1 Overview of the V4L2 driver framework
2 =====================================
4 This text documents the various structures provided by the V4L2 framework and
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
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
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).
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.
30 There is also a lot of common code that could never be refactored due to
31 the lack of a framework.
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.
41 All drivers have the following structure:
43 1) A struct for each device instance containing the device state.
45 2) A way of initializing and commanding sub-devices (if any).
47 3) Creating V4L2 device nodes (/dev/videoX, /dev/vbiX, /dev/radioX and
48 /dev/vtxX) and keeping track of device-node specific data.
50 4) Filehandle-specific structs containing per-filehandle data;
52 5) video buffer handling.
54 This is a rough schematic of how it all relates:
58 +-sub-device instances
62 \-filehandle instances
65 Structure of the framework
66 --------------------------
68 The framework closely resembles the driver structure: it has a v4l2_device
69 struct for the device instance data, a v4l2_subdev struct to refer to
70 sub-device instances, the video_device struct stores V4L2 device node data
71 and in the future a v4l2_fh struct will keep track of filehandle instances
72 (this is not yet implemented).
78 Each device instance is represented by a struct v4l2_device (v4l2-device.h).
79 Very simple devices can just allocate this struct, but most of the time you
80 would embed this struct inside a larger struct.
82 You must register the device instance:
84 v4l2_device_register(struct device *dev, struct v4l2_device *v4l2_dev);
86 Registration will initialize the v4l2_device struct and link dev->driver_data
87 to v4l2_dev. If v4l2_dev->name is empty then it will be set to a value derived
88 from dev (driver name followed by the bus_id, to be precise). If you set it
89 up before calling v4l2_device_register then it will be untouched. If dev is
90 NULL, then you *must* setup v4l2_dev->name before calling v4l2_device_register.
92 The first 'dev' argument is normally the struct device pointer of a pci_dev,
93 usb_device or platform_device. It is rare for dev to be NULL, but it happens
94 with ISA devices or when one device creates multiple PCI devices, thus making
95 it impossible to associate v4l2_dev with a particular parent.
97 You can also supply a notify() callback that can be called by sub-devices to
98 notify you of events. Whether you need to set this depends on the sub-device.
99 Any notifications a sub-device supports must be defined in a header in
100 include/media/<subdevice>.h.
104 v4l2_device_unregister(struct v4l2_device *v4l2_dev);
106 Unregistering will also automatically unregister all subdevs from the device.
108 Sometimes you need to iterate over all devices registered by a specific
109 driver. This is usually the case if multiple device drivers use the same
110 hardware. E.g. the ivtvfb driver is a framebuffer driver that uses the ivtv
111 hardware. The same is true for alsa drivers for example.
113 You can iterate over all registered devices as follows:
115 static int callback(struct device *dev, void *p)
117 struct v4l2_device *v4l2_dev = dev_get_drvdata(dev);
119 /* test if this device was inited */
120 if (v4l2_dev == NULL)
128 struct device_driver *drv;
131 /* Find driver 'ivtv' on the PCI bus.
132 pci_bus_type is a global. For USB busses use usb_bus_type. */
133 drv = driver_find("ivtv", &pci_bus_type);
134 /* iterate over all ivtv device instances */
135 err = driver_for_each_device(drv, NULL, p, callback);
140 Sometimes you need to keep a running counter of the device instance. This is
141 commonly used to map a device instance to an index of a module option array.
143 The recommended approach is as follows:
145 static atomic_t drv_instance = ATOMIC_INIT(0);
147 static int __devinit drv_probe(struct pci_dev *pdev,
148 const struct pci_device_id *pci_id)
151 state->instance = atomic_inc_return(&drv_instance) - 1;
158 Many drivers need to communicate with sub-devices. These devices can do all
159 sort of tasks, but most commonly they handle audio and/or video muxing,
160 encoding or decoding. For webcams common sub-devices are sensors and camera
163 Usually these are I2C devices, but not necessarily. In order to provide the
164 driver with a consistent interface to these sub-devices the v4l2_subdev struct
165 (v4l2-subdev.h) was created.
167 Each sub-device driver must have a v4l2_subdev struct. This struct can be
168 stand-alone for simple sub-devices or it might be embedded in a larger struct
169 if more state information needs to be stored. Usually there is a low-level
170 device struct (e.g. i2c_client) that contains the device data as setup
171 by the kernel. It is recommended to store that pointer in the private
172 data of v4l2_subdev using v4l2_set_subdevdata(). That makes it easy to go
173 from a v4l2_subdev to the actual low-level bus-specific device data.
