Linux input event interface numbering

Question

In Linux, how do you create an input event interface with a user specified event number and map that to a specific device event?

I'm using the gpio-keys driver to translate key presses from a keypad. I define the keys to be used in my board configuration source file as shown below

static struct gpio_keys_button ev_keys[] = {
    [0] = {
        .type           = EV_KEY,
        .active_low     = 1,
        .wakeup         = 0,
        .debounce_interval  = KEYS_DEBOUNCE_MS,
        .code           = KEY_MUTE,
       .desc           = "mute", 
       .gpio           = PUSHBUTTON_MUTE, 
    }
};

and register this with the kernel.

And I enable the event interface and GPIO buttons when building the kernel.

Device Drivers ---> Input device support --> Event interface

Device Drivers ---> Input device support --> Keyboards --> GPIO buttons

This creates a node to the event at /dev/input/event0 to which the GPIO button events are mapped. In a system that uses only one event interface I can call poll() on the fd to /dev/input/event0 and everything works as expected.

Now, I have second peripheral on my system that uses /dev/input/event0 by default and I need to map the events from the gpio-keys driver to another event. Any suggestions on how I go about creating an event with a number/id I can specify and then map this to the gpio-keys events?

Thanks.

JvO · Accepted Answer · 2013-09-11 10:51:06Z

If you mean by "mapping" specifying the name of the /dev/input/eventX 'file', you should use Udev. The kernel assigns the number of the event device, it is a bad idea and probably impossible to try and force this number since you never know which other device may have gotten this number first.

My recommendation would be to let Udev create a symlink that points to your device; you can choose your own name and use that in your program (i.e. /dev/my_first_keypad). For example, my Wacom tablet is assigned /dev/wacom with the following udev rule:

KERNEL=="event*", SUBSYSTEM=="input", SUBSYSTEMS=="input", ATTRS{name}=="Wacom Volito", SYMLINK+="wacom"

The trick is to find the proper set of variables to exactly specify your keypad. If it is USB based, the vender/product ID are a good start. Otherwise, use udevadm info --export-db to get a full dump of the Udev database. Udev rules go in files in /etc/udev/rules.d/ or /lib/udev.d, depending on the Linux distribution you are using.

Mikhail Boiko · Accepted Answer · 2013-09-12 10:30:07Z

0

You can check System.map file for functions that register event interface. The one that comes first, usually gets lowest eventX number and later functions gets eventX number increased by one. IMO, its ok to rely on static device node file for embedded device where device configuration is static and is not going to change during operation, but generally you should use udev for you purposes.

answered Sep 12, 2013 at 10:30

Mikhail Boiko

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