How Linux creates device files under /dev

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本文详细阐述了Linux系统如何在/dev目录下创建设备文件，包括字符设备、平台设备的创建过程，以及创建方法的区别与实例分析。

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 is device 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.