175 You also need a way to go from the low-level struct to v4l2_subdev. For the
176 common i2c_client struct the i2c_set_clientdata() call is used to store a
177 v4l2_subdev pointer, for other busses you may have to use other methods.
179 From the bridge driver perspective you load the sub-device module and somehow
180 obtain the v4l2_subdev pointer. For i2c devices this is easy: you call
181 i2c_get_clientdata(). For other busses something similar needs to be done.
182 Helper functions exists for sub-devices on an I2C bus that do most of this
185 Each v4l2_subdev contains function pointers that sub-device drivers can
186 implement (or leave NULL if it is not applicable). Since sub-devices can do
187 so many different things and you do not want to end up with a huge ops struct
188 of which only a handful of ops are commonly implemented, the function pointers
189 are sorted according to category and each category has its own ops struct.
191 The top-level ops struct contains pointers to the category ops structs, which
192 may be NULL if the subdev driver does not support anything from that category.
196 struct v4l2_subdev_core_ops {
197 int (*g_chip_ident)(struct v4l2_subdev *sd, struct v4l2_dbg_chip_ident *chip);
198 int (*log_status)(struct v4l2_subdev *sd);
199 int (*init)(struct v4l2_subdev *sd, u32 val);
203 struct v4l2_subdev_tuner_ops {
207 struct v4l2_subdev_audio_ops {
211 struct v4l2_subdev_video_ops {
215 struct v4l2_subdev_ops {
216 const struct v4l2_subdev_core_ops *core;
217 const struct v4l2_subdev_tuner_ops *tuner;
218 const struct v4l2_subdev_audio_ops *audio;
219 const struct v4l2_subdev_video_ops *video;
222 The core ops are common to all subdevs, the other categories are implemented
223 depending on the sub-device. E.g. a video device is unlikely to support the
224 audio ops and vice versa.
226 This setup limits the number of function pointers while still making it easy
227 to add new ops and categories.
229 A sub-device driver initializes the v4l2_subdev struct using:
231 v4l2_subdev_init(sd, &ops);
233 Afterwards you need to initialize subdev->name with a unique name and set the
234 module owner. This is done for you if you use the i2c helper functions.
236 A device (bridge) driver needs to register the v4l2_subdev with the
239 int err = v4l2_device_register_subdev(v4l2_dev, sd);
241 This can fail if the subdev module disappeared before it could be registered.
242 After this function was called successfully the subdev->dev field points to
245 You can unregister a sub-device using:
247 v4l2_device_unregister_subdev(sd);
249 Afterwards the subdev module can be unloaded and sd->dev == NULL.
251 You can call an ops function either directly:
253 err = sd->ops->core->g_chip_ident(sd, &chip);
255 but it is better and easier to use this macro:
257 err = v4l2_subdev_call(sd, core, g_chip_ident, &chip);
259 The macro will to the right NULL pointer checks and returns -ENODEV if subdev
260 is NULL, -ENOIOCTLCMD if either subdev->core or subdev->core->g_chip_ident is
261 NULL, or the actual result of the subdev->ops->core->g_chip_ident ops.
263 It is also possible to call all or a subset of the sub-devices:
265 v4l2_device_call_all(v4l2_dev, 0, core, g_chip_ident, &chip);
267 Any subdev that does not support this ops is skipped and error results are
268 ignored. If you want to check for errors use this:
270 err = v4l2_device_call_until_err(v4l2_dev, 0, core, g_chip_ident, &chip);
272 Any error except -ENOIOCTLCMD will exit the loop with that error. If no
273 errors (except -ENOIOCTLCMD) occured, then 0 is returned.
275 The second argument to both calls is a group ID. If 0, then all subdevs are
276 called. If non-zero, then only those whose group ID match that value will
277 be called. Before a bridge driver registers a subdev it can set sd->grp_id
278 to whatever value it wants (it's 0 by default). This value is owned by the
279 bridge driver and the sub-device driver will never modify or use it.
281 The group ID gives the bridge driver more control how callbacks are called.
282 For example, there may be multiple audio chips on a board, each capable of
283 changing the volume. But usually only one will actually be used when the
284 user want to change the volume. You can set the group ID for that subdev to
285 e.g. AUDIO_CONTROLLER and specify that as the group ID value when calling
286 v4l2_device_call_all(). That ensures that it will only go to the subdev
289 If the sub-device needs to notify its v4l2_device parent of an event, then
290 it can call v4l2_subdev_notify(sd, notification, arg). This macro checks
291 whether there is a notify() callback defined and returns -ENODEV if not.
