summaryrefslogtreecommitdiff
path: root/Documentation/video4linux/v4l2-framework.txt
blob: e1e6a01d7ac6f26abcccb3ffbd3847381247f267 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
Overview of the V4L2 driver framework
=====================================

This text documents the various structures provided by the V4L2 framework and
their relationships.


Introduction
------------

The V4L2 drivers tend to be very complex due to the complexity of the
hardware: most devices have multiple ICs, export multiple device nodes in
/dev, and create also non-V4L2 devices such as DVB, ALSA, FB, I2C and input
(IR) devices.

Especially the fact that V4L2 drivers have to setup supporting ICs to
do audio/video muxing/encoding/decoding makes it more complex than most.
Usually these ICs are connected to the main bridge driver through one or
more I2C busses, but other busses can also be used. Such devices are
called 'sub-devices'.

For a long time the framework was limited to the video_device struct for
creating V4L device nodes and video_buf for handling the video buffers
(note that this document does not discuss the video_buf framework).

This meant that all drivers had to do the setup of device instances and
connecting to sub-devices themselves. Some of this is quite complicated
to do right and many drivers never did do it correctly.

There is also a lot of common code that could never be refactored due to
the lack of a framework.

So this framework sets up the basic building blocks that all drivers
need and this same framework should make it much easier to refactor
common code into utility functions shared by all drivers.


Structure of a driver
---------------------

All drivers have the following structure:

1) A struct for each device instance containing the device state.

2) A way of initializing and commanding sub-devices (if any).

3) Creating V4L2 device nodes (/dev/videoX, /dev/vbiX and /dev/radioX)
   and keeping track of device-node specific data.

4) Filehandle-specific structs containing per-filehandle data;

5) video buffer handling.

This is a rough schematic of how it all relates:

    device instances
      |
      +-sub-device instances
      |
      \-V4L2 device nodes
	  |
	  \-filehandle instances


Structure of the framework
--------------------------

The framework closely resembles the driver structure: it has a v4l2_device
struct for the device instance data, a v4l2_subdev struct to refer to
sub-device instances, the video_device struct stores V4L2 device node data
and in the future a v4l2_fh struct will keep track of filehandle instances
(this is not yet implemented).

The V4L2 framework also optionally integrates with the media framework. If a
driver sets the struct v4l2_device mdev field, sub-devices and video nodes
will automatically appear in the media framework as entities.


struct v4l2_device
------------------

Each device instance is represented by a struct v4l2_device (v4l2-device.h).
Very simple devices can just allocate this struct, but most of the time you
would embed this struct inside a larger struct.

You must register the device instance:

	v4l2_device_register(struct device *dev, struct v4l2_device *v4l2_dev);

Registration will initialize the v4l2_device struct. If the dev->driver_data
field is NULL, it will be linked to v4l2_dev.

Drivers that want integration with the media device framework need to set
dev->driver_data manually to point to the driver-specific device structure
that embed the struct v4l2_device instance. This is achieved by a
dev_set_drvdata() call before registering the V4L2 device instance. They must
also set the struct v4l2_device mdev field to point to a properly initialized
and registered media_device instance.

If v4l2_dev->name is empty then it will be set to a value derived from dev
(driver name followed by the bus_id, to be precise). If you set it up before
calling v4l2_device_register then it will be untouched. If dev is NULL, then
you *must* setup v4l2_dev->name before calling v4l2_device_register.

You can use v4l2_device_set_name() to set the name based on a driver name and
a driver-global atomic_t instance. This will generate names like ivtv0, ivtv1,
etc. If the name ends with a digit, then it will insert a dash: cx18-0,
cx18-1, etc. This function returns the instance number.

The first 'dev' argument is normally the struct device pointer of a pci_dev,
usb_interface or platform_device. It is rare for dev to be NULL, but it happens
with ISA devices or when one device creates multiple PCI devices, thus making
it impossible to associate v4l2_dev with a particular parent.

You can also supply a notify() callback that can be called by sub-devices to
notify you of events. Whether you need to set this depends on the sub-device.
Any notifications a sub-device supports must be defined in a header in
include/media/<subdevice>.h.

You unregister with:

	v4l2_device_unregister(struct v4l2_device *v4l2_dev);

If the dev->driver_data field points to v4l2_dev, it will be reset to NULL.
Unregistering will also automatically unregister all subdevs from the device.

