1 ========================== 1 ==========================
2 The Basic Device Structure 2 The Basic Device Structure
3 ========================== 3 ==========================
4 4
5 See the kerneldoc for the struct device. 5 See the kerneldoc for the struct device.
6 6
7 7
8 Programming Interface 8 Programming Interface
9 ~~~~~~~~~~~~~~~~~~~~~ 9 ~~~~~~~~~~~~~~~~~~~~~
10 The bus driver that discovers the device uses 10 The bus driver that discovers the device uses this to register the
11 device with the core:: 11 device with the core::
12 12
13 int device_register(struct device * dev); 13 int device_register(struct device * dev);
14 14
15 The bus should initialize the following fields 15 The bus should initialize the following fields:
16 16
17 - parent 17 - parent
18 - name 18 - name
19 - bus_id 19 - bus_id
20 - bus 20 - bus
21 21
22 A device is removed from the core when its ref 22 A device is removed from the core when its reference count goes to
23 0. The reference count can be adjusted using:: 23 0. The reference count can be adjusted using::
24 24
25 struct device * get_device(struct device * d 25 struct device * get_device(struct device * dev);
26 void put_device(struct device * dev); 26 void put_device(struct device * dev);
27 27
28 get_device() will return a pointer to the stru 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 29 if the reference is not already 0 (if it's in the process of being
30 removed already). 30 removed already).
31 31
32 A driver can access the lock in the device str 32 A driver can access the lock in the device structure using::
33 33
34 void lock_device(struct device * dev); 34 void lock_device(struct device * dev);
35 void unlock_device(struct device * dev); 35 void unlock_device(struct device * dev);
36 36
37 37
38 Attributes 38 Attributes
39 ~~~~~~~~~~ 39 ~~~~~~~~~~
40 40
41 :: 41 ::
42 42
43 struct device_attribute { 43 struct device_attribute {
44 struct attribute attr; 44 struct attribute attr;
45 ssize_t (*show)(struct device *dev, st 45 ssize_t (*show)(struct device *dev, struct device_attribute *attr,
46 char *buf); 46 char *buf);
47 ssize_t (*store)(struct device *dev, s 47 ssize_t (*store)(struct device *dev, struct device_attribute *attr,
48 const char *buf, size 48 const char *buf, size_t count);
49 }; 49 };
50 50
51 Attributes of devices can be exported by a dev 51 Attributes of devices can be exported by a device driver through sysfs.
52 52
53 Please see Documentation/filesystems/sysfs.rst 53 Please see Documentation/filesystems/sysfs.rst for more information
54 on how sysfs works. 54 on how sysfs works.
55 55
56 As explained in Documentation/core-api/kobject 56 As explained in Documentation/core-api/kobject.rst, device attributes must be
57 created before the KOBJ_ADD uevent is generate 57 created before the KOBJ_ADD uevent is generated. The only way to realize
58 that is by defining an attribute group. 58 that is by defining an attribute group.
59 59
60 Attributes are declared using a macro called D 60 Attributes are declared using a macro called DEVICE_ATTR::
61 61
62 #define DEVICE_ATTR(name,mode,show,store) 62 #define DEVICE_ATTR(name,mode,show,store)
63 63
64 Example::: 64 Example:::
65 65
66 static DEVICE_ATTR(type, 0444, type_show, NU 66 static DEVICE_ATTR(type, 0444, type_show, NULL);
67 static DEVICE_ATTR(power, 0644, power_show, 67 static DEVICE_ATTR(power, 0644, power_show, power_store);
68 68
69 Helper macros are available for common values 69 Helper macros are available for common values of mode, so the above examples
70 can be simplified to::: 70 can be simplified to:::
71 71
72 static DEVICE_ATTR_RO(type); 72 static DEVICE_ATTR_RO(type);
73 static DEVICE_ATTR_RW(power); 73 static DEVICE_ATTR_RW(power);
74 74
75 This declares two structures of type struct de 75 This declares two structures of type struct device_attribute with respective
76 names 'dev_attr_type' and 'dev_attr_power'. Th 76 names 'dev_attr_type' and 'dev_attr_power'. These two attributes can be
77 organized as follows into a group:: 77 organized as follows into a group::
78 78
79 static struct attribute *dev_attrs[] = { 79 static struct attribute *dev_attrs[] = {
80 &dev_attr_type.attr, 80 &dev_attr_type.attr,
81 &dev_attr_power.attr, 81 &dev_attr_power.attr,
82 NULL, 82 NULL,
83 }; 83 };
84 84
85 static struct attribute_group dev_group = { 85 static struct attribute_group dev_group = {
86 .attrs = dev_attrs, 86 .attrs = dev_attrs,
87 }; 87 };
88 88
89 static const struct attribute_group *dev_gro 89 static const struct attribute_group *dev_groups[] = {
90 &dev_group, 90 &dev_group,
91 NULL, 91 NULL,
92 }; 92 };
93 93
94 A helper macro is available for the common cas 94 A helper macro is available for the common case of a single group, so the
95 above two structures can be declared using::: 95 above two structures can be declared using:::
96 96
97 ATTRIBUTE_GROUPS(dev); 97 ATTRIBUTE_GROUPS(dev);
98 98
99 This array of groups can then be associated wi 99 This array of groups can then be associated with a device by setting the
100 group pointer in struct device before device_r 100 group pointer in struct device before device_register() is invoked::
101 101
102 dev->groups = dev_groups; 102 dev->groups = dev_groups;
103 device_register(dev); 103 device_register(dev);
104 104
105 The device_register() function will use the 'g 105 The device_register() function will use the 'groups' pointer to create the
106 device attributes and the device_unregister() 106 device attributes and the device_unregister() function will use this pointer
107 to remove the device attributes. 107 to remove the device attributes.
108 108
109 Word of warning: While the kernel allows devi 109 Word of warning: While the kernel allows device_create_file() and
110 device_remove_file() to be called on a device 110 device_remove_file() to be called on a device at any time, userspace has
111 strict expectations on when attributes get cre 111 strict expectations on when attributes get created. When a new device is
112 registered in the kernel, a uevent is generate 112 registered in the kernel, a uevent is generated to notify userspace (like
113 udev) that a new device is available. If attr 113 udev) that a new device is available. If attributes are added after the
114 device is registered, then userspace won't get 114 device is registered, then userspace won't get notified and userspace will
115 not know about the new attributes. 115 not know about the new attributes.
116 116
117 This is important for device driver that need 117 This is important for device driver that need to publish additional
118 attributes for a device at driver probe time. 118 attributes for a device at driver probe time. If the device driver simply
119 calls device_create_file() on the device struc 119 calls device_create_file() on the device structure passed to it, then
120 userspace will never be notified of the new at 120 userspace will never be notified of the new attributes.
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