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1 | Overview of the V4L2 driver framework |
2 | ===================================== | |
3 | ||
4 | This text documents the various structures provided by the V4L2 framework and | |
5 | their relationships. | |
6 | ||
7 | ||
8 | Introduction | |
9 | ------------ | |
10 | ||
11 | The V4L2 drivers tend to be very complex due to the complexity of the | |
12 | hardware: most devices have multiple ICs, export multiple device nodes in | |
13 | /dev, and create also non-V4L2 devices such as DVB, ALSA, FB, I2C and input | |
14 | (IR) devices. | |
15 | ||
16 | Especially the fact that V4L2 drivers have to setup supporting ICs to | |
17 | do audio/video muxing/encoding/decoding makes it more complex than most. | |
18 | Usually these ICs are connected to the main bridge driver through one or | |
19 | more I2C busses, but other busses can also be used. Such devices are | |
20 | called 'sub-devices'. | |
21 | ||
22 | For a long time the framework was limited to the video_device struct for | |
23 | creating V4L device nodes and video_buf for handling the video buffers | |
24 | (note that this document does not discuss the video_buf framework). | |
25 | ||
26 | This meant that all drivers had to do the setup of device instances and | |
27 | connecting to sub-devices themselves. Some of this is quite complicated | |
28 | to do right and many drivers never did do it correctly. | |
29 | ||
30 | There is also a lot of common code that could never be refactored due to | |
31 | the lack of a framework. | |
32 | ||
33 | So this framework sets up the basic building blocks that all drivers | |
34 | need and this same framework should make it much easier to refactor | |
35 | common code into utility functions shared by all drivers. | |
36 | ||
37 | ||
38 | Structure of a driver | |
39 | --------------------- | |
40 | ||
41 | All drivers have the following structure: | |
42 | ||
43 | 1) A struct for each device instance containing the device state. | |
44 | ||
45 | 2) A way of initializing and commanding sub-devices (if any). | |
46 | ||
47 | 3) Creating V4L2 device nodes (/dev/videoX, /dev/vbiX, /dev/radioX and | |
48 | /dev/vtxX) and keeping track of device-node specific data. | |
49 | ||
50 | 4) Filehandle-specific structs containing per-filehandle data. | |
51 | ||
52 | This is a rough schematic of how it all relates: | |
53 | ||
54 | device instances | |
55 | | | |
56 | +-sub-device instances | |
57 | | | |
58 | \-V4L2 device nodes | |
59 | | | |
60 | \-filehandle instances | |
61 | ||
62 | ||
63 | Structure of the framework | |
64 | -------------------------- | |
65 | ||
66 | The framework closely resembles the driver structure: it has a v4l2_device | |
67 | struct for the device instance data, a v4l2_subdev struct to refer to | |
68 | sub-device instances, the video_device struct stores V4L2 device node data | |
69 | and in the future a v4l2_fh struct will keep track of filehandle instances | |
70 | (this is not yet implemented). | |
71 | ||
72 | ||
73 | struct v4l2_device | |
74 | ------------------ | |
75 | ||
76 | Each device instance is represented by a struct v4l2_device (v4l2-device.h). | |
77 | Very simple devices can just allocate this struct, but most of the time you | |
78 | would embed this struct inside a larger struct. | |
79 | ||
80 | You must register the device instance: | |
81 | ||
82 | v4l2_device_register(struct device *dev, struct v4l2_device *v4l2_dev); | |
83 | ||
84 | Registration will initialize the v4l2_device struct and link dev->driver_data | |
85 | to v4l2_dev. Registration will also set v4l2_dev->name to a value derived from | |
86 | dev (driver name followed by the bus_id, to be precise). You may change the | |
87 | name after registration if you want. | |
88 | ||
89 | You unregister with: | |
90 | ||
91 | v4l2_device_unregister(struct v4l2_device *v4l2_dev); | |
92 | ||
93 | Unregistering will also automatically unregister all subdevs from the device. | |
94 | ||
95 | Sometimes you need to iterate over all devices registered by a specific | |
