How Linux creates device files under /dev

This post is used to describe how Linux creates device files under /dev, illustrated with devices attached to TCC8935's DISPLAY bus.

First of all, the "create device file" procedures are device-type specific:

1. Char device.   Example: VIQE //drivers/video/tcc/viqe/tcc_vioc_viqe.c

    When the driver (module) is loaded in:

    static int __init tcc_viqe_init(void)
    {
    int res;

    res = register_chrdev(VIQE_DEV_MAJOR, VIQE_DEV_NAME, &tcc_viqe_fops);
    if (res < 0)
        return res;
    viqe_class = class_create(THIS_MODULE, VIQE_DEV_NAME);
    device_create(viqe_class, NULL, MKDEV(VIQE_DEV_MAJOR, VIQE_DEV_MINOR), NULL, VIQE_DEV_NAME);

    printk(KERN_INFO "%s\n", __FUNCTION__);
    return 0;
    }

   The routine device_create() creates  a node under /dev according to MAJOR / MINOR / NAME parameters, /dev/viqe.

   BTW, routine class_create() creates a class "viqe" under /sys/class.

   Similar examples include WMIXER / SCALER. //drivers/char/tcc_wmixer.c, tcc_scaler.c

2. Platform device. 

     According to other blogs, Linux creates device node for platform devices when the DEVICE is registered, rather than the driver is registered.

     For example:

     platform_device_register(&tcc_lcd_device); (the struct tcc_lcd_device's NAME is "tccfb", which is identical with it's driver's name.

     CALLS platform_device_add(dev) -> CALLS device_add()

     routine device_add() calls device_create_file() to create files under /sys

     and

     calls devtmpfs_create_node() to create device node under /dev.

     NOTE: see blow blogs for details:  http://jieohsong.blog.163.com/blog/static/110530884201343054853424/ and

     I'm not sure if it's correct but i do cannot find a "tccfb" node under /dev.

     Well, my understanding is totally different. The driver creates device node within PROBE phase, and, the approach isdevice specific.

     Example 1: tccfb  //drivers/video/tcc/tcc_vioc_fb.c

     This is a PLATFORM device since the driver is registered via platform_driver_register().

     In routine probe(), calls register_framebuffer() -> do_register_framebuffer() , in do_register_framebuffer(),

     fb_info->dev = device_create(fb_class, fb_info->device,
                     MKDEV(FB_MAJOR, i), NULL, "fb%d", i);

     The fb_class is "graphics", FB_MAJOR is 29.

     Here, we see device_create() again, exact the same as a char device above. And, there is a node under /dev/graphics, fb0, major number is 29.

     Example 2:  overlay //drivers/video/tcc/tcc_overlay.c

     This is a PLATFORM device.

     static struct platform_device tcc_overlay_device = {
    .name    = "tcc_overlay",
    .dev    = {
        .release     = NULL,
     },
    .id    = 0,
     };

     But also a MISC device.

     static struct miscdevice overlay_misc_device =
     {
    DEV_MINOR,
    DEVICE_NAME,
    &tcc_overlay_fops,
     };

    At the end of probe(), within misc_register(&overlay_misc_device), calls

    misc->this_device = device_create(misc_class, misc->parent, dev,
                      misc, "%s", misc->name);

    Familiar? That it!  It creates /dev/overlay.