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1==========================
2The Basic Device Structure
3==========================
4
5See the kerneldoc for the struct device.
6
7
8Programming Interface
9~~~~~~~~~~~~~~~~~~~~~
10The bus driver that discovers the device uses this to register the
11device with the core::
12
13 int device_register(struct device * dev);
14
15The bus should initialize the following fields:
16
17 - parent
18 - name
19 - bus_id
20 - bus
21
22A device is removed from the core when its reference count goes to
230. The reference count can be adjusted using::
24
25 struct device * get_device(struct device * dev);
26 void put_device(struct device * dev);
27
28get_device() will return a pointer to the struct device passed to it
29if the reference is not already 0 (if it's in the process of being
30removed already).
31
32A driver can access the lock in the device structure using::
33
34 void lock_device(struct device * dev);
35 void unlock_device(struct device * dev);
36
37
38Attributes
39~~~~~~~~~~
40
41::
42
43 struct device_attribute {
44 struct attribute attr;
45 ssize_t (*show)(struct device *dev, struct device_attribute *attr,
46 char *buf);
47 ssize_t (*store)(struct device *dev, struct device_attribute *attr,
48 const char *buf, size_t count);
49 };
50
51Attributes of devices can be exported by a device driver through sysfs.
52
53Please see Documentation/filesystems/sysfs.rst for more information
54on how sysfs works.
55
56As explained in Documentation/core-api/kobject.rst, device attributes must be
57created before the KOBJ_ADD uevent is generated. The only way to realize
58that is by defining an attribute group.
59
60Attributes are declared using a macro called DEVICE_ATTR::
61
62 #define DEVICE_ATTR(name,mode,show,store)
63
64Example:::
65
66 static DEVICE_ATTR(type, 0444, type_show, NULL);
67 static DEVICE_ATTR(power, 0644, power_show, power_store);
68
69Helper macros are available for common values of mode, so the above examples
70can be simplified to:::
71
72 static DEVICE_ATTR_RO(type);
73 static DEVICE_ATTR_RW(power);
74
75This declares two structures of type struct device_attribute with respective
76names 'dev_attr_type' and 'dev_attr_power'. These two attributes can be
77organized as follows into a group::
78
79 static struct attribute *dev_attrs[] = {
80 &dev_attr_type.attr,
81 &dev_attr_power.attr,
82 NULL,
83 };
84
85 static struct attribute_group dev_group = {
86 .attrs = dev_attrs,
87 };
88
89 static const struct attribute_group *dev_groups[] = {
90 &dev_group,
91 NULL,
92 };
93
94A helper macro is available for the common case of a single group, so the
95above two structures can be declared using:::
96
97 ATTRIBUTE_GROUPS(dev);
98
99This array of groups can then be associated with a device by setting the
100group pointer in struct device before device_register() is invoked::
101
102 dev->groups = dev_groups;
103 device_register(dev);
104
105The device_register() function will use the 'groups' pointer to create the
106device attributes and the device_unregister() function will use this pointer
107to remove the device attributes.
108
109Word of warning: While the kernel allows device_create_file() and
110device_remove_file() to be called on a device at any time, userspace has
111strict expectations on when attributes get created. When a new device is
112registered in the kernel, a uevent is generated to notify userspace (like
113udev) that a new device is available. If attributes are added after the
114device is registered, then userspace won't get notified and userspace will
115not know about the new attributes.
116
117This is important for device driver that need to publish additional
118attributes for a device at driver probe time. If the device driver simply
119calls device_create_file() on the device structure passed to it, then
120userspace will never be notified of the new attributes.