292 Otherwise the result of the notify() call is returned.
294 The advantage of using v4l2_subdev is that it is a generic struct and does
295 not contain any knowledge about the underlying hardware. So a driver might
296 contain several subdevs that use an I2C bus, but also a subdev that is
297 controlled through GPIO pins. This distinction is only relevant when setting
298 up the device, but once the subdev is registered it is completely transparent.
301 I2C sub-device drivers
302 ----------------------
304 Since these drivers are so common, special helper functions are available to
305 ease the use of these drivers (v4l2-common.h).
307 The recommended method of adding v4l2_subdev support to an I2C driver is to
308 embed the v4l2_subdev struct into the state struct that is created for each
309 I2C device instance. Very simple devices have no state struct and in that case
310 you can just create a v4l2_subdev directly.
312 A typical state struct would look like this (where 'chipname' is replaced by
313 the name of the chip):
315 struct chipname_state {
316 struct v4l2_subdev sd;
317 ... /* additional state fields */
320 Initialize the v4l2_subdev struct as follows:
322 v4l2_i2c_subdev_init(&state->sd, client, subdev_ops);
324 This function will fill in all the fields of v4l2_subdev and ensure that the
325 v4l2_subdev and i2c_client both point to one another.
327 You should also add a helper inline function to go from a v4l2_subdev pointer
328 to a chipname_state struct:
330 static inline struct chipname_state *to_state(struct v4l2_subdev *sd)
332 return container_of(sd, struct chipname_state, sd);
335 Use this to go from the v4l2_subdev struct to the i2c_client struct:
337 struct i2c_client *client = v4l2_get_subdevdata(sd);
339 And this to go from an i2c_client to a v4l2_subdev struct:
341 struct v4l2_subdev *sd = i2c_get_clientdata(client);
343 Finally you need to make a command function to make driver->command()
344 call the right subdev_ops functions:
346 static int subdev_command(struct i2c_client *client, unsigned cmd, void *arg)
348 return v4l2_subdev_command(i2c_get_clientdata(client), cmd, arg);
351 If driver->command is never used then you can leave this out. Eventually the
352 driver->command usage should be removed from v4l.
354 Make sure to call v4l2_device_unregister_subdev(sd) when the remove() callback
355 is called. This will unregister the sub-device from the bridge driver. It is
356 safe to call this even if the sub-device was never registered.
358 You need to do this because when the bridge driver destroys the i2c adapter
359 the remove() callbacks are called of the i2c devices on that adapter.
360 After that the corresponding v4l2_subdev structures are invalid, so they
361 have to be unregistered first. Calling v4l2_device_unregister_subdev(sd)
362 from the remove() callback ensures that this is always done correctly.
365 The bridge driver also has some helper functions it can use:
367 struct v4l2_subdev *sd = v4l2_i2c_new_subdev(adapter, "module_foo", "chipid", 0x36);
369 This loads the given module (can be NULL if no module needs to be loaded) and
370 calls i2c_new_device() with the given i2c_adapter and chip/address arguments.
371 If all goes well, then it registers the subdev with the v4l2_device. It gets
372 the v4l2_device by calling i2c_get_adapdata(adapter), so you should make sure
373 to call i2c_set_adapdata(adapter, v4l2_device) when you setup the i2c_adapter
376 You can also use v4l2_i2c_new_probed_subdev() which is very similar to
377 v4l2_i2c_new_subdev(), except that it has an array of possible I2C addresses
378 that it should probe. Internally it calls i2c_new_probed_device().
380 Both functions return NULL if something went wrong.
382 Note that the chipid you pass to v4l2_i2c_new_(probed_)subdev() is usually
383 the same as the module name. It allows you to specify a chip variant, e.g.
384 "saa7114" or "saa7115". In general though the i2c driver autodetects this.
385 The use of chipid is something that needs to be looked at more closely at a
386 later date. It differs between i2c drivers and as such can be confusing.
387 To see which chip variants are supported you can look in the i2c driver code
388 for the i2c_device_id table. This lists all the possibilities.
394 The actual device nodes in the /dev directory are created using the
395 video_device struct (v4l2-dev.h). This struct can either be allocated
396 dynamically or embedded in a larger struct.
