Re: Linux General Joystick Max buttons limitation 80

[Wei Shuai <cpuwolf@gmail.com>]

Dear Dmitry Torokhov,

I have tried this change on Ubuntu, it really works. The small fix
will break the limitation of the max button number of Joystick.
Please help to review the patch, I have sent the patch in a different
email thread.

Wei Shuai <cpuwolf@gmail.com> 于2020年7月10日周五 上午7:10写道：
>
> Dear Joystick Maintainer,
>
> we found Linux kernel has max joystick buttons up to 80, no more
>
> We are selling a USB joystick device QMCP737C for flight simulator,
> which is nothing but a common USB joystick. it has 104 buttons
>
> Product link
> https://x-plane.vip/quickmade/qmcp737c/
>
> it works good on Windows, Mac, but not on Linux.
> Linux kernel exposes /dev/input/js0 max to 80 buttons, but we have 104 buttons.
>
> I did a lot of google search, but nothing I got. then I have to look
> at Kernel source, to find out where this number max 80 comes from
>
> Eventually I found the final limitation
>
> #define BTN_JOYSTICK 0x120
>
> #define BTN_DEAD 0x12f
>
> #define BTN_TRIGGER_HAPPY 0x2c0
>
> #define KEY_MAX 0x2ff
>
> include/uapi/linux/input-event-codes.h
>
> according to function hidinput_configure_usage() in file drivers/hid/hid-input.c
>
> the joystick button mapping is not a continuous space, general speak
> the mapping space is from
>
> BTN_JOYSTICK~BTN_DEAD
> BTN_TRIGGER_HAPPY~KEY_MAX
>
> and finally I got the max limitation is 80.
>
> My question is why KEY_MAX is 0x2ff?  this number 0x2ff looks like it
> does not align with char?integer? the answer is no
>
> so may I ask to expand KEY_MAX to such as 0x4ff? or you need me to
> submit patch for it, I am OK
>
> Thank you