If you have a hotpluggable device (e.g. a USB device), then when a disconnect
happens the parent device becomes invalid. Since v4l2_device has a pointer to
that parent device it has to be cleared as well to mark that the parent is
gone. To do this call:

	v4l2_device_disconnect(struct v4l2_device *v4l2_dev);

This does *not* unregister the subdevs, so you still need to call the
v4l2_device_unregister() function for that. If your driver is not hotpluggable,
then there is no need to call v4l2_device_disconnect().

Sometimes you need to iterate over all devices registered by a specific
driver. This is usually the case if multiple device drivers use the same
hardware. E.g. the ivtvfb driver is a framebuffer driver that uses the ivtv
hardware. The same is true for alsa drivers for example.

You can iterate over all registered devices as follows:

static int callback(struct device *dev, void *p)
{
	struct v4l2_device *v4l2_dev = dev_get_drvdata(dev);

	/* test if this device was inited */
	if (v4l2_dev == NULL)
		return 0;
	...
	return 0;
}

int iterate(void *p)
{
	struct device_driver *drv;
	int err;

	/* Find driver 'ivtv' on the PCI bus.
	   pci_bus_type is a global. For USB busses use usb_bus_type. */
	drv = driver_find("ivtv", &pci_bus_type);
	/* iterate over all ivtv device instances */
	err = driver_for_each_device(drv, NULL, p, callback);
	put_driver(drv);
	return err;
}

Sometimes you need to keep a running counter of the device instance. This is
commonly used to map a device instance to an index of a module option array.

The recommended approach is as follows:

static atomic_t drv_instance = ATOMIC_INIT(0);

static int __devinit drv_probe(struct pci_dev *pdev,
				const struct pci_device_id *pci_id)
{
	...
	state->instance = atomic_inc_return(&drv_instance) - 1;
}

If you have multiple device nodes then it can be difficult to know when it is
safe to unregister v4l2_device for hotpluggable devices. For this purpose
v4l2_device has refcounting support. The refcount is increased whenever
video_register_device is called and it is decreased whenever that device node
is released. When the refcount reaches zero, then the v4l2_device release()
callback is called. You can do your final cleanup there.

If other device nodes (e.g. ALSA) are created, then you can increase and
decrease the refcount manually as well by calling:

void v4l2_device_get(struct v4l2_device *v4l2_dev);

or:

int v4l2_device_put(struct v4l2_device *v4l2_dev);

Since the initial refcount is 1 you also need to call v4l2_device_put in the
disconnect() callback (for USB devices) or in the remove() callback (for e.g.
PCI devices), otherwise the refcount will never reach 0.

struct v4l2_subdev
------------------

Many drivers need to communicate with sub-devices. These devices can do all
sort of tasks, but most commonly they handle audio and/or video muxing,
encoding or decoding. For webcams common sub-devices are sensors and camera
controllers.

Usually these are I2C devices, but not necessarily. In order to provide the
driver with a consistent interface to these sub-devices the v4l2_subdev struct
(v4l2-subdev.h) was created.

Each sub-device driver must have a v4l2_subdev struct. This struct can be
stand-alone for simple sub-devices or it might be embedded in a larger struct
if more state information needs to be stored. Usually there is a low-level
device struct (e.g. i2c_client) that contains the device data as setup
by the kernel. It is recommended to store that pointer in the private
data of v4l2_subdev using v4l2_set_subdevdata(). That makes it easy to go
from a v4l2_subdev to the actual low-level bus-specific device data.

You also need a way to go from the low-level struct to v4l2_subdev. For the
common i2c_client struct the i2c_set_clientdata() call is used to store a
v4l2_subdev pointer, for other busses you may have to use other methods.

Bridges might also need to store per-subdev private data, such as a pointer to
bridge-specific per-subdev private data. The v4l2_subdev structure provides
host private data for that purpose that can be accessed with
v4l2_get_subdev_hostdata() and v4l2_set_subdev_hostdata().

From the bridge driver perspective you load the sub-device module and somehow
obtain the v4l2_subdev pointer. For i2c devices this is easy: you call
i2c_get_clientdata(). For other busses something similar needs to be done.
Helper functions exists for sub-devices on an I2C bus that do most of this
tricky work for you.

Each v4l2_subdev contains function pointers that sub-device drivers can
implement (or leave NULL if it is not applicable). Since sub-devices can do
so many different things and you do not want to end up with a huge ops struct
of which only a handful of ops are commonly implemented, the function pointers
are sorted according to category and each category has its own ops struct.

The top-level ops struct contains pointers to the category ops structs, which
may be NULL if the subdev driver does not support anything from that category.