96 | driver. This is usually the case if multiple device drivers use the same | |
97 | hardware. E.g. the ivtvfb driver is a framebuffer driver that uses the ivtv | |
98 | hardware. The same is true for alsa drivers for example. | |
99 | ||
100 | You can iterate over all registered devices as follows: | |
101 | ||
102 | static int callback(struct device *dev, void *p) | |
103 | { | |
104 | struct v4l2_device *v4l2_dev = dev_get_drvdata(dev); | |
105 | ||
106 | /* test if this device was inited */ | |
107 | if (v4l2_dev == NULL) | |
108 | return 0; | |
109 | ... | |
110 | return 0; | |
111 | } | |
112 | ||
113 | int iterate(void *p) | |
114 | { | |
115 | struct device_driver *drv; | |
116 | int err; | |
117 | ||
118 | /* Find driver 'ivtv' on the PCI bus. | |
119 | pci_bus_type is a global. For USB busses use usb_bus_type. */ | |
120 | drv = driver_find("ivtv", &pci_bus_type); | |
121 | /* iterate over all ivtv device instances */ | |
122 | err = driver_for_each_device(drv, NULL, p, callback); | |
123 | put_driver(drv); | |
124 | return err; | |
125 | } | |
126 | ||
127 | Sometimes you need to keep a running counter of the device instance. This is | |
128 | commonly used to map a device instance to an index of a module option array. | |
129 | ||
130 | The recommended approach is as follows: | |
131 | ||
132 | static atomic_t drv_instance = ATOMIC_INIT(0); | |
133 | ||
134 | static int __devinit drv_probe(struct pci_dev *dev, | |
135 | const struct pci_device_id *pci_id) | |
136 | { | |
137 | ... | |
138 | state->instance = atomic_inc_return(&drv_instance) - 1; | |
139 | } | |
140 | ||
141 | ||
142 | struct v4l2_subdev | |
143 | ------------------ | |
144 | ||
145 | Many drivers need to communicate with sub-devices. These devices can do all | |
146 | sort of tasks, but most commonly they handle audio and/or video muxing, | |
147 | encoding or decoding. For webcams common sub-devices are sensors and camera | |
148 | controllers. | |
149 | ||
150 | Usually these are I2C devices, but not necessarily. In order to provide the | |
151 | driver with a consistent interface to these sub-devices the v4l2_subdev struct | |
152 | (v4l2-subdev.h) was created. | |
153 | ||
154 | Each sub-device driver must have a v4l2_subdev struct. This struct can be | |
155 | stand-alone for simple sub-devices or it might be embedded in a larger struct | |
156 | if more state information needs to be stored. Usually there is a low-level | |
157 | device struct (e.g. i2c_client) that contains the device data as setup | |
158 | by the kernel. It is recommended to store that pointer in the private | |
159 | data of v4l2_subdev using v4l2_set_subdevdata(). That makes it easy to go | |
160 | from a v4l2_subdev to the actual low-level bus-specific device data. | |
161 | ||
162 | You also need a way to go from the low-level struct to v4l2_subdev. For the | |
163 | common i2c_client struct the i2c_set_clientdata() call is used to store a | |
164 | v4l2_subdev pointer, for other busses you may have to use other methods. | |
165 | ||
166 | From the bridge driver perspective you load the sub-device module and somehow | |
167 | obtain the v4l2_subdev pointer. For i2c devices this is easy: you call | |
168 | i2c_get_clientdata(). For other busses something similar needs to be done. | |
169 | Helper functions exists for sub-devices on an I2C bus that do most of this | |
170 | tricky work for you. | |
171 | ||
172 | Each v4l2_subdev contains function pointers that sub-device drivers can | |
173 | implement (or leave NULL if it is not applicable). Since sub-devices can do | |
174 | so many different things and you do not want to end up with a huge ops struct | |
175 | of which only a handful of ops are commonly implemented, the function pointers | |
176 | are sorted according to category and each category has its own ops struct. | |
177 | ||
178 | The top-level ops struct contains pointers to the category ops structs, which | |
179 | may be NULL if the subdev driver does not support anything from that category. | |
180 | ||
