linux-kernel - Re: [PATCH] Input: break joystick limitation of maximum 80 buttons

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Date:   Mon, 13 Jul 2020 18:29:00 +0800
From:   Wei Shuai <cpuwolf@...il.com>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc:     "open list:INPUT (KEYBOARD, MOUSE, JOYSTICK, TOUCHSCREEN)..." 
        <linux-input@...r.kernel.org>,
        Dmitry Torokhov <dmitry.torokhov@...il.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jeffrey Hugo <jhugo@...eaurora.org>,
        Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>,
        Borislav Petkov <bp@...e.de>, Mattias Jacobsson <2pi@....nu>,
        Mark Gross <mgross@...ux.intel.com>,
        Russell King <rmk+kernel@...linux.org.uk>,
        Erwan Velu <e.velu@...teo.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] Input: break joystick limitation of maximum 80 buttons

hi Andy,
sorry from a newbie

we found Linux kernel has max joystick buttons up to 80, no more

We are selling USB joystick device QMCP737C for flight simulator,
which is nothing but common USB joystick. it has 104 buttons

Product link
https://x-plane.vip/quickmade/qmcp737c/

it works well on Windows, Mac, but not on Linux.
Linux kernel exposes /dev/input/js0 max to 80 button, 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 continues space, generally 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 not
align with char? integer? the answer is no

so may I ask to expand KEY_MAX to such as 0x4ff?

Andy Shevchenko <andriy.shevchenko@...ux.intel.com> 于2020年7月13日周一 下午5:06写道：
>
> On Sun, Jul 12, 2020 at 07:54:54AM +0800, Wei Shuai wrote:
> > The joystick max buttons 80 limitation comes from
> >
> > #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 continues space
> > generally speaking, the mapping space is from
> >
> > 1. BTN_JOYSTICK~BTN_DEAD
> > 2. BTN_TRIGGER_HAPPY~KEY_MAX
> >
> > Finally, I got the max limitation is 80.
> > The patch is expanding KEY_MAX from 0x2ff to 4ff
> > and the change has been verified on 104 button USB HID device on Ubuntu
>
>
> If you send and resend this patch too often, nobody will take you seriously.
> Also, you have to provide versioning and changelog in each submission.
> Besides that, above doesn't answer to question "why?". Why are you doing this
> at all? What the problem?
>
>
> --
> With Best Regards,
> Andy Shevchenko
>
>