It looks like this:

struct v4l2_subdev_core_ops {
	int (*g_chip_ident)(struct v4l2_subdev *sd, struct v4l2_dbg_chip_ident *chip);
	int (*log_status)(struct v4l2_subdev *sd);
	int (*init)(struct v4l2_subdev *sd, u32 val);
	...
};

struct v4l2_subdev_tuner_ops {
	...
};

struct v4l2_subdev_audio_ops {
	...
};

struct v4l2_subdev_video_ops {
	...
};

struct v4l2_subdev_pad_ops {
	...
};

struct v4l2_subdev_ops {
	const struct v4l2_subdev_core_ops  *core;
	const struct v4l2_subdev_tuner_ops *tuner;
	const struct v4l2_subdev_audio_ops *audio;
	const struct v4l2_subdev_video_ops *video;
	const struct v4l2_subdev_pad_ops *video;
};

The core ops are common to all subdevs, the other categories are implemented
depending on the sub-device. E.g. a video device is unlikely to support the
audio ops and vice versa.

This setup limits the number of function pointers while still making it easy
to add new ops and categories.

A sub-device driver initializes the v4l2_subdev struct using:

	v4l2_subdev_init(sd, &ops);

Afterwards you need to initialize subdev->name with a unique name and set the
module owner. This is done for you if you use the i2c helper functions.

If integration with the media framework is needed, you must initialize the
media_entity struct embedded in the v4l2_subdev struct (entity field) by
calling media_entity_init():

	struct media_pad *pads = &my_sd->pads;
	int err;

	err = media_entity_init(&sd->entity, npads, pads, 0);

The pads array must have been previously initialized. There is no need to
manually set the struct media_entity type and name fields, but the revision
field must be initialized if needed.

A reference to the entity will be automatically acquired/released when the
subdev device node (if any) is opened/closed.

Don't forget to cleanup the media entity before the sub-device is destroyed:

	media_entity_cleanup(&sd->entity);

If the subdev driver intends to process video and integrate with the media
framework, it must implement format related functionality using
v4l2_subdev_pad_ops instead of v4l2_subdev_video_ops.

In that case, the subdev driver may set the link_validate field to provide
its own link validation function. The link validation function is called for
every link in the pipeline where both of the ends of the links are V4L2
sub-devices. The driver is still responsible for validating the correctness
of the format configuration between sub-devices and video nodes.

If link_validate op is not set, the default function
v4l2_subdev_link_validate_default() is used instead. This function ensures
that width, height and the media bus pixel code are equal on both source and
sink of the link. Subdev drivers are also free to use this function to
perform the checks mentioned above in addition to their own checks.

A device (bridge) driver needs to register the v4l2_subdev with the
v4l2_device:

	int err = v4l2_device_register_subdev(v4l2_dev, sd);

This can fail if the subdev module disappeared before it could be registered.
After this function was called successfully the subdev->dev field points to
the v4l2_device.

If the v4l2_device parent device has a non-NULL mdev field, the sub-device
entity will be automatically registered with the media device.

You can unregister a sub-device using:

	v4l2_device_unregister_subdev(sd);

Afterwards the subdev module can be unloaded and sd->dev == NULL.

You can call an ops function either directly:

	err = sd->ops->core->g_chip_ident(sd, &chip);

but it is better and easier to use this macro:

	err = v4l2_subdev_call(sd, core, g_chip_ident, &chip);

The macro will to the right NULL pointer checks and returns -ENODEV if subdev
is NULL, -ENOIOCTLCMD if either subdev->core or subdev->core->g_chip_ident is
NULL, or the actual result of the subdev->ops->core->g_chip_ident ops.

It is also possible to call all or a subset of the sub-devices:

	v4l2_device_call_all(v4l2_dev, 0, core, g_chip_ident, &chip);

Any subdev that does not support this ops is skipped and error results are
ignored. If you want to check for errors use this:

	err = v4l2_device_call_until_err(v4l2_dev, 0, core, g_chip_ident, &chip);

Any error except -ENOIOCTLCMD will exit the loop with that error. If no
errors (except -ENOIOCTLCMD) occurred, then 0 is returned.

The second argument to both calls is a group ID. If 0, then all subdevs are
called. If non-zero, then only those whose group ID match that value will
be called. Before a bridge driver registers a subdev it can set sd->grp_id
to whatever value it wants (it's 0 by default). This value is owned by the
bridge driver and the sub-device driver will never modify or use it.

The group ID gives the bridge driver more control how callbacks are called.
For example, there may be multiple audio chips on a board, each capable of
changing the volume. But usually only one will actually be used when the
user want to change the volume. You can set the group ID for that subdev to
e.g. AUDIO_CONTROLLER and specify that as the group ID value when calling
v4l2_device_call_all(). That ensures that it will only go to the subdev
that needs it.

