sysfs- The filesystem for exporting kernel objects

sysfs - _The_ filesystem for exporting kernel objects.

Patrick Mochel    
Mike Murphy

Revised:    22 February 2009
Original:   10 January 2003


What it is:
~~~~~~~~~~~

sysfs is a ram-based filesystem initially based on ramfs. It provides a means to export kernel data structures, their attributes, and the linkages between them to userspace.

sysfs is tied inherently to the kobject infrastructure. Please read Documentation/kobject.txt for more information concerning the kobject interface.


Using sysfs
~~~~~~~~~~~

sysfs is always compiled in if CONFIG_SYSFS is defined. You can access it by doing:

    mount -t sysfs sysfs /sys


Directory Creation
~~~~~~~~~~~~~~~~~~

For every kobject that is registered with the system, a directory is created for it in sysfs. That directory is created as a subdirectory of the kobject's parent, expressing internal object hierarchies to userspace. Top-level directories in sysfs represent the common
ancestors of object hierarchies; i.e. the subsystems the objects belong to.

Sysfs internally stores the kobject that owns the directory in the ->d_fsdata pointer of the directory's dentry. This allows sysfs to do reference counting directly on the kobject when the file is opened and closed.


Attributes
~~~~~~~~~~

Attributes can be exported for kobjects in the form of regular files in the filesystem. Sysfs forwards file I/O operations to methods defined for the attributes, providing a means to read and write kernel
attributes.

Attributes should be ASCII text files, preferably with only one value per file. It is noted that it may not be efficient to contain only one value per file, so it is socially acceptable to express an array of values of the same type.

Mixing types, expressing multiple lines of data, and doing fancy formatting of data is heavily frowned upon. Doing these things may get you publically humiliated and your code rewritten without notice.


An attribute definition is simply:

struct attribute {
        char                    * name;
        struct module        *owner;
        mode_t                  mode;
};


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


A bare attribute contains no means to read or write the value of the attribute. Subsystems are encouraged to define their own attribute structure and wrapper functions for adding and removing attributes for a specific object type.

For example, the driver model defines struct device_attribute like:

struct device_attribute {
    struct attribute    attr;
    ssize_t (*show)(struct device *dev, struct device_attribute *attr,
            char *buf);
    ssize_t (*store)(struct device *dev, struct device_attribute *attr,
             const char *buf, size_t count);
};

int device_create_file(struct device *, const struct device_attribute *);
void device_remove_file(struct device *, const struct device_attribute *);

It also defines this helper for defining device attributes:

#define DEVICE_ATTR(_name, _mode, _show, _store) \
struct device_attribute dev_attr_##_name = __ATTR(_name, _mode, _show, _store)

For example, declaring

static DEVICE_ATTR(foo, S_IWUSR | S_IRUGO, show_foo, store_foo);

is equivalent to doing:

static struct device_attribute dev_attr_foo = {
       .attr    = {
        .name = "foo",
        .mode = S_IWUSR | S_IRUGO,
        .show = show_foo,
        .store = store_foo,
    },
};


