Why is my device a touchpad and a mouse and a keyboard?

If you have spent any time around HID devices under Linux (for example if you are an avid mouse, touchpad or keyboard user) then you may have noticed that your single physical device actually shows up as multiple device nodes (for free! and nothing happens for free these days!). If you haven't noticed this, run libinput record and you may be part of the lucky roughly 50% who get free extra event nodes.

The pattern is always the same. Assuming you have a device named FooBar ExceptionalDog 2000 AI[1] what you will see are multiple devices

/dev/input/event0: FooBar ExceptionalDog 2000 AI Mouse
/dev/input/event1: FooBar ExceptionalDog 2000 AI Keybard 
/dev/input/event2: FooBar ExceptionalDog 2000 AI Consumer Control 

The Mouse/Keyboard/Consumer Control/... suffixes are a quirk of the kernel's HID implementation which splits out a device based on the Application Collection. [2]

A HID report descriptor may use collections to group things together. A "Physical Collection" indicates "these things are (on) the same physical thingy". A "Logical Collection" indicates "these things belong together". And you can of course nest these things near-indefinitely so e.g. a logical collection inside a physical collection is a common thing.

An "Application Collection" is a high-level abstractions to group something together so it can be detected by software. The "something" is defined by the HID usage for this collection. For example, you'll never guess what this device might be based on the hid-recorder output:

# 0x05, 0x01,                    // Usage Page (Generic Desktop)              0
# 0x09, 0x06,                    // Usage (Keyboard)                          2
# 0xa1, 0x01,                    // Collection (Application)                  4
...
# 0xc0,                          // End Collection                            74

Yep, it's a keyboard. Pop the champagne[3] and hooray, you deserve it.

The kernel, ever eager to help, takes top-level application collections (i.e. those not inside another collection) and applies a usage-specific suffix to the device. For the above Generic Desktop/Keyboard usage you get "Keyboard", the other ones currently supported are "Keypad" and "Mouse" as well as the slightly more niche "System Control", "Consumer Control" and "Wireless Radio Control" and "System Multi Axis". In the Digitizer usage page we have "Stylus", "Pen", "Touchscreen" and "Touchpad". Any other Application Collection is currently unsuffixed (though see [2] again, e.g. the hid-uclogic driver uses "Touch Strip" and other suffixes).

This suffix is necessary because the kernel also splits out the data sent within each collection as separate evdev event node. Since HID is (mostly) hidden from userspace this makes it much easier for userspace to identify different devices because you can look at a event node and say "well, it has buttons and x/y, so must be a mouse" (this is exactly what udev does when applying the various ID_INPUT properties, with varying levels of success).

The side effect of this however is that your device may show up as multiple devices and most of those extra devices will never send events. Sometimes that is due to the device supporting multiple modes (e.g. a touchpad may by default emulate a mouse for backwards compatibility but once the kernel toggles it to touchpad mode the mouse feature is mute). Sometimes it's just laziness when vendors re-use the same firmware and leave unused bits in place.

It's largely a cosmetic problem only, e.g. libinput treats every event node as individual device and if there is a device that never sends events it won't affect the other event nodes. It can cause user confusion though: "why does my laptop say there's a mouse?" and in some cases it can cause functional degradation - the two I can immediately recall are udev detecting the mouse node of a touchpad as pointing stick (because i2c mice aren't a thing), hence the pointing stick configuration may show up in unexpected places. And fake mouse devices prevent features like "disable touchpad if a mouse is plugged in" from working correctly. At the moment we don't have a good solution for detecting these fake devices - short of shipping giant databases with product-specific entries we cannot easily detect which device is fake. After all, a Keyboard node on a gaming mouse may only send events if the user configured the firmware to send keyboard events, and the same is true for a Mouse node on a gaming keyboard.

So for now, the only solution to those is a per-user udev rule to ignore a device. If we ever figure out a better fix, expect to find a gloating blog post in this very space.

[1] input device naming is typically bonkers, so I'm just sticking with precedence here
[2] if there's a custom kernel driver this may not apply and there are quirks to change this so this isn't true for all devices
[3] or sparkling wine, let's not be regionist here