If the sub-device needs to notify its v4l2_device parent of an event, then
it can call v4l2_subdev_notify(sd, notification, arg). This macro checks
whether there is a notify() callback defined and returns -ENODEV if not.
Otherwise the result of the notify() call is returned.

The advantage of using v4l2_subdev is that it is a generic struct and does
not contain any knowledge about the underlying hardware. So a driver might
contain several subdevs that use an I2C bus, but also a subdev that is
controlled through GPIO pins. This distinction is only relevant when setting
up the device, but once the subdev is registered it is completely transparent.


V4L2 sub-device userspace API
-----------------------------

Beside exposing a kernel API through the v4l2_subdev_ops structure, V4L2
sub-devices can also be controlled directly by userspace applications.

Device nodes named v4l-subdevX can be created in /dev to access sub-devices
directly. If a sub-device supports direct userspace configuration it must set
the V4L2_SUBDEV_FL_HAS_DEVNODE flag before being registered.

After registering sub-devices, the v4l2_device driver can create device nodes
for all registered sub-devices marked with V4L2_SUBDEV_FL_HAS_DEVNODE by calling
v4l2_device_register_subdev_nodes(). Those device nodes will be automatically
removed when sub-devices are unregistered.

The device node handles a subset of the V4L2 API.

VIDIOC_QUERYCTRL
VIDIOC_QUERYMENU
VIDIOC_G_CTRL
VIDIOC_S_CTRL
VIDIOC_G_EXT_CTRLS
VIDIOC_S_EXT_CTRLS
VIDIOC_TRY_EXT_CTRLS

	The controls ioctls are identical to the ones defined in V4L2. They
	behave identically, with the only exception that they deal only with
	controls implemented in the sub-device. Depending on the driver, those
	controls can be also be accessed through one (or several) V4L2 device
	nodes.

VIDIOC_DQEVENT
VIDIOC_SUBSCRIBE_EVENT
VIDIOC_UNSUBSCRIBE_EVENT

	The events ioctls are identical to the ones defined in V4L2. They
	behave identically, with the only exception that they deal only with
	events generated by the sub-device. Depending on the driver, those
	events can also be reported by one (or several) V4L2 device nodes.

	Sub-device drivers that want to use events need to set the
	V4L2_SUBDEV_USES_EVENTS v4l2_subdev::flags and initialize
	v4l2_subdev::nevents to events queue depth before registering the
	sub-device. After registration events can be queued as usual on the
	v4l2_subdev::devnode device node.

	To properly support events, the poll() file operation is also
	implemented.

Private ioctls

	All ioctls not in the above list are passed directly to the sub-device
	driver through the core::ioctl operation.


I2C sub-device drivers
----------------------

Since these drivers are so common, special helper functions are available to
ease the use of these drivers (v4l2-common.h).

The recommended method of adding v4l2_subdev support to an I2C driver is to
embed the v4l2_subdev struct into the state struct that is created for each
I2C device instance. Very simple devices have no state struct and in that case
you can just create a v4l2_subdev directly.

A typical state struct would look like this (where 'chipname' is replaced by
the name of the chip):

struct chipname_state {
	struct v4l2_subdev sd;
	...  /* additional state fields */
};

Initialize the v4l2_subdev struct as follows:

	v4l2_i2c_subdev_init(&state->sd, client, subdev_ops);

This function will fill in all the fields of v4l2_subdev and ensure that the
v4l2_subdev and i2c_client both point to one another.

You should also add a helper inline function to go from a v4l2_subdev pointer
to a chipname_state struct:

static inline struct chipname_state *to_state(struct v4l2_subdev *sd)
{
	return container_of(sd, struct chipname_state, sd);
}

Use this to go from the v4l2_subdev struct to the i2c_client struct:

	struct i2c_client *client = v4l2_get_subdevdata(sd);

And this to go from an i2c_client to a v4l2_subdev struct:

	struct v4l2_subdev *sd = i2c_get_clientdata(client);

Make sure to call v4l2_device_unregister_subdev(sd) when the remove() callback
is called. This will unregister the sub-device from the bridge driver. It is
safe to call this even if the sub-device was never registered.

You need to do this because when the bridge driver destroys the i2c adapter
the remove() callbacks are called of the i2c devices on that adapter.
After that the corresponding v4l2_subdev structures are invalid, so they
have to be unregistered first. Calling v4l2_device_unregister_subdev(sd)
from the remove() callback ensures that this is always done correctly.


