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The sysfs filesystem is an in-memory virtual filesystem that provides a view of the kobject object hierarchy. It enables users to view the device topology of their system as a simple filesystem. Using attributes, kobjects can export files that allow kernel variables to be read from and optionally written to.

Although the intended purpose of the device model was initially to provide a device topology for power management reasons, a quite spectacular offshoot was sysfs. To facilitate debugging, the device model's developer decided to export the tree as a file-system. This quickly proved quite useful, at first as a replacement for device-related files that previously found themselves in /proc, and later as a powerful view into the system's object hierarchy. Indeed, sysfs, originally called driverfs, predated kobjects. Eventually sysfs made it clear that a new object model would be quite beneficial, and kobject was born. Today, every system with a 2.6 kernel has sysfs and nearly all have it mounted.

The magic behind sysfs is simply tying kobjects to directory entries via the dentry variable inside each kobject. Recall from Chapter 12, "The Virtual Filesystem," that the dentry structure represents directory entries. By linking kobjects to dentries, kobjects trivially map to directories. Exporting the kobjects as a filesystem is now as easy as building a tree of the dentries in memory. But wait! kobjects already form a tree, our beloved device model. With kobjects mapping to dentries and the object hierarchy already forming an in-memory tree, sysfs became trivial.

Figure 17.2 is a partial view of the sysfs filesystem as mounted at /sys.

Figure 17.2. A view of part of the /sys tree.

The root of the sysfs contains seven directories: block, bus, class, devices, firmware, module, and power. The block directory contains one directory for each of the registered block devices on the system. Each of those directories, in turn, contains any partitions on the block device. The bus directory provides a view of the system buses. The class directory contains a view of the devices on the system organized by high-level function. The devices directory is a view of the device topology of the system. It maps directly to the hierarchy of device structures inside the kernel. The firmware directory contains a system-specific tree of low-level subsystems such as ACPI, EDD, EFI, and so on. The power directory contains system-wide power management data.

The most important directory is devices, which exports the device model to the world. The directory structure is the actual device topology of the system. Much of the data in other directories is simply alternative organizations of the data in the devices directory. For example, /sys/class/net/ organizes devices by the high-level concept of registered network interfaces. Inside this directory might be the subdirectory eth0, which contains the symlink device back to the actual device directory in devices.

Take a look at /sys on any Linux systems that you have access to. Such an accurate view into the system's device is pretty neat, and seeing the interconnection between the high-level concepts in class versus the low-level physical devices in devices and the actual drivers in bus is very informative. The whole experience is even more rewarding when you realize that this data is free, that this is the very representation of the system maintained inside the kernel[1].

[1] If you find sysfs interesting, you might be interested in HAL, a hardware abstraction layer, which can be found at HAL builds an in-memory database based on the data in sysfs, linking together the concepts of class, device, and driver. On top of this data, HAL provides a rich API allowing for smarter, more aware applications.

Adding and Removing kobjects from sysfs

Initialized kobjects are not automatically exported to sysfs. To represent a kobject to sysfs, you use kobject_add():

int kobject_add(struct kobject *kobj);

The kobject's location in sysfs depends on the kobject's location in the object hierarchy. If the kobject's parent pointer is set, the kobject will map to a subdirectory in sysfs inside its parent. If the parent pointer is not set, the kobject will map to a subdirectory inside kset->kobj. If neither the parent nor the kset fields are set in the given kobject, the kobject is assumed to have no parent and will map to a root-level directory in sysfs. This is almost assuredly what you want, so one or both of parent and kset should be set appropriately before kobject_add() is called. Regardless, the name of the directory representing the kobject in sysfs is given by kobj->k_name.

Rather than call both kobject_init() and kobject_add(), you can call kobject_register():

int kobject_register(struct kobject *kobj)

This function both initializes the given kobject and adds it to the object hierarchy.

Removing a kobject's sysfs representation is done via kobject_del():

void kobject_del(struct kobject *kobj);

The function kobject_unregister() combines both kobject_del() and kobject_put():

void kobject_unregister(struct kobject * kobj)

All four of these functions are defined in lib/kobject.c and declared in <linux/kobject.h>.

Adding Files to sysfs

kobjects map to directories, and the complete object hierarchy maps nicely to the complete sysfs directory structure, but what about files? sysfs is nothing but a pretty tree without files to provide actual data.

Default Attributes

A default set of files is provided via the ktype field in kobjects and ksets. Consequently, all kobjects of the same type have the same default set of files populating their sysfs directories. The kobj_type structure contains a member, default_attrs, that is an array of attribute structures. Attributes map kernel data to files in sysfs.

The attribute structure is defined in <linux/sysfs.h>:

/* attribute structure - attributes map kernel data to a sysfs file */
struct attribute {
        char             *name;    /* attribute's name */
        struct module    *owner;   /* owning module, if any */
        mode_t           mode;     /* permissions */

The name member provides the name of this attribute. This will be the filename of the resulting file in sysfs. The owner member points to a module structure representing the owning module, if any. If a module does not own this attribute, this field is NULL. The mode member is a mode_t type that specifies the permissions for the file in sysfs. Read-only attributes probably want to set this to S_IRUGO if they are world readable and S_IRUSR if they are only owner readable. Writable attributes probably want to set mode to S_IRUGO | S_IWUSR. All files and directories in sysfs are owned by uid zero and gid zero.

