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1da177e4 LT |
1 | |
2 | The Basic Device Structure | |
3 | ~~~~~~~~~~~~~~~~~~~~~~~~~~ | |
4 | ||
63dc355a | 5 | See the kerneldoc for the struct device. |
1da177e4 LT |
6 | |
7 | ||
8 | Programming Interface | |
9 | ~~~~~~~~~~~~~~~~~~~~~ | |
10 | The bus driver that discovers the device uses this to register the | |
11 | device with the core: | |
12 | ||
13 | int device_register(struct device * dev); | |
14 | ||
15 | The bus should initialize the following fields: | |
16 | ||
17 | - parent | |
18 | - name | |
19 | - bus_id | |
20 | - bus | |
21 | ||
22 | A device is removed from the core when its reference count goes to | |
23 | 0. The reference count can be adjusted using: | |
24 | ||
25 | struct device * get_device(struct device * dev); | |
26 | void put_device(struct device * dev); | |
27 | ||
28 | get_device() will return a pointer to the struct device passed to it | |
29 | if the reference is not already 0 (if it's in the process of being | |
30 | removed already). | |
31 | ||
32 | A 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 | ||
38 | Attributes | |
39 | ~~~~~~~~~~ | |
40 | struct device_attribute { | |
245127db MM |
41 | struct attribute attr; |
42 | ssize_t (*show)(struct device *dev, struct device_attribute *attr, | |
43 | char *buf); | |
44 | ssize_t (*store)(struct device *dev, struct device_attribute *attr, | |
45 | const char *buf, size_t count); | |
1da177e4 LT |
46 | }; |
47 | ||
48 | Attributes of devices can be exported via drivers using a simple | |
49 | procfs-like interface. | |
50 | ||
51 | Please see Documentation/filesystems/sysfs.txt for more information | |
52 | on how sysfs works. | |
53 | ||
54 | Attributes are declared using a macro called DEVICE_ATTR: | |
55 | ||
56 | #define DEVICE_ATTR(name,mode,show,store) | |
57 | ||
58 | Example: | |
59 | ||
60 | DEVICE_ATTR(power,0644,show_power,store_power); | |
61 | ||
62 | This declares a structure of type struct device_attribute named | |
63 | 'dev_attr_power'. This can then be added and removed to the device's | |
64 | directory using: | |
65 | ||
66 | int device_create_file(struct device *device, struct device_attribute * entry); | |
67 | void device_remove_file(struct device * dev, struct device_attribute * attr); | |
68 | ||
69 | Example: | |
70 | ||
71 | device_create_file(dev,&dev_attr_power); | |
72 | device_remove_file(dev,&dev_attr_power); | |
73 | ||
74 | The file name will be 'power' with a mode of 0644 (-rw-r--r--). | |
75 | ||
b22813b3 GL |
76 | Word of warning: While the kernel allows device_create_file() and |
77 | device_remove_file() to be called on a device at any time, userspace has | |
78 | strict expectations on when attributes get created. When a new device is | |
79 | registered in the kernel, a uevent is generated to notify userspace (like | |
80 | udev) that a new device is available. If attributes are added after the | |
81 | device is registered, then userspace won't get notified and userspace will | |
82 | not know about the new attributes. | |
83 | ||
84 | This is important for device driver that need to publish additional | |
85 | attributes for a device at driver probe time. If the device driver simply | |
86 | calls device_create_file() on the device structure passed to it, then | |
87 | userspace will never be notified of the new attributes. Instead, it should | |
88 | probably use class_create() and class->dev_attrs to set up a list of | |
89 | desired attributes in the modules_init function, and then in the .probe() | |
90 | hook, and then use device_create() to create a new device as a child | |
91 | of the probed device. The new device will generate a new uevent and | |
92 | properly advertise the new attributes to userspace. | |
93 | ||
94 | For example, if a driver wanted to add the following attributes: | |
95 | struct device_attribute mydriver_attribs[] = { | |
96 | __ATTR(port_count, 0444, port_count_show), | |
97 | __ATTR(serial_number, 0444, serial_number_show), | |
98 | NULL | |
99 | }; | |
100 | ||
101 | Then in the module init function is would do: | |
102 | mydriver_class = class_create(THIS_MODULE, "my_attrs"); | |
103 | mydriver_class.dev_attr = mydriver_attribs; | |
104 | ||
105 | And assuming 'dev' is the struct device passed into the probe hook, the driver | |
106 | probe function would do something like: | |
b6badddc | 107 | device_create(&mydriver_class, dev, chrdev, &private_data, "my_name"); |