The bridge driver also has some helper functions it can use:

struct v4l2_subdev *sd = v4l2_i2c_new_subdev(v4l2_dev, adapter,
	       "module_foo", "chipid", 0x36, NULL);

This loads the given module (can be NULL if no module needs to be loaded) and
calls i2c_new_device() with the given i2c_adapter and chip/address arguments.
If all goes well, then it registers the subdev with the v4l2_device.

You can also use the last argument of v4l2_i2c_new_subdev() to pass an array
of possible I2C addresses that it should probe. These probe addresses are
only used if the previous argument is 0. A non-zero argument means that you
know the exact i2c address so in that case no probing will take place.

Both functions return NULL if something went wrong.

Note that the chipid you pass to v4l2_i2c_new_subdev() is usually
the same as the module name. It allows you to specify a chip variant, e.g.
"saa7114" or "saa7115". In general though the i2c driver autodetects this.
The use of chipid is something that needs to be looked at more closely at a
later date. It differs between i2c drivers and as such can be confusing.
To see which chip variants are supported you can look in the i2c driver code
for the i2c_device_id table. This lists all the possibilities.

There are two more helper functions:

v4l2_i2c_new_subdev_cfg: this function adds new irq and platform_data
arguments and has both 'addr' and 'probed_addrs' arguments: if addr is not
0 then that will be used (non-probing variant), otherwise the probed_addrs
are probed.

For example: this will probe for address 0x10:

struct v4l2_subdev *sd = v4l2_i2c_new_subdev_cfg(v4l2_dev, adapter,
	       "module_foo", "chipid", 0, NULL, 0, I2C_ADDRS(0x10));

v4l2_i2c_new_subdev_board uses an i2c_board_info struct which is passed
to the i2c driver and replaces the irq, platform_data and addr arguments.

If the subdev supports the s_config core ops, then that op is called with
the irq and platform_data arguments after the subdev was setup. The older
v4l2_i2c_new_(probed_)subdev functions will call s_config as well, but with
irq set to 0 and platform_data set to NULL.

struct video_device
-------------------

The actual device nodes in the /dev directory are created using the
video_device struct (v4l2-dev.h). This struct can either be allocated
dynamically or embedded in a larger struct.

To allocate it dynamically use:

	struct video_device *vdev = video_device_alloc();

	if (vdev == NULL)
		return -ENOMEM;

	vdev->release = video_device_release;

If you embed it in a larger struct, then you must set the release()
callback to your own function:

	struct video_device *vdev = &my_vdev->vdev;

	vdev->release = my_vdev_release;

The release callback must be set and it is called when the last user
of the video device exits.

The default video_device_release() callback just calls kfree to free the
allocated memory.

You should also set these fields:

- v4l2_dev: set to the v4l2_device parent device.

- name: set to something descriptive and unique.

- fops: set to the v4l2_file_operations struct.

- ioctl_ops: if you use the v4l2_ioctl_ops to simplify ioctl maintenance
  (highly recommended to use this and it might become compulsory in the
  future!), then set this to your v4l2_ioctl_ops struct.

- lock: leave to NULL if you want to do all the locking in the driver.
  Otherwise you give it a pointer to a struct mutex_lock and before any
  of the v4l2_file_operations is called this lock will be taken by the
  core and released afterwards. See the next section for more details.

- prio: keeps track of the priorities. Used to implement VIDIOC_G/S_PRIORITY.
  If left to NULL, then it will use the struct v4l2_prio_state in v4l2_device.
  If you want to have a separate priority state per (group of) device node(s),
  then you can point it to your own struct v4l2_prio_state.

- parent: you only set this if v4l2_device was registered with NULL as
  the parent device struct. This only happens in cases where one hardware
  device has multiple PCI devices that all share the same v4l2_device core.

  The cx88 driver is an example of this: one core v4l2_device struct, but
  it is used by both an raw video PCI device (cx8800) and a MPEG PCI device
  (cx8802). Since the v4l2_device cannot be associated with a particular
  PCI device it is setup without a parent device. But when the struct
  video_device is setup you do know which parent PCI device to use.

- flags: optional. Set to V4L2_FL_USE_FH_PRIO if you want to let the framework
  handle the VIDIOC_G/S_PRIORITY ioctls. This requires that you use struct
  v4l2_fh. Eventually this flag will disappear once all drivers use the core
  priority handling. But for now it has to be set explicitly.

If you use v4l2_ioctl_ops, then you should set .unlocked_ioctl to video_ioctl2
in your v4l2_file_operations struct.

Do not use .ioctl! This is deprecated and will go away in the future.