398 To allocate it dynamically use:
400 struct video_device *vdev = video_device_alloc();
405 vdev->release = video_device_release;
407 If you embed it in a larger struct, then you must set the release()
408 callback to your own function:
410 struct video_device *vdev = &my_vdev->vdev;
412 vdev->release = my_vdev_release;
414 The release callback must be set and it is called when the last user
415 of the video device exits.
417 The default video_device_release() callback just calls kfree to free the
420 You should also set these fields:
422 - v4l2_dev: set to the v4l2_device parent device.
423 - name: set to something descriptive and unique.
424 - fops: set to the v4l2_file_operations struct.
425 - ioctl_ops: if you use the v4l2_ioctl_ops to simplify ioctl maintenance
426 (highly recommended to use this and it might become compulsory in the
427 future!), then set this to your v4l2_ioctl_ops struct.
428 - parent: you only set this if v4l2_device was registered with NULL as
429 the parent device struct. This only happens in cases where one hardware
430 device has multiple PCI devices that all share the same v4l2_device core.
432 The cx88 driver is an example of this: one core v4l2_device struct, but
433 it is used by both an raw video PCI device (cx8800) and a MPEG PCI device
434 (cx8802). Since the v4l2_device cannot be associated with a particular
435 PCI device it is setup without a parent device. But when the struct
436 video_device is setup you do know which parent PCI device to use.
438 If you use v4l2_ioctl_ops, then you should set either .unlocked_ioctl or
439 .ioctl to video_ioctl2 in your v4l2_file_operations struct.
441 The v4l2_file_operations struct is a subset of file_operations. The main
442 difference is that the inode argument is omitted since it is never used.
445 video_device registration
446 -------------------------
448 Next you register the video device: this will create the character device
451 err = video_register_device(vdev, VFL_TYPE_GRABBER, -1);
453 video_device_release(vdev); /* or kfree(my_vdev); */
457 Which device is registered depends on the type argument. The following
460 VFL_TYPE_GRABBER: videoX for video input/output devices
461 VFL_TYPE_VBI: vbiX for vertical blank data (i.e. closed captions, teletext)
462 VFL_TYPE_RADIO: radioX for radio tuners
463 VFL_TYPE_VTX: vtxX for teletext devices (deprecated, don't use)
465 The last argument gives you a certain amount of control over the device
466 kernel number used (i.e. the X in videoX). Normally you will pass -1 to
467 let the v4l2 framework pick the first free number. But if a driver creates
468 many devices, then it can be useful to have different video devices in
469 separate ranges. For example, video capture devices start at 0, video
470 output devices start at 16.
472 So you can use the last argument to specify a minimum kernel number and
473 the v4l2 framework will try to pick the first free number that is equal
474 or higher to what you passed. If that fails, then it will just pick the
477 Whenever a device node is created some attributes are also created for you.
478 If you look in /sys/class/video4linux you see the devices. Go into e.g.
479 video0 and you will see 'name' and 'index' attributes. The 'name' attribute
480 is the 'name' field of the video_device struct. The 'index' attribute is
481 a device node index that can be assigned by the driver, or that is calculated
484 If you call video_register_device(), then the index is just increased by
485 1 for each device node you register. The first video device node you register
486 always starts off with 0.
488 Alternatively you can call video_register_device_index() which is identical
489 to video_register_device(), but with an extra index argument. Here you can
490 pass a specific index value (between 0 and 31) that should be used.
492 Users can setup udev rules that utilize the index attribute to make fancy
493 device names (e.g. 'mpegX' for MPEG video capture device nodes).
495 After the device was successfully registered, then you can use these fields:
497 - vfl_type: the device type passed to video_register_device.
498 - minor: the assigned device minor number.
499 - num: the device kernel number (i.e. the X in videoX).
500 - index: the device index number (calculated or set explicitly using
501 video_register_device_index).
503 If the registration failed, then you need to call video_device_release()
504 to free the allocated video_device struct, or free your own struct if the
505 video_device was embedded in it. The vdev->release() callback will never
506 be called if the registration failed, nor should you ever attempt to
507 unregister the device if the registration failed.
