>                                                                  
  <optionList>                                                                   
    <group allowMultipleSelection="true">                                        
      <configItem>                                                               
        <name>custom</name>                                                      
        <description>Custom options</description>                                
      </configItem>                                                              
      <option>                                                                   
      <configItem>                                                               
        <name>custom:foo</name>                                                  
        <description>Map Tilde to nothing</description>                          
      </configItem>                                                              
      </option>                                                                  
      <option>                                                                   
      <configItem>                                                               
        <name>custom:baz</name>                                                  
        <description>Map Z to K</description>                                    
      </configItem>                                                              
      </option>                                                                  
    </group>                                                                     
  </optionList>                                                                  
</xkbConfigRegistry>   
This looks more complicated than it is: we have models (not shown here), layouts which can have multiple variants and options which are grouped together in option group (to make options mutually exclusive). libxkbregistry will merge this with the system layouts in what is supposed to be the most obvious merge algorithm. The simple summary of that is that you can add to existing system layouts but you can't modify those - the above example will add a "banana" variant to the US keyboard layout without modifying "us" itself or any of its other variants. The second part adds two new options based on my previous post.

Now, all that is needed is to change every user of evdev.xml to use libxkbregistry. The gnome-desktop merge request is here for a start.

[1] technically something that goes around something else doesn't bypass it but the earth is flat, the moon is made of cheese, facts don't matter anymore and stop being so pedantic about things already!
[2] it's amazing what you can handwave away with "for historical reasons". Life would be better if there was less history to choose reasons from.
[3] there's also evdev.extras.xml for very niche layouts which is a separate file for historical reasons [2], despite there being a "popularity" XML attribute