1==============
2Device Drivers
3==============
4
5See the kerneldoc for the struct device_driver.
6
7Allocation
8~~~~~~~~~~
9
10Device drivers are statically allocated structures. Though there may
11be multiple devices in a system that a driver supports, struct
12device_driver represents the driver as a whole (not a particular
13device instance).
14
15Initialization
16~~~~~~~~~~~~~~
17
18The driver must initialize at least the name and bus fields. It should
19also initialize the devclass field (when it arrives), so it may obtain
20the proper linkage internally. It should also initialize as many of
21the callbacks as possible, though each is optional.
22
23Declaration
24~~~~~~~~~~~
25
26As stated above, struct device_driver objects are statically
27allocated. Below is an example declaration of the eepro100
28driver. This declaration is hypothetical only; it relies on the driver
29being converted completely to the new model::
30
31  static struct device_driver eepro100_driver = {
32         .name		= "eepro100",
33         .bus		= &pci_bus_type,
34
35         .probe		= eepro100_probe,
36         .remove		= eepro100_remove,
37         .suspend		= eepro100_suspend,
38         .resume		= eepro100_resume,
39  };
40
41Most drivers will not be able to be converted completely to the new
42model because the bus they belong to has a bus-specific structure with
43bus-specific fields that cannot be generalized.
44
45The most common example of this are device ID structures. A driver
46typically defines an array of device IDs that it supports. The format
47of these structures and the semantics for comparing device IDs are
48completely bus-specific. Defining them as bus-specific entities would
49sacrifice type-safety, so we keep bus-specific structures around.
50
51Bus-specific drivers should include a generic struct device_driver in
52the definition of the bus-specific driver. Like this::
53
54  struct pci_driver {
55         const struct pci_device_id *id_table;
56         struct device_driver	  driver;
57  };
58
59A definition that included bus-specific fields would look like
60(using the eepro100 driver again)::
61
62  static struct pci_driver eepro100_driver = {
63         .id_table       = eepro100_pci_tbl,
64         .driver	       = {
65		.name		= "eepro100",
66		.bus		= &pci_bus_type,
67		.probe		= eepro100_probe,
68		.remove		= eepro100_remove,
69		.suspend	= eepro100_suspend,
70		.resume		= eepro100_resume,
71         },
72  };
73
74Some may find the syntax of embedded struct initialization awkward or
75even a bit ugly. So far, it's the best way we've found to do what we want...
76
77Registration
78~~~~~~~~~~~~
79
80::
81
82  int driver_register(struct device_driver *drv);
83
84The driver registers the structure on startup. For drivers that have
85no bus-specific fields (i.e. don't have a bus-specific driver
86structure), they would use driver_register and pass a pointer to their
87struct device_driver object.
88
89Most drivers, however, will have a bus-specific structure and will
90need to register with the bus using something like pci_driver_register.
91
92It is important that drivers register their driver structure as early as
93possible. Registration with the core initializes several fields in the
94struct device_driver object, including the reference count and the
95lock. These fields are assumed to be valid at all times and may be
96used by the device model core or the bus driver.
97
98
99Transition Bus Drivers
100~~~~~~~~~~~~~~~~~~~~~~
101
102By defining wrapper functions, the transition to the new model can be
103made easier. Drivers can ignore the generic structure altogether and
104let the bus wrapper fill in the fields. For the callbacks, the bus can
105define generic callbacks that forward the call to the bus-specific
106callbacks of the drivers.
107
108This solution is intended to be only temporary. In order to get class
109information in the driver, the drivers must be modified anyway. Since
110converting drivers to the new model should reduce some infrastructural
111complexity and code size, it is recommended that they are converted as
112class information is added.
113
114Access
115~~~~~~
116
117Once the object has been registered, it may access the common fields of
118the object, like the lock and the list of devices::
119
120  int driver_for_each_dev(struct device_driver *drv, void *data,
121			  int (*callback)(struct device *dev, void *data));
122
123The devices field is a list of all the devices that have been bound to
124the driver. The LDM core provides a helper function to operate on all
125the devices a driver controls. This helper locks the driver on each
126node access, and does proper reference counting on each device as it
127accesses it.
128
129
130sysfs
131~~~~~
132
133When a driver is registered, a sysfs directory is created in its
134bus's directory. In this directory, the driver can export an interface
135to userspace to control operation of the driver on a global basis;
136e.g. toggling debugging output in the driver.
137
138A future feature of this directory will be a 'devices' directory. This
139directory will contain symlinks to the directories of devices it
140supports.
141
142
143
144Callbacks
145~~~~~~~~~
146
147::
148
149	int	(*probe)	(struct device *dev);
150
151The probe() entry is called in task context, with the bus's rwsem locked
152and the driver partially bound to the device.  Drivers commonly use
153container_of() to convert "dev" to a bus-specific type, both in probe()
154and other routines.  That type often provides device resource data, such
155as pci_dev.resource[] or platform_device.resources, which is used in
156addition to dev->platform_data to initialize the driver.
