linux-next/Documentation/filesystems/sysfs.rst

.. SPDX-License-Identifier: GPL-2.0

=====================================================
sysfs - _The_ filesystem for exporting kernel objects
=====================================================

Patrick Mochel	<mochel@osdl.org>

Mike Murphy <mamurph@cs.clemson.edu>

:Revised:    16 August 2011
: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/core-api/kobject.rst 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 a pointer to the kobject that implements a
directory in the kernfs_node object associated with the directory. In
the past this kobject pointer has been used by sysfs to do reference
counting directly on the kobject whenever the file is opened or closed.
With the current sysfs implementation the kobject reference count is
only modified directly by the function sysfs_schedule_callback().


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 publicly humiliated and your code rewritten without notice.


An attribute definition is simply::

    struct attribute {
	    char                    * name;
	    struct module		*owner;
	    umode_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,
    };

Note as stated in include/linux/kernel.h "OTHER_WRITABLE?  Generally
considered a bad idea." so trying to set a sysfs file writable for
everyone will fail reverting to RO mode for "Others".

For the common cases sysfs.h provides convenience macros to make
defining attributes easier as well as making code more concise and
readable. The above case could be shortened to:

static struct device_attribute dev_attr_foo = __ATTR_RW(foo);

the list of helpers available to define your wrapper function is:

__ATTR_RO(name):
		 assumes default name_show and mode 0444
__ATTR_WO(name):
		 assumes a name_store only and is restricted to mode
                 0200 that is root write access only.
__ATTR_RO_MODE(name, mode):
	         fore more restrictive RO access currently
                 only use case is the EFI System Resource Table
                 (see drivers/firmware/efi/esrt.c)
__ATTR_RW(name):
	         assumes default name_show, name_store and setting
                 mode to 0644.
__ATTR_NULL:
	         which sets the name to NULL and is used as end of list
                 indicator (see: kernel/workqueue.c)

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 *, size_t);
    };

[ 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(obj) container_of(obj, struct device, kobj)
    #define to_dev_attr(_attr) container_of(_attr, struct device_attribute, attr)

    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 = -EIO;

	    if (dev_attr->show)
		    ret = dev_attr->show(dev, dev_attr, buf);
	    if (ret >= (ssize_t)PAGE_SIZE) {
		    printk("dev_attr_show: %pS returned bad count\n",
				    dev_attr->show);
	    }
	    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, size_t count);

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.
  A terminating null is added after the data on stores. This makes
  functions like sysfs_streq() safe to use.

  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 scnprintf().

- show() must not use snprintf() when formatting the value to be
  returned to user space. If you can guarantee that an overflow
  will never happen you can use sprintf() otherwise you must use
  scnprintf().

- store() should return the number of bytes used from the buffer. If the
  entire buffer has been used, just return the count argument.

- 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 scnprintf(buf, PAGE_SIZE, "%s\n", dev->name);
    }

    static ssize_t store_name(struct device *dev, struct device_attribute *attr,
			    const char *buf, size_t count)
    {
	    snprintf(dev->name, sizeof(dev->name), "%.*s",
		    (int)min(count, sizeof(dev->name) - 1), buf);
	    return count;
    }

    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 <major>:<minor>.  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-api/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, size_t count);
    };

Declaring::

    static BUS_ATTR_RW(name);
    static BUS_ATTR_RO(name);
    static BUS_ATTR_WO(name);

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_RO(_name)
    DRIVER_ATTR_RW(_name)

Creation/Removal::

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


Documentation
~~~~~~~~~~~~~

The sysfs directory structure and the attributes in each directory define an
ABI between the kernel and user space. As for any ABI, it is important that
this ABI is stable and properly documented. All new sysfs attributes must be
documented in Documentation/ABI. See also Documentation/ABI/README for more
information.