No input events for rudder pedals on Linux

[MrMinimal]

Godot version:
3.1

OS/device including version:
Linux

Issue description:
Godot does not recognize joysticks/gamepads with axis only (e.g. rudder pedals). As a result no input events reach the game logic. Happens when compiling with both udev=yes and udev=no.

Code which produces this bug:
https://github.com/godotengine/godot/blob/master/platform/x11/joypad_linux.cpp#L342

(reformatted with values from a gamepad vs my rudder pedals):

The offending line discards any events coming from joysticks that don't have any buttons(test1).
Removing the line fixed the problem for me, but is only a temporary solution.

Steps to reproduce:

1. Plug in a joystick without any buttons, only axis
2. Have a GDScript with an input function like so:

func _input(event):
    print(event)

3. Move axis on joystick
4. Observe no InputEventJoypadMotion events occuring

[MrMinimal]

I would like to hand in a PR, but this comment is problematic:

//check if the device supports basic gamepad events, prevents certain keyboards from
//being detected as joypads

What are the edge cases the boolean logic is supposed to prevent?
I would have to rewrite the logic in that line, but how do I make sure all edge cases are still covered?

[akien-mga]

CC @Hinsbart

[groud]

Have you checked if it was detected with another software ? Something like jstest ?
To be sure the problem does not come from a lack of drivers on your side.

[MrMinimal]

@groud Yes I did, that's how I found the offending source code in the first place. I updated the issue to reflect that the issue is fixed by commenting out the linked line.
But that won't do for a PR, gotta find the edge cases to write proper handling logic.

[timower]

I have the exact same issue with a usb RC controller as it has no buttons.
Commenting out the line that checks for buttons will cause my mouse, touchscreen en pen tablet to be recognized as joypads.
But I've searched for an alternative way to check if the device is a joystick and couldn't think of one that wouldn't rule out some potential input devices.
The questions is: is it so bad that a touch screen is seen as a potential joystick input devices, the axis make sense as they are the x and y position of my finger on the screen.

[KingKone]

Same here:
Godot on Windows detects my RC-controller
Godot on Linux does not see it.
Also my controller has only axis, no buttons.
@timower nice project you got there

[supertin]

+1 on Linux. I've currently got CH rudder pedals (3 axis, 0 buttons) as /dev/js0 and a CH throttle quad (6 axis, 12 buttons) as /dev/js1. As a long time Linux user, I immediately assumed "Device 0" meant "js0", and spent about half an hour getting increasingly frustrated with why Godot was refusing to see my button presses on "Device 1" (answer - it didn't exist).

This issue would also effect things like standalone handbrake levers, higher end standalone driving pedals, custom devices for flight, train or industrial machinery simulation, etc.

[akien-mga]

CC @madmiraal, though I doubt it's easy to debug/fix without hardware to test on.