Although default_attrs lists the default attributes, sysfs_ops describes how to use them. The sysfs_ops member is a pointer to a structure of the same name, which is defined in <linux/sysfs.h>:

struct sysfs_ops {
        /* method invoked on read of a sysfs file */
        ssize_t (*show) (struct kobject *kobj,
                         struct attribute *attr,
                         char *buffer);

        /* method invoked on write of a sysfs file */
        ssize_t (*store) (struct kobject *kobj,
                          struct attribute *attr,
                          const char *buffer,
                          size_t size);

The show() method is invoked on read. It must copy the value of the attribute given by attr into the buffer provided by buffer. The buffer is PAGE_SIZE bytes in length; on x86, PAGE_SIZE is 4096 bytes. The function should return the size in bytes of data actually written into buffer on success or a negative error code on failure.

The store() method is invoked on write. It must read the size bytes from buffer into the variable represented by the attribute attr. The size of the buffer is always PAGE_SIZE or smaller. The function should return the size in bytes of data read from buffer on success or a negative error code on failure.

Because this single set of functions must handle file I/O requests on all attributes, they typically need to maintain some sort of generic mapping to invoke a handler specific to each attribute.

Creating New Attributes

Generally, the default attributes provided by the ktype associated with a kobject are sufficient. Indeed, because all the kobjects of the same ktype are supposed to be relatedif not identical in nature as in, say, all partitionsthe same set of attributes should satisfy all kobjects. This not only simplifies life, but also provides code consolidation and a uniform look and feel to sysfs directories of related objects.

Nonetheless, often some specific instance of a kobject is somehow special. It wants or even needs its own attributesperhaps to provide data or functionality not shared by the more general ktype. To this end, the kernel provides the sysfs_create_file() interface for adding new attributes on top of the default set:

int sysfs_create_file(struct kobject *kobj, const struct attribute *attr);

This badboy will associate the attribute structure pointed at by attr with the kobject pointed at by kobj. Before it is invoked, the given attribute should be filled out. This function returns zero on success and a negative error code otherwise.

Note that the sysfs_ops specified in the kobject's ktype is invoked to handle this new attribute. The existing default show() and store() methods must be capable of handling the newly created attribute.

In addition to creating actual files, it is possible to create symbolic links. Creating a symlink in sysfs is very easy:

int sysfs_create_link(struct kobject *kobj, struct kobject *target, char *name);

This function creates a link named name in the directory mapped from kobj to the directory mapped from target. This function returns zero on success and a negative error code otherwise.

Destroying New Attributes

Removing an attribute is handled via sysfs_remove_file():

void sysfs_remove_file(struct kobject *kobj, const struct attribute *attr);

Upon call return, the given attribute will no longer appear in the given kobject's directory.

Symbolic links created with sysfs_create_link() may be removed with sysfs_remove_link():

void sysfs_remove_link(struct kobject *kobj, char *name);

Upon return, the symbolic link name in the directory mapped from kobj is removed.

All four of these functions are declared in <linux/kobject.h>. The sysfs_create_file() and sysfs_remove_file() functions are defined in fs/sysfs/file.c. The sysfs_create_link() and sysfs_remove_link() functions are defined in fs/sysfs/symlink.c.

sysfs Conventions

The sysfs filesystem is currently the place for implementing functionality previously reserved for ioctl() calls on device nodes or the procfs filesystem. These days, the chic thing to do is implement such functionality as sysfs attributes in the appropriate directory. For example, instead of a new ioctl() on a device node, add a sysfs attribute in the driver's sysfs directory. Such an approach avoids the type-unsafe use of obscure ioctl() arguments and the haphazard mess of /proc.

To keep sysfs clean and intuitive, however, developers must follow certain conventions.

First, sysfs attributes should export one value per file. Values should be text-based and map to simple C types. The goal is to avoid the highly structured or highly messy representation of data we have today in /proc. Providing one value per file makes reading and writing trivial from the command line and enables C programs to easily slurp the kernel's data from sysfs into their own variables. In situations where the one-value-per-file rule results in an inefficient representation of data, it is acceptable to place multiple values of the same type in one file. Delineate them as appropriate; a simple space probably makes the most sense. Ultimately, think of sysfs attributes as mapping to individual kernel variables (as they usually do) and keep in mind ease of manipulation from user-space, particularly from the shell.

Second, organize data in sysfs in a clean hierarchy. Correctly parent kobjects so that they map intuitively into the sysfs tree. Associate attributes with the correct kobject and keep in mind that the kobject hierarchy exists not only in the kernel but also as an exported tree to user-space. Keep the sysfs tree sane!

Finally, keep in mind that sysfs provides a kernel-to-user service and is thus some sort of user-space ABI. User programs may come to rely on the existence, location, value, and behavior of sysfs directories and files. Changing existing files in any way is discouraged, and modifying the behavior of a given attribute but keeping its name and location is surely begging for trouble.

These simple conventions should allow sysfs to provide a rich and intuitive interface to user-space. Use sysfs correctly and user-space developers will not curse your existence but praise your beautiful code.

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