Hans Verkuil | 2a1fcdf | 2008-11-29 21:36:58 -0300 | [diff] [blame^] | 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. |