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-
-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/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 a pointer to the kobject that implements a
-directory in the sysfs_dirent 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,
- },
-};
-
-
-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) {
- print_symbol("dev_attr_show: %s returned bad count\n",
- (unsigned long)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.
-
- 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() should always 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-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:
-
-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 *);
-
-
-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.