The v4l2_file_operations struct is a subset of file_operations. The main
difference is that the inode argument is omitted since it is never used.

If integration with the media framework is needed, you must initialize the
media_entity struct embedded in the video_device struct (entity field) by
calling media_entity_init():

	struct media_pad *pad = &my_vdev->pad;
	int err;

	err = media_entity_init(&vdev->entity, 1, pad, 0);

The pads array must have been previously initialized. There is no need to
manually set the struct media_entity type and name fields.

A reference to the entity will be automatically acquired/released when the
video device is opened/closed.

v4l2_file_operations and locking
--------------------------------

You can set a pointer to a mutex_lock in struct video_device. Usually this
will be either a top-level mutex or a mutex per device node. By default this
lock will be used for each file operation and ioctl, but you can disable
locking for selected ioctls by calling:

	void v4l2_dont_use_lock(struct video_device *vdev, unsigned int cmd);

E.g.: v4l2_dont_use_lock(vdev, VIDIOC_DQBUF);

You have to call this before you register the video_device.

Particularly with USB drivers where certain commands such as setting controls
can take a long time you may want to do your own locking for the buffer queuing
ioctls.

If you want still finer-grained locking then you have to set mutex_lock to NULL
and do you own locking completely.

It is up to the driver developer to decide which method to use. However, if
your driver has high-latency operations (for example, changing the exposure
of a USB webcam might take a long time), then you might be better off with
doing your own locking if you want to allow the user to do other things with
the device while waiting for the high-latency command to finish.

If a lock is specified then all file operations will be serialized on that
lock. If you use videobuf then you must pass the same lock to the videobuf
queue initialize function: if videobuf has to wait for a frame to arrive, then
it will temporarily unlock the lock and relock it afterwards. If your driver
also waits in the code, then you should do the same to allow other processes
to access the device node while the first process is waiting for something.

In the case of videobuf2 you will need to implement the wait_prepare and
wait_finish callbacks to unlock/lock if applicable. In particular, if you use
the lock in struct video_device then you must unlock/lock this mutex in
wait_prepare and wait_finish.

The implementation of a hotplug disconnect should also take the lock before
calling v4l2_device_disconnect.

video_device registration
-------------------------

Next you register the video device: this will create the character device
for you.

	err = video_register_device(vdev, VFL_TYPE_GRABBER, -1);
	if (err) {
		video_device_release(vdev); /* or kfree(my_vdev); */
		return err;
	}

If the v4l2_device parent device has a non-NULL mdev field, the video device
entity will be automatically registered with the media device.

Which device is registered depends on the type argument. The following
types exist:

VFL_TYPE_GRABBER: videoX for video input/output devices
VFL_TYPE_VBI: vbiX for vertical blank data (i.e. closed captions, teletext)
VFL_TYPE_RADIO: radioX for radio tuners

The last argument gives you a certain amount of control over the device
device node number used (i.e. the X in videoX). Normally you will pass -1
to let the v4l2 framework pick the first free number. But sometimes users
want to select a specific node number. It is common that drivers allow
the user to select a specific device node number through a driver module
option. That number is then passed to this function and video_register_device
will attempt to select that device node number. If that number was already
in use, then the next free device node number will be selected and it
will send a warning to the kernel log.

Another use-case is if a driver creates many devices. In that case it can
be useful to place different video devices in separate ranges. For example,
video capture devices start at 0, video output devices start at 16.
So you can use the last argument to specify a minimum device node number
and the v4l2 framework will try to pick the first free number that is equal
or higher to what you passed. If that fails, then it will just pick the
first free number.

Since in this case you do not care about a warning about not being able
to select the specified device node number, you can call the function
video_register_device_no_warn() instead.

Whenever a device node is created some attributes are also created for you.
If you look in /sys/class/video4linux you see the devices. Go into e.g.
video0 and you will see 'name' and 'index' attributes. The 'name' attribute
is the 'name' field of the video_device struct.

The 'index' attribute is the index of the device node: for each call to
video_register_device() the index is just increased by 1. The first video
device node you register always starts with index 0.

Users can setup udev rules that utilize the index attribute to make fancy
device names (e.g. 'mpegX' for MPEG video capture device nodes).

After the device was successfully registered, then you can use these fields:

- vfl_type: the device type passed to video_register_device.
- minor: the assigned device minor number.
- num: the device node number (i.e. the X in videoX).
- index: the device index number.

