Linux kernel file system -sysfs analysis

First, SYSFS Introduction

sysfsIt is a pseudo file system. Does not represent real physical devices, in the Linux kernel,sysfsThe file system will exist for a long time in memory.sysfsUsed to model specific kernel objects (e.g., physical devices) and provide a method associated with device and device driver. use

ls -l /sys

Commands can be viewedsysfsWhich kernel objects have been exported in the file system. As shown below:

It can be seen from the above figure,sysfsBlock, Bus, Class, Dev, Devices, Firmware, FS, Kernel, Module, and Power kernel objects (different kernel configurations are different).

From the kernel source code. HerebusThe kernel object is an example, the kernel source will bebuses_init()Call in the functionkset_create_and_add()The kernel function creates a BUS object collection and adds it to the SYSFS file system.

Developers can usesysfsTo determine useful information about the running kernel, such as: The kernel finds which devices are found on each bus (BUS), and which drivers that are bound each device can be known from the SYSFS file system. In addition, SYSFS can also be used for debugging, optimizing devices, and other kernel subsystems.

Current block (block) subsystem usessysfsTo mount the root partition. If SYSFS is disabled, you must specify the boot device in the kernel boot command line. For example, for / dev / hda1 devices, the command can beroot=03:01。

For designers of embedded systems, according to actual use scenarios, can be disabledsysfsFile system to save system resources.

Second, SYSFS file system source code analysis

First, the Linux kernelsysfsThe source code of the file system is located in the (/ fs / sysfs) directory. As shown below (the source code has been compiled):

fromMakefileThe file can be seen that 5 source files will be compiled during the compilation process:file.o dir.o symlink.o mount.o group.o

This section does not analyzesysfsThe internal implementation details of the file system, mainly analyzes the following two questions:

（1）sysfsHow does the file system mounted in the Linux kernel?

(2) How to add the kernel object set to how the Linux kernelsysfsFile system?

（2-1）sysfsHow to mount the file system in the Linux kernel

existstart_kernel()The function will be calledvfs_caches_init(totalram_pages);The function is initialized by the virtual file system, and this function will be called.mnt_init()Function, inmnt_init()The function will be calledsysfs_init()rightsysfsThe file system is mounted. The following picture:

code segment:

void __init mnt_init(void)
{
    //...
    
	kernfs_init();

	err = sysfs_init();
	if (err)
		printk(KERN_WARNING "%s: sysfs_init error: %d\n",
			__func__, err);

    //....
}

（/fs/sysfs/mount.c）

static struct file_system_type sysfs_fs_type = {
	.name		= "sysfs",
	.mount		= sysfs_mount,
	.kill_sb	= sysfs_kill_sb,
	.fs_flags	= FS_USERNS_VISIBLE | FS_USERNS_MOUNT,
};

int __init sysfs_init(void)
{
	int err;

	sysfs_root = kernfs_create_root(NULL, KERNFS_ROOT_EXTRA_OPEN_PERM_CHECK,
					NULL);
	if (IS_ERR(sysfs_root))
		return PTR_ERR(sysfs_root);

	sysfs_root_kn = sysfs_root->kn;

	err = register_filesystem(&sysfs_fs_type);
	if (err) {
		kernfs_destroy_root(sysfs_root);
		return err;
	}

	return 0;
}

The above code first-line code, defines the type of SYSFS file system description, specified.name、.mount、.kill_sb、.fs_flags。

existsysfs_init()Call in the functionkernfs_create_root()（kernfsIt is a universal kernel virtual file system, which is more complex this article not to do excessive analysis) created Kernfs_Root. Then callregister_filesystem()Register to kernelsysfsFile system.

(2-2) How to add the kernel object set to how the Linux kernelsysfsFile system?

The above writes, the Linux kernel is called.kset_create_and_add()The kernel function creates a kernel object set to add the kernel object set to the SYSFS file system, this section will look at the function.

This function has the following important function call relationships:

It can be seen from the above figure,kset_create_and_add()The function will last callsysfs_create_dir_ns()This function. Thereby create a SYSFS file system directory. The code snippet is as follows:

（/fs/sysfs/dir.c）

int sysfs_create_dir_ns(struct kobject *kobj, const void *ns)
{
	struct kernfs_node *parent, *kn;

	BUG_ON(!kobj);

	if (kobj->parent)
		parent = kobj->parent->sd;
	else
		parent = sysfs_root_kn;

	if (!parent)
		return -ENOENT;

	kn = kernfs_create_dir_ns(parent, kobject_name(kobj),
				  S_IRWXU | S_IRUGO | S_IXUGO, kobj, ns);
	if (IS_ERR(kn)) {
		if (PTR_ERR(kn) == -EEXIST)
			sysfs_warn_dup(parent, kobject_name(kobj));
		return PTR_ERR(kn);
	}

	kobj->sd = kn;
	return 0;
}

Third, summary

This article describes the starting point from Linux kernel code and the actual system operation.sysfsSome functions of the file system, and analyze how the Linux kernel initializes and mountsysfsFile system, and how the INUX kernel adds the kernel object set tosysfsDocument system.

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