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Linux/Documentation/driver-api/driver-model/device.rst

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Diff markup

Differences between /Documentation/driver-api/driver-model/device.rst (Version linux-6.12-rc7) and /Documentation/driver-api/driver-model/device.rst (Version linux-5.19.17)


  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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