If the registration failed, then you need to call video_device_release()
to free the allocated video_device struct, or free your own struct if the
video_device was embedded in it. The vdev->release() callback will never
be called if the registration failed, nor should you ever attempt to
unregister the device if the registration failed.


video_device cleanup
--------------------

When the video device nodes have to be removed, either during the unload
of the driver or because the USB device was disconnected, then you should
unregister them:

	video_unregister_device(vdev);

This will remove the device nodes from sysfs (causing udev to remove them
from /dev).

After video_unregister_device() returns no new opens can be done. However,
in the case of USB devices some application might still have one of these
device nodes open. So after the unregister all file operations (except
release, of course) will return an error as well.

When the last user of the video device node exits, then the vdev->release()
callback is called and you can do the final cleanup there.

Don't forget to cleanup the media entity associated with the video device if
it has been initialized:

	media_entity_cleanup(&vdev->entity);

This can be done from the release callback.


video_device helper functions
-----------------------------

There are a few useful helper functions:

- file/video_device private data

You can set/get driver private data in the video_device struct using:

void *video_get_drvdata(struct video_device *vdev);
void video_set_drvdata(struct video_device *vdev, void *data);

Note that you can safely call video_set_drvdata() before calling
video_register_device().

And this function:

struct video_device *video_devdata(struct file *file);

returns the video_device belonging to the file struct.

The video_drvdata function combines video_get_drvdata with video_devdata:

void *video_drvdata(struct file *file);

You can go from a video_device struct to the v4l2_device struct using:

struct v4l2_device *v4l2_dev = vdev->v4l2_dev;

- Device node name

The video_device node kernel name can be retrieved using

const char *video_device_node_name(struct video_device *vdev);

The name is used as a hint by userspace tools such as udev. The function
should be used where possible instead of accessing the video_device::num and
video_device::minor fields.


video buffer helper functions
-----------------------------

The v4l2 core API provides a set of standard methods (called "videobuf")
for dealing with video buffers. Those methods allow a driver to implement
read(), mmap() and overlay() in a consistent way.  There are currently
methods for using video buffers on devices that supports DMA with
scatter/gather method (videobuf-dma-sg), DMA with linear access
(videobuf-dma-contig), and vmalloced buffers, mostly used on USB drivers
(videobuf-vmalloc).

Please see Documentation/video4linux/videobuf for more information on how
to use the videobuf layer.

struct v4l2_fh
--------------

struct v4l2_fh provides a way to easily keep file handle specific data
that is used by the V4L2 framework. New drivers must use struct v4l2_fh
since it is also used to implement priority handling (VIDIOC_G/S_PRIORITY)
if the video_device flag V4L2_FL_USE_FH_PRIO is also set.

The users of v4l2_fh (in the V4L2 framework, not the driver) know
whether a driver uses v4l2_fh as its file->private_data pointer by
testing the V4L2_FL_USES_V4L2_FH bit in video_device->flags. This bit is
set whenever v4l2_fh_init() is called.

struct v4l2_fh is allocated as a part of the driver's own file handle
structure and file->private_data is set to it in the driver's open
function by the driver.

In many cases the struct v4l2_fh will be embedded in a larger structure.
In that case you should call v4l2_fh_init+v4l2_fh_add in open() and
v4l2_fh_del+v4l2_fh_exit in release().

Drivers can extract their own file handle structure by using the container_of
macro. Example:

struct my_fh {
	int blah;
	struct v4l2_fh fh;
};

...

int my_open(struct file *file)
{
	struct my_fh *my_fh;
	struct video_device *vfd;
	int ret;

	...

	my_fh = kzalloc(sizeof(*my_fh), GFP_KERNEL);

	...

	v4l2_fh_init(&my_fh->fh, vfd);

	...

	file->private_data = &my_fh->fh;
	v4l2_fh_add(&my_fh->fh);
	return 0;
}

int my_release(struct file *file)
{
	struct v4l2_fh *fh = file->private_data;
	struct my_fh *my_fh = container_of(fh, struct my_fh, fh);

	...
	v4l2_fh_del(&my_fh->fh);
	v4l2_fh_exit(&my_fh->fh);
	kfree(my_fh);
	return 0;
}

Below is a short description of the v4l2_fh functions used:

void v4l2_fh_init(struct v4l2_fh *fh, struct video_device *vdev)

  Initialise the file handle. This *MUST* be performed in the driver's
  v4l2_file_operations->open() handler.

void v4l2_fh_add(struct v4l2_fh *fh)

  Add a v4l2_fh to video_device file handle list. Must be called once the
  file handle is completely initialized.

void v4l2_fh_del(struct v4l2_fh *fh)

  Unassociate the file handle from video_device(). The file handle
  exit function may now be called.

void v4l2_fh_exit(struct v4l2_fh *fh)

  Uninitialise the file handle. After uninitialisation the v4l2_fh
  memory can be freed.