157
158This callback holds the driver-specific logic to bind the driver to a
159given device.  That includes verifying that the device is present, that
160it's a version the driver can handle, that driver data structures can
161be allocated and initialized, and that any hardware can be initialized.
162Drivers often store a pointer to their state with dev_set_drvdata().
163When the driver has successfully bound itself to that device, then probe()
164returns zero and the driver model code will finish its part of binding
165the driver to that device.
166
167A driver's probe() may return a negative errno value to indicate that
168the driver did not bind to this device, in which case it should have
169released all resources it allocated.
170
171Optionally, probe() may return -EPROBE_DEFER if the driver depends on
172resources that are not yet available (e.g., supplied by a driver that
173hasn't initialized yet).  The driver core will put the device onto the
174deferred probe list and will try to call it again later. If a driver
175must defer, it should return -EPROBE_DEFER as early as possible to
176reduce the amount of time spent on setup work that will need to be
177unwound and reexecuted at a later time.
178
179.. warning::
180      -EPROBE_DEFER must not be returned if probe() has already created
181      child devices, even if those child devices are removed again
182      in a cleanup path. If -EPROBE_DEFER is returned after a child
183      device has been registered, it may result in an infinite loop of
184      .probe() calls to the same driver.
185
186::
187
188	void	(*sync_state)	(struct device *dev);
189
190sync_state is called only once for a device. It's called when all the consumer
191devices of the device have successfully probed. The list of consumers of the
192device is obtained by looking at the device links connecting that device to its
193consumer devices.
194
195The first attempt to call sync_state() is made during late_initcall_sync() to
196give firmware and drivers time to link devices to each other. During the first
197attempt at calling sync_state(), if all the consumers of the device at that
198point in time have already probed successfully, sync_state() is called right
199away. If there are no consumers of the device during the first attempt, that
200too is considered as "all consumers of the device have probed" and sync_state()
201is called right away.
202
203If during the first attempt at calling sync_state() for a device, there are
204still consumers that haven't probed successfully, the sync_state() call is
205postponed and reattempted in the future only when one or more consumers of the
206device probe successfully. If during the reattempt, the driver core finds that
207there are one or more consumers of the device that haven't probed yet, then
208sync_state() call is postponed again.
209
210A typical use case for sync_state() is to have the kernel cleanly take over
211management of devices from the bootloader. For example, if a device is left on
212and at a particular hardware configuration by the bootloader, the device's
213driver might need to keep the device in the boot configuration until all the
214consumers of the device have probed. Once all the consumers of the device have
215probed, the device's driver can synchronize the hardware state of the device to
216match the aggregated software state requested by all the consumers. Hence the
217name sync_state().
218
219While obvious examples of resources that can benefit from sync_state() include
220resources such as regulator, sync_state() can also be useful for complex
221resources like IOMMUs. For example, IOMMUs with multiple consumers (devices
222whose addresses are remapped by the IOMMU) might need to keep their mappings
223fixed at (or additive to) the boot configuration until all its consumers have
224probed.
225
226While the typical use case for sync_state() is to have the kernel cleanly take
227over management of devices from the bootloader, the usage of sync_state() is
228not restricted to that. Use it whenever it makes sense to take an action after
229all the consumers of a device have probed::
230
231	int 	(*remove)	(struct device *dev);
232
233remove is called to unbind a driver from a device. This may be
234called if a device is physically removed from the system, if the
235driver module is being unloaded, during a reboot sequence, or
236in other cases.
237
238It is up to the driver to determine if the device is present or
239not. It should free any resources allocated specifically for the
240device; i.e. anything in the device's driver_data field.
241
242If the device is still present, it should quiesce the device and place
243it into a supported low-power state.
244
245::
246
247	int	(*suspend)	(struct device *dev, pm_message_t state);
248
249suspend is called to put the device in a low power state.
250
251::
252
253	int	(*resume)	(struct device *dev);
254
255Resume is used to bring a device back from a low power state.
256
257
258Attributes
259~~~~~~~~~~
260
261::
262
263  struct driver_attribute {
264          struct attribute        attr;
265          ssize_t (*show)(struct device_driver *driver, char *buf);
266          ssize_t (*store)(struct device_driver *, const char *buf, size_t count);
267  };
268
269Device drivers can export attributes via their sysfs directories.
270Drivers can declare attributes using a DRIVER_ATTR_RW and DRIVER_ATTR_RO
271macro that works identically to the DEVICE_ATTR_RW and DEVICE_ATTR_RO
272macros.
273
274Example::
275
276	DRIVER_ATTR_RW(debug);
277
278This is equivalent to declaring::
279
280	struct driver_attribute driver_attr_debug;
281
282This can then be used to add and remove the attribute from the
283driver's directory using::
284
285  int driver_create_file(struct device_driver *, const struct driver_attribute *);
286  void driver_remove_file(struct device_driver *, const struct driver_attribute *);
287