TEXT   67

device.txt

Guest on 1st August 2021 06:07:00 AM

  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.

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