If struct v4l2_fh is not embedded, then you can use these helper functions:

int v4l2_fh_open(struct file *filp)

  This allocates a struct v4l2_fh, initializes it and adds it to the struct
  video_device associated with the file struct.

int v4l2_fh_release(struct file *filp)

  This deletes it from the struct video_device associated with the file
  struct, uninitialised the v4l2_fh and frees it.

These two functions can be plugged into the v4l2_file_operation's open() and
release() ops.


Several drivers need to do something when the first file handle is opened and
when the last file handle closes. Two helper functions were added to check
whether the v4l2_fh struct is the only open filehandle of the associated
device node:

int v4l2_fh_is_singular(struct v4l2_fh *fh)

  Returns 1 if the file handle is the only open file handle, else 0.

int v4l2_fh_is_singular_file(struct file *filp)

  Same, but it calls v4l2_fh_is_singular with filp->private_data.


V4L2 events
-----------

The V4L2 events provide a generic way to pass events to user space.
The driver must use v4l2_fh to be able to support V4L2 events.

Events are defined by a type and an optional ID. The ID may refer to a V4L2
object such as a control ID. If unused, then the ID is 0.

When the user subscribes to an event the driver will allocate a number of
kevent structs for that event. So every (type, ID) event tuple will have
its own set of kevent structs. This guarantees that if a driver is generating
lots of events of one type in a short time, then that will not overwrite
events of another type.

But if you get more events of one type than the number of kevents that were
reserved, then the oldest event will be dropped and the new one added.

Furthermore, the internal struct v4l2_subscribed_event has merge() and
replace() callbacks which drivers can set. These callbacks are called when
a new event is raised and there is no more room. The replace() callback
allows you to replace the payload of the old event with that of the new event,
merging any relevant data from the old payload into the new payload that
replaces it. It is called when this event type has only one kevent struct
allocated. The merge() callback allows you to merge the oldest event payload
into that of the second-oldest event payload. It is called when there are two
or more kevent structs allocated.

This way no status information is lost, just the intermediate steps leading
up to that state.

A good example of these replace/merge callbacks is in v4l2-event.c:
ctrls_replace() and ctrls_merge() callbacks for the control event.

Note: these callbacks can be called from interrupt context, so they must be
fast.

Useful functions:

void v4l2_event_queue(struct video_device *vdev, const struct v4l2_event *ev)

  Queue events to video device. The driver's only responsibility is to fill
  in the type and the data fields. The other fields will be filled in by
  V4L2.

int v4l2_event_subscribe(struct v4l2_fh *fh,
			 struct v4l2_event_subscription *sub, unsigned elems,
			 const struct v4l2_subscribed_event_ops *ops)

  The video_device->ioctl_ops->vidioc_subscribe_event must check the driver
  is able to produce events with specified event id. Then it calls
  v4l2_event_subscribe() to subscribe the event.

  The elems argument is the size of the event queue for this event. If it is 0,
  then the framework will fill in a default value (this depends on the event
  type).

  The ops argument allows the driver to specify a number of callbacks:
  * add:     called when a new listener gets added (subscribing to the same
             event twice will only cause this callback to get called once)
  * del:     called when a listener stops listening
  * replace: replace event 'old' with event 'new'.
  * merge:   merge event 'old' into event 'new'.
  All 4 callbacks are optional, if you don't want to specify any callbacks
  the ops argument itself maybe NULL.

int v4l2_event_unsubscribe(struct v4l2_fh *fh,
			   struct v4l2_event_subscription *sub)

  vidioc_unsubscribe_event in struct v4l2_ioctl_ops. A driver may use
  v4l2_event_unsubscribe() directly unless it wants to be involved in
  unsubscription process.

  The special type V4L2_EVENT_ALL may be used to unsubscribe all events. The
  drivers may want to handle this in a special way.

int v4l2_event_pending(struct v4l2_fh *fh)

  Returns the number of pending events. Useful when implementing poll.

Events are delivered to user space through the poll system call. The driver
can use v4l2_fh->wait (a wait_queue_head_t) as the argument for poll_wait().

There are standard and private events. New standard events must use the
smallest available event type. The drivers must allocate their events from
their own class starting from class base. Class base is
V4L2_EVENT_PRIVATE_START + n * 1000 where n is the lowest available number.
The first event type in the class is reserved for future use, so the first
available event type is 'class base + 1'.

An example on how the V4L2 events may be used can be found in the OMAP
3 ISP driver (drivers/media/video/omap3isp).