evdev: Distinguish between full keyboards and devices that merely have some keys

[smcv]

In both udev and steam-runtime-tools, the equivalent of SDL_UDEV_DEVICE_KEYBOARD is split into two:

• KEYBOARD is a reasonably full alphanumeric keyboard that you could use to type commands or your password;
• HAS_KEYS is anything that can produce keyboard events, including USB numeric keypads, Bluetooth headset volume buttons, ACPI power buttons and even a few game controllers

At the moment SDL_UDEV_DEVICE_KEYBOARD is basically udev's HAS_KEYS.

If we want to change this in SDL 3, I would suggest renaming SDL_UDEV_DEVICE_KEYBOARD to SDL_UDEV_DEVICE_HAS_KEYS and introducing a new SDL_UDEV_DEVICE_FULL_KEYBOARD, so that anyone porting SDL 2 code is forced to decide which one they meant.

The logic in steam-runtime-tools, and I think also udev, is that a keyboard is assumed to be a full alphanumeric keyboard if it has at least Escape, 0-9 and Q-D, so the end of SDL_EVDEV_GuessDeviceClass would change to something like this:

    keyboard_mask = 0xFFFFFFFE;
    if ((bitmask_key[0] & keyboard_mask) != 0) {
        devclass |= SDL_UDEV_DEVICE_HAS_KEYS;
    }

    if ((bitmask_key[0] & keyboard_mask) == keyboard_mask) {
        devclass |= SDL_UDEV_DEVICE_FULL_KEYBOARD;
    }

[smcv]

(As discussed briefly on #7826)

[slouken]

Looking more closely I think that SDL's current behavior is intended. I think we wanted to support things like headsets with additional multimedia keys and so forth. Right now, if you plug in a controller that generates keys as well as gamepad input, we'll send the keys as keyboard input to the application. I don't think there's any reason to prevent that.

[slouken]

Also, the SDL udev support is entirely internal, there's no SDL 2 application code that would be affected by any change we make here.

[smcv]

That's fine, that would be HAS_KEYS in udev-speak. The thing that makes KEYBOARD more useful is for ruling out keyboards from slightly shaky heuristics, like this pseudocode:

if (!(flags & KEYBOARD) && weak_indication_of_being_a_gamepad) {
    do gamepad things;
}