181 | It looks like this: | |
182 | ||
183 | struct v4l2_subdev_core_ops { | |
184 | int (*g_chip_ident)(struct v4l2_subdev *sd, struct v4l2_chip_ident *chip); | |
185 | int (*log_status)(struct v4l2_subdev *sd); | |
186 | int (*init)(struct v4l2_subdev *sd, u32 val); | |
187 | ... | |
188 | }; | |
189 | ||
190 | struct v4l2_subdev_tuner_ops { | |
191 | ... | |
192 | }; | |
193 | ||
194 | struct v4l2_subdev_audio_ops { | |
195 | ... | |
196 | }; | |
197 | ||
198 | struct v4l2_subdev_video_ops { | |
199 | ... | |
200 | }; | |
201 | ||
202 | struct v4l2_subdev_ops { | |
203 | const struct v4l2_subdev_core_ops *core; | |
204 | const struct v4l2_subdev_tuner_ops *tuner; | |
205 | const struct v4l2_subdev_audio_ops *audio; | |
206 | const struct v4l2_subdev_video_ops *video; | |
207 | }; | |
208 | ||
209 | The core ops are common to all subdevs, the other categories are implemented | |
210 | depending on the sub-device. E.g. a video device is unlikely to support the | |
211 | audio ops and vice versa. | |
212 | ||
213 | This setup limits the number of function pointers while still making it easy | |
214 | to add new ops and categories. | |
215 | ||
216 | A sub-device driver initializes the v4l2_subdev struct using: | |
217 | ||
218 | v4l2_subdev_init(subdev, &ops); | |
219 | ||
220 | Afterwards you need to initialize subdev->name with a unique name and set the | |
221 | module owner. This is done for you if you use the i2c helper functions. | |
222 | ||
223 | A device (bridge) driver needs to register the v4l2_subdev with the | |
224 | v4l2_device: | |
225 | ||
226 | int err = v4l2_device_register_subdev(device, subdev); | |
227 | ||
228 | This can fail if the subdev module disappeared before it could be registered. | |
229 | After this function was called successfully the subdev->dev field points to | |
230 | the v4l2_device. | |
231 | ||
232 | You can unregister a sub-device using: | |
233 | ||
234 | v4l2_device_unregister_subdev(subdev); | |
235 | ||
236 | Afterwards the subdev module can be unloaded and subdev->dev == NULL. | |
237 | ||
238 | You can call an ops function either directly: | |
239 | ||
240 | err = subdev->ops->core->g_chip_ident(subdev, &chip); | |
241 | ||
242 | but it is better and easier to use this macro: | |
243 | ||
244 | err = v4l2_subdev_call(subdev, core, g_chip_ident, &chip); | |
245 | ||
246 | The macro will to the right NULL pointer checks and returns -ENODEV if subdev | |
247 | is NULL, -ENOIOCTLCMD if either subdev->core or subdev->core->g_chip_ident is | |
248 | NULL, or the actual result of the subdev->ops->core->g_chip_ident ops. | |
249 | ||
250 | It is also possible to call all or a subset of the sub-devices: | |
251 | ||
252 | v4l2_device_call_all(dev, 0, core, g_chip_ident, &chip); | |
253 | ||
254 | Any subdev that does not support this ops is skipped and error results are | |
255 | ignored. If you want to check for errors use this: | |
256 | ||
257 | err = v4l2_device_call_until_err(dev, 0, core, g_chip_ident, &chip); | |
258 | ||
259 | Any error except -ENOIOCTLCMD will exit the loop with that error. If no | |
260 | errors (except -ENOIOCTLCMD) occured, then 0 is returned. | |
261 | ||
262 | The second argument to both calls is a group ID. If 0, then all subdevs are | |
263 | called. If non-zero, then only those whose group ID match that value will | |
264 | be called. Before a bridge driver registers a subdev it can set subdev->grp_id | |
265 | to whatever value it wants (it's 0 by default). This value is owned by the | |
266 | bridge driver and the sub-device driver will never modify or use it. | |
267 | ||
268 | The group ID gives the bridge driver more control how callbacks are called. | |
269 | For example, there may be multiple audio chips on a board, each capable of | |
270 | changing the volume. But usually only one will actually be used when the | |
271 | user want to change the volume. You can set the group ID for that subdev to | |
272 | e.g. AUDIO_CONTROLLER and specify that as the group ID value when calling | |
273 | v4l2_device_call_all(). That ensures that it will only go to the subdev | |