Subsystem-Specific Callbacks
~~~~~~~~~~~~~~~~~~~~~~~~~~~~

When a subsystem defines a new attribute type, it must implement a set of sysfs operations for forwarding read and write calls to the show and store methods of the attribute owners.

struct sysfs_ops {
        ssize_t (*show)(struct kobject *, struct attribute *, char *);
        ssize_t (*store)(struct kobject *, struct attribute *, const char *);
};

[ Subsystems should have already defined a struct kobj_type as a descriptor for this type, which is where the sysfs_ops pointer is stored. See the kobject documentation for more information. ]

When a file is read or written, sysfs calls the appropriate method for the type. The method then translates the generic struct kobject and struct attribute pointers to the appropriate pointer types, and calls the associated methods.


To illustrate:

#define to_dev_attr(_attr) container_of(_attr, struct device_attribute, attr)
#define to_dev(d) container_of(d, struct device, kobj)

static ssize_t
dev_attr_show(struct kobject * kobj, struct attribute * attr, char * buf)
{
        struct device_attribute * dev_attr = to_dev_attr(attr);
        struct device * dev = to_dev(kobj);
        ssize_t ret = 0;

        if (dev_attr->show)
                ret = dev_attr->show(dev, buf);
        return ret;
}



Reading/Writing Attribute Data
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

To read or write attributes, show() or store() methods must be specified when declaring the attribute. The method types should be as simple as those defined for device attributes:

ssize_t (*show)(struct device * dev, struct device_attribute * attr,
                char * buf);
ssize_t (*store)(struct device * dev, struct device_attribute * attr,
                 const char * buf);

IOW, they should take only an object, an attribute, and a buffer as parameters.


sysfs allocates a buffer of size (PAGE_SIZE) and passes it to the method. Sysfs will call the method exactly once for each read or write. This forces the following behavior on the method implementations:

- On read(2), the show() method should fill the entire buffer. Recall that an attribute should only be exporting one value, or an array of similar values, so this shouldn't be that expensive.

  This allows userspace to do partial reads and forward seeks arbitrarily over the entire file at will. If userspace seeks back to zero or does a pread(2) with an offset of '0' the show() method will be called again, rearmed, to fill the buffer.

- On write(2), sysfs expects the entire buffer to be passed during the first write. Sysfs then passes the entire buffer to the store() method.
 
  When writing sysfs files, userspace processes should first read the entire file, modify the values it wishes to change, then write the entire buffer back.

  Attribute method implementations should operate on an identical buffer when reading and writing values.

Other notes:

- Writing causes the show() method to be rearmed regardless of current file position.

- The buffer will always be PAGE_SIZE bytes in length. On i386, this
  is 4096.

- show() methods should return the number of bytes printed into the buffer. This is the return value of snprintf().

- show() should always use snprintf().

- store() should return the number of bytes used from the buffer. This can be done using strlen().

- show() or store() can always return errors. If a bad value comes through, be sure to return an error.

- The object passed to the methods will be pinned in       memory via sysfs referencing counting its embedded object. However, the physical entity (e.g. device) the object represents may not be present. Be sure to have a way to check this, if necessary.


A very simple (and naive) implementation of a device attribute is:

static ssize_t show_name(struct device *dev, struct device_attribute *attr, char *buf)
{
    return snprintf(buf, PAGE_SIZE, "%s\n", dev->name);
}

static ssize_t store_name(struct device * dev, const char * buf)
{
    sscanf(buf, "%20s", dev->name);
    return strnlen(buf, PAGE_SIZE);
}

static DEVICE_ATTR(name, S_IRUGO, show_name, store_name);


(Note that the real implementation doesn't allow userspace to set the name for a device.)


Top Level Directory Layout
~~~~~~~~~~~~~~~~~~~~~~~~~~

The sysfs directory arrangement exposes the relationship of kernel data structures.

The top level sysfs directory looks like:

block/
bus/
class/
dev/
devices/
firmware/
net/
fs/

devices/ contains a filesystem representation of the device tree. It maps directly to the internal kernel device tree, which is a hierarchy of struct device.

bus/ contains flat directory layout of the various bus types in the kernel. Each bus's directory contains two subdirectories:

    devices/
    drivers/

devices/ contains symlinks for each device discovered in the system that point to the device's directory under root/.

drivers/ contains a directory for each device driver that is loaded for devices on that particular bus (this assumes that drivers do not span multiple bus types).

fs/ contains a directory for some filesystems.  Currently each filesystem wanting to export attributes must create its own hierarchy below fs/ (see ./fuse.txt for an example).

dev/ contains two directories char/ and block/. Inside these two directories there are symlinks named :.  These symlinks point to the sysfs directory for the given device.  /sys/dev provides a quick way to lookup the sysfs interface for a device from the result of a stat(2) operation.

More information can driver-model specific features can be found in Documentation/driver-model/.


TODO: Finish this section.


Current Interfaces
~~~~~~~~~~~~~~~~~~

The following interface layers currently exist in sysfs:


- devices (include/linux/device.h)
----------------------------------
Structure:

struct device_attribute {
    struct attribute    attr;
    ssize_t (*show)(struct device *dev, struct device_attribute *attr,
            char *buf);
    ssize_t (*store)(struct device *dev, struct device_attribute *attr,
             const char *buf, size_t count);
};

Declaring:

DEVICE_ATTR(_name, _mode, _show, _store);

Creation/Removal:

int device_create_file(struct device *dev, const struct device_attribute * attr);
void device_remove_file(struct device *dev, const struct device_attribute * attr);


- bus drivers (include/linux/device.h)
--------------------------------------
Structure:

struct bus_attribute {
        struct attribute        attr;
        ssize_t (*show)(struct bus_type *, char * buf);
        ssize_t (*store)(struct bus_type *, const char * buf);
};

Declaring:

BUS_ATTR(_name, _mode, _show, _store)

Creation/Removal:

int bus_create_file(struct bus_type *, struct bus_attribute *);
void bus_remove_file(struct bus_type *, struct bus_attribute *);


- device drivers (include/linux/device.h)
-----------------------------------------

Structure:

struct driver_attribute {
        struct attribute        attr;
        ssize_t (*show)(struct device_driver *, char * buf);
        ssize_t (*store)(struct device_driver *, const char * buf,
                         size_t count);
};

Declaring:

DRIVER_ATTR(_name, _mode, _show, _store)

Creation/Removal:

int driver_create_file(struct device_driver *, const struct driver_attribute *);
void driver_remove_file(struct device_driver *, const struct driver_attribute *);