Adding upstream version 5.10.209.upstream/5.10.209upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

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+.. 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_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 = kobj_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.
+
+- show() should only use sysfs_emit() or sysfs_emit_at() when formatting
+  the value to be returned to user space.
+
+- 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.