274 | that needs it. | |
275 | ||
276 | The advantage of using v4l2_subdev is that it is a generic struct and does | |
277 | not contain any knowledge about the underlying hardware. So a driver might | |
278 | contain several subdevs that use an I2C bus, but also a subdev that is | |
279 | controlled through GPIO pins. This distinction is only relevant when setting | |
280 | up the device, but once the subdev is registered it is completely transparent. | |
281 | ||
282 | ||
283 | I2C sub-device drivers | |
284 | ---------------------- | |
285 | ||
286 | Since these drivers are so common, special helper functions are available to | |
287 | ease the use of these drivers (v4l2-common.h). | |
288 | ||
289 | The recommended method of adding v4l2_subdev support to an I2C driver is to | |
290 | embed the v4l2_subdev struct into the state struct that is created for each | |
291 | I2C device instance. Very simple devices have no state struct and in that case | |
292 | you can just create a v4l2_subdev directly. | |
293 | ||
294 | A typical state struct would look like this (where 'chipname' is replaced by | |
295 | the name of the chip): | |
296 | ||
297 | struct chipname_state { | |
298 | struct v4l2_subdev sd; | |
299 | ... /* additional state fields */ | |
300 | }; | |
301 | ||
302 | Initialize the v4l2_subdev struct as follows: | |
303 | ||
304 | v4l2_i2c_subdev_init(&state->sd, client, subdev_ops); | |
305 | ||
306 | This function will fill in all the fields of v4l2_subdev and ensure that the | |
307 | v4l2_subdev and i2c_client both point to one another. | |
308 | ||
309 | You should also add a helper inline function to go from a v4l2_subdev pointer | |
310 | to a chipname_state struct: | |
311 | ||
312 | static inline struct chipname_state *to_state(struct v4l2_subdev *sd) | |
313 | { | |
314 | return container_of(sd, struct chipname_state, sd); | |
315 | } | |
316 | ||
317 | Use this to go from the v4l2_subdev struct to the i2c_client struct: | |
318 | ||
319 | struct i2c_client *client = v4l2_get_subdevdata(sd); | |
320 | ||
321 | And this to go from an i2c_client to a v4l2_subdev struct: | |
322 | ||
323 | struct v4l2_subdev *sd = i2c_get_clientdata(client); | |
324 | ||
325 | Finally you need to make a command function to make driver->command() | |
326 | call the right subdev_ops functions: | |
327 | ||
328 | static int subdev_command(struct i2c_client *client, unsigned cmd, void *arg) | |
329 | { | |
330 | return v4l2_subdev_command(i2c_get_clientdata(client), cmd, arg); | |
331 | } | |
332 | ||
333 | If driver->command is never used then you can leave this out. Eventually the | |
334 | driver->command usage should be removed from v4l. | |
335 | ||
336 | Make sure to call v4l2_device_unregister_subdev(sd) when the remove() callback | |
337 | is called. This will unregister the sub-device from the bridge driver. It is | |
338 | safe to call this even if the sub-device was never registered. | |
339 | ||
340 | ||
341 | The bridge driver also has some helper functions it can use: | |
342 | ||
343 | struct v4l2_subdev *sd = v4l2_i2c_new_subdev(adapter, "module_foo", "chipid", 0x36); | |
344 | ||
345 | This loads the given module (can be NULL if no module needs to be loaded) and | |
346 | calls i2c_new_device() with the given i2c_adapter and chip/address arguments. | |
347 | If all goes well, then it registers the subdev with the v4l2_device. It gets | |
348 | the v4l2_device by calling i2c_get_adapdata(adapter), so you should make sure | |
349 | that adapdata is set to v4l2_device when you setup the i2c_adapter in your | |
350 | driver. | |
351 | ||
352 | You can also use v4l2_i2c_new_probed_subdev() which is very similar to | |
353 | v4l2_i2c_new_subdev(), except that it has an array of possible I2C addresses | |
354 | that it should probe. Internally it calls i2c_new_probed_device(). | |
355 | ||
356 | Both functions return NULL if something went wrong. | |
357 | ||
358 | ||
359 | struct video_device | |
360 | ------------------- | |
361 | ||
362 | Not yet documented. |