Based on kernel version 4.16.1. Page generated on 2018-04-09 11:53 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 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.