513 When the video device nodes have to be removed, either during the unload
514 of the driver or because the USB device was disconnected, then you should
517 video_unregister_device(vdev);
519 This will remove the device nodes from sysfs (causing udev to remove them
522 After video_unregister_device() returns no new opens can be done.
524 However, in the case of USB devices some application might still have one
525 of these device nodes open. You should block all new accesses to read,
526 write, poll, etc. except possibly for certain ioctl operations like
529 When the last user of the video device node exits, then the vdev->release()
530 callback is called and you can do the final cleanup there.
533 video_device helper functions
534 -----------------------------
536 There are a few useful helper functions:
538 You can set/get driver private data in the video_device struct using:
540 void *video_get_drvdata(struct video_device *vdev);
541 void video_set_drvdata(struct video_device *vdev, void *data);
543 Note that you can safely call video_set_drvdata() before calling
544 video_register_device().
548 struct video_device *video_devdata(struct file *file);
550 returns the video_device belonging to the file struct.
552 The final helper function combines video_get_drvdata with
555 void *video_drvdata(struct file *file);
557 You can go from a video_device struct to the v4l2_device struct using:
559 struct v4l2_device *v4l2_dev = vdev->v4l2_dev;
561 video buffer helper functions
562 -----------------------------
564 The v4l2 core API provides a standard method for dealing with video
565 buffers. Those methods allow a driver to implement read(), mmap() and
566 overlay() on a consistent way.
568 There are currently methods for using video buffers on devices that
569 supports DMA with scatter/gather method (videobuf-dma-sg), DMA with
570 linear access (videobuf-dma-contig), and vmalloced buffers, mostly
571 used on USB drivers (videobuf-vmalloc).
573 Any driver using videobuf should provide operations (callbacks) for
576 ops->buf_setup - calculates the size of the video buffers and avoid they
577 to waste more than some maximum limit of RAM;
578 ops->buf_prepare - fills the video buffer structs and calls
579 videobuf_iolock() to alloc and prepare mmaped memory;
580 ops->buf_queue - advices the driver that another buffer were
581 requested (by read() or by QBUF);
582 ops->buf_release - frees any buffer that were allocated.
584 In order to use it, the driver need to have a code (generally called at
585 interrupt context) that will properly handle the buffer request lists,
586 announcing that a new buffer were filled.
588 The irq handling code should handle the videobuf task lists, in order
589 to advice videobuf that a new frame were filled, in order to honor to a
590 request. The code is generally like this one:
591 if (list_empty(&dma_q->active))
594 buf = list_entry(dma_q->active.next, struct vbuffer, vb.queue);
596 if (!waitqueue_active(&buf->vb.done))
599 /* Some logic to handle the buf may be needed here */
601 list_del(&buf->vb.queue);
602 do_gettimeofday(&buf->vb.ts);
603 wake_up(&buf->vb.done);
605 Those are the videobuffer functions used on drivers, implemented on
608 - Videobuf init functions
609 videobuf_queue_sg_init()
610 Initializes the videobuf infrastructure. This function should be
611 called before any other videobuf function on drivers that uses DMA
612 Scatter/Gather buffers.
614 videobuf_queue_dma_contig_init
615 Initializes the videobuf infrastructure. This function should be
616 called before any other videobuf function on drivers that need DMA
619 videobuf_queue_vmalloc_init()
620 Initializes the videobuf infrastructure. This function should be
621 called before any other videobuf function on USB (and other drivers)
622 that need a vmalloced type of videobuf.
625 Prepares the videobuf memory for the proper method (read, mmap, overlay).
627 - videobuf_queue_is_busy()
628 Checks if a videobuf is streaming.
630 - videobuf_queue_cancel()
631 Stops video handling.
633 - videobuf_mmap_free()
637 Stops video handling, ends mmap and frees mmap and other buffers.
639 - V4L2 api functions. Those functions correspond to VIDIOC_foo ioctls:
640 videobuf_reqbufs(), videobuf_querybuf(), videobuf_qbuf(),
641 videobuf_dqbuf(), videobuf_streamon(), videobuf_streamoff().
643 - V4L1 api function (corresponds to VIDIOCMBUF ioctl):
645 This function is used to provide backward compatibility with V4L1
648 - Some help functions for read()/poll() operations:
649 videobuf_read_stream()
650 For continuous stream read()
653 videobuf_poll_stream()
654 polling help function
656 The better way to understand it is to take a look at vivi driver. One
657 of the main reasons for vivi is to be a videobuf usage example. the
658 vivi_thread_tick() does the task that the IRQ callback would do on PCI
659 drivers (or the irq callback on USB).