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Documentation / driver-model / device.txt




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Based on kernel version 3.15.4. Page generated on 2014-07-07 09:02 EST.

1	
2	The Basic Device Structure
3	~~~~~~~~~~~~~~~~~~~~~~~~~~
4	
5	See the kerneldoc for the struct device.
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 {
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);
46	};
47	
48	Attributes of devices can be exported by a device driver through sysfs.
49	
50	Please see Documentation/filesystems/sysfs.txt for more information
51	on how sysfs works.
52	
53	As explained in Documentation/kobject.txt, device attributes must be be
54	created before the KOBJ_ADD uevent is generated. The only way to realize
55	that is by defining an attribute group.
56	
57	Attributes are declared using a macro called DEVICE_ATTR:
58	
59	#define DEVICE_ATTR(name,mode,show,store)
60	
61	Example:
62	
63	static DEVICE_ATTR(type, 0444, show_type, NULL);
64	static DEVICE_ATTR(power, 0644, show_power, store_power);
65	
66	This declares two structures of type struct device_attribute with respective
67	names 'dev_attr_type' and 'dev_attr_power'. These two attributes can be
68	organized as follows into a group:
69	
70	static struct attribute *dev_attrs[] = {
71		&dev_attr_type.attr,
72		&dev_attr_power.attr,
73		NULL,
74	};
75	
76	static struct attribute_group dev_attr_group = {
77		.attrs = dev_attrs,
78	};
79	
80	static const struct attribute_group *dev_attr_groups[] = {
81		&dev_attr_group,
82		NULL,
83	};
84	
85	This array of groups can then be associated with a device by setting the
86	group pointer in struct device before device_register() is invoked:
87	
88	      dev->groups = dev_attr_groups;
89	      device_register(dev);
90	
91	The device_register() function will use the 'groups' pointer to create the
92	device attributes and the device_unregister() function will use this pointer
93	to remove the device attributes.
94	
95	Word of warning:  While the kernel allows device_create_file() and
96	device_remove_file() to be called on a device at any time, userspace has
97	strict expectations on when attributes get created.  When a new device is
98	registered in the kernel, a uevent is generated to notify userspace (like
99	udev) that a new device is available.  If attributes are added after the
100	device is registered, then userspace won't get notified and userspace will
101	not know about the new attributes.
102	
103	This is important for device driver that need to publish additional
104	attributes for a device at driver probe time.  If the device driver simply
105	calls device_create_file() on the device structure passed to it, then
106	userspace will never be notified of the new attributes.
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