evdev

The evdev driver reads touchscreen, mouse and keyboard input straight from Linux /dev/input event devices, with no user-space input library required.

Overview

The Linux event device (evdev) interface exposes input events from a touchscreen, mouse, keyboard or anything else the kernel recognizes, through device files under /dev/input/.

It's the lightest input option on Linux and the natural pairing for the framebuffer and DRM display drivers, neither of which brings its own input.

Prerequisites

An input device under /dev/input/, typically /dev/input/event0, and permission to read it.

Configuration

Enable LV_USE_EVDEV.

Building

The driver always requires libevdev.

Usage

Create an input device with lv_evdev_create, giving it the device type and node path, then attach it to a display with lv_indev_set_display.

lv_display_t * disp = lv_linux_fbdev_create();
lv_linux_fbdev_set_file(disp, "/dev/fb0");

lv_indev_t * touch = lv_evdev_create(LV_INDEV_TYPE_POINTER, "/dev/input/event0");
lv_indev_set_display(touch, disp);

The type is LV_INDEV_TYPE_POINTER for mice and touchscreens, or LV_INDEV_TYPE_KEYPAD for keyboards. lv_evdev_create_fd takes an already open file descriptor instead of a path, and lv_evdev_delete removes a device.

Finding your input device

List what the kernel sees:

cat /proc/bus/input/devices

Entries mention an event handler. event1 corresponds to /dev/input/event1. Confirm you picked the right one by watching its events:

evtest /dev/input/event1

Calibrating a touchscreen

Raw touchscreens report device coordinates, not screen coordinates. Map them with lv_evdev_set_calibration, and correct a transposed panel with lv_evdev_set_swap_axes.

lv_evdev_set_swap_axes(touch, true);
lv_evdev_set_calibration(touch, min_x, min_y, max_x, max_y);

Automatic device discovery

Rather than naming device nodes, let the driver find them. lv_evdev_discovery_start scans /dev/input/ and keeps watching it, so devices plugged in later are added automatically. Passing NULL for both arguments is enough to enable it:

lv_evdev_discovery_start(NULL, NULL);

Supply a callback when you need to react to a new device — to attach a cursor image to a mouse, for example:

#include "lvgl/src/core/lv_global.h"

static void indev_deleted_cb(lv_event_t * e)
{
    if(LV_GLOBAL_DEFAULT()->deinit_in_progress) return;
    lv_obj_t * cursor_obj = lv_event_get_user_data(e);
    lv_obj_delete(cursor_obj);
}

static void discovery_cb(lv_indev_t * indev, lv_evdev_type_t type, void * user_data)
{
    LV_LOG_USER("new '%s' device discovered", type == LV_EVDEV_TYPE_REL ? "REL" :
                                             type == LV_EVDEV_TYPE_ABS ? "ABS" :
                                             type == LV_EVDEV_TYPE_KEY ? "KEY" :
                                             "unknown");

    if(type == LV_EVDEV_TYPE_REL) {
        LV_IMAGE_DECLARE(mouse_cursor_icon);
        lv_obj_t * cursor_obj = lv_image_create(lv_screen_active());
        lv_image_set_src(cursor_obj, &mouse_cursor_icon);
        lv_indev_set_cursor(indev, cursor_obj);
        lv_indev_add_event_cb(indev, indev_deleted_cb, LV_EVENT_DELETE, cursor_obj);
    }
}

int main(void)
{
    /* ... */
    lv_evdev_discovery_start(discovery_cb, NULL);
    /* ... */
}

lv_evdev_discovery_stop turns it off again.

Raw key codes

For keys LVGL does not map to one of its own, lv_evdev_is_raw_key tells you an event carries a raw code and lv_evdev_get_raw_key returns it. Use these when you need keys outside LVGL's navigation set.

Support

See Also

• libinput - richer input stack, with XKB keyboard support
• Linux Framebuffer - display driver commonly paired with evdev
• DRM - display driver commonly paired with evdev
• Dependency Management - how LVGL resolves the evdev library