Root/Documentation/video4linux/v4l2-framework.txt

1Overview of the V4L2 driver framework
2=====================================
3
4This text documents the various structures provided by the V4L2 framework and
5their relationships.
6
7
8Introduction
9------------
10
11The V4L2 drivers tend to be very complex due to the complexity of the
12hardware: 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
16Especially the fact that V4L2 drivers have to setup supporting ICs to
17do audio/video muxing/encoding/decoding makes it more complex than most.
18Usually these ICs are connected to the main bridge driver through one or
19more I2C busses, but other busses can also be used. Such devices are
20called 'sub-devices'.
21
22For a long time the framework was limited to the video_device struct for
23creating V4L device nodes and video_buf for handling the video buffers
24(note that this document does not discuss the video_buf framework).
25
26This meant that all drivers had to do the setup of device instances and
27connecting to sub-devices themselves. Some of this is quite complicated
28to do right and many drivers never did do it correctly.
29
30There is also a lot of common code that could never be refactored due to
31the lack of a framework.
32
33So this framework sets up the basic building blocks that all drivers
34need and this same framework should make it much easier to refactor
35common code into utility functions shared by all drivers.
36
37
38Structure of a driver
39---------------------
40
41All drivers have the following structure:
42
431) A struct for each device instance containing the device state.
44
452) A way of initializing and commanding sub-devices (if any).
46
473) Creating V4L2 device nodes (/dev/videoX, /dev/vbiX, /dev/radioX and
48   /dev/vtxX) and keeping track of device-node specific data.
49
504) Filehandle-specific structs containing per-filehandle data;
51
525) video buffer handling.
53
54This is a rough schematic of how it all relates:
55
56    device instances
57      |
58      +-sub-device instances
59      |
60      \-V4L2 device nodes
61      |
62      \-filehandle instances
63
64
65Structure of the framework
66--------------------------
67
68The framework closely resembles the driver structure: it has a v4l2_device
69struct for the device instance data, a v4l2_subdev struct to refer to
70sub-device instances, the video_device struct stores V4L2 device node data
71and in the future a v4l2_fh struct will keep track of filehandle instances
72(this is not yet implemented).
73
74
75struct v4l2_device
76------------------
77
78Each device instance is represented by a struct v4l2_device (v4l2-device.h).
79Very simple devices can just allocate this struct, but most of the time you
80would embed this struct inside a larger struct.
81
82You must register the device instance:
83
84    v4l2_device_register(struct device *dev, struct v4l2_device *v4l2_dev);
85
86Registration will initialize the v4l2_device struct and link dev->driver_data
87to v4l2_dev. If v4l2_dev->name is empty then it will be set to a value derived
88from dev (driver name followed by the bus_id, to be precise). If you set it
89up before calling v4l2_device_register then it will be untouched. If dev is
90NULL, then you *must* setup v4l2_dev->name before calling v4l2_device_register.
91
92You can use v4l2_device_set_name() to set the name based on a driver name and
93a driver-global atomic_t instance. This will generate names like ivtv0, ivtv1,
94etc. If the name ends with a digit, then it will insert a dash: cx18-0,
95cx18-1, etc. This function returns the instance number.
96
97The first 'dev' argument is normally the struct device pointer of a pci_dev,
98usb_interface or platform_device. It is rare for dev to be NULL, but it happens
99with ISA devices or when one device creates multiple PCI devices, thus making
100it impossible to associate v4l2_dev with a particular parent.
101
102You can also supply a notify() callback that can be called by sub-devices to
103notify you of events. Whether you need to set this depends on the sub-device.
104Any notifications a sub-device supports must be defined in a header in
105include/media/<subdevice>.h.
106
107You unregister with:
108
109    v4l2_device_unregister(struct v4l2_device *v4l2_dev);
110
111Unregistering will also automatically unregister all subdevs from the device.
112
113If you have a hotpluggable device (e.g. a USB device), then when a disconnect
114happens the parent device becomes invalid. Since v4l2_device has a pointer to
115that parent device it has to be cleared as well to mark that the parent is
116gone. To do this call:
117
118    v4l2_device_disconnect(struct v4l2_device *v4l2_dev);
119
120This does *not* unregister the subdevs, so you still need to call the
121v4l2_device_unregister() function for that. If your driver is not hotpluggable,
122then there is no need to call v4l2_device_disconnect().
123
124Sometimes you need to iterate over all devices registered by a specific
125driver. This is usually the case if multiple device drivers use the same
126hardware. E.g. the ivtvfb driver is a framebuffer driver that uses the ivtv
127hardware. The same is true for alsa drivers for example.
128
129You can iterate over all registered devices as follows:
130
131static int callback(struct device *dev, void *p)
132{
133    struct v4l2_device *v4l2_dev = dev_get_drvdata(dev);
134
135    /* test if this device was inited */
136    if (v4l2_dev == NULL)
137        return 0;
138    ...
139    return 0;
140}
141
142int iterate(void *p)
143{
144    struct device_driver *drv;
145    int err;
146
147    /* Find driver 'ivtv' on the PCI bus.
148       pci_bus_type is a global. For USB busses use usb_bus_type. */
149    drv = driver_find("ivtv", &pci_bus_type);
150    /* iterate over all ivtv device instances */
151    err = driver_for_each_device(drv, NULL, p, callback);
152    put_driver(drv);
153    return err;
154}
155
156Sometimes you need to keep a running counter of the device instance. This is
157commonly used to map a device instance to an index of a module option array.
158
159The recommended approach is as follows:
160
161static atomic_t drv_instance = ATOMIC_INIT(0);
162
163static int __devinit drv_probe(struct pci_dev *pdev,
164                const struct pci_device_id *pci_id)
165{
166    ...
167    state->instance = atomic_inc_return(&drv_instance) - 1;
168}
169
170
171struct v4l2_subdev
172------------------
173
174Many drivers need to communicate with sub-devices. These devices can do all
175sort of tasks, but most commonly they handle audio and/or video muxing,
176encoding or decoding. For webcams common sub-devices are sensors and camera
177controllers.
178
179Usually these are I2C devices, but not necessarily. In order to provide the
180driver with a consistent interface to these sub-devices the v4l2_subdev struct
181(v4l2-subdev.h) was created.
182
183Each sub-device driver must have a v4l2_subdev struct. This struct can be
184stand-alone for simple sub-devices or it might be embedded in a larger struct
185if more state information needs to be stored. Usually there is a low-level
186device struct (e.g. i2c_client) that contains the device data as setup
187by the kernel. It is recommended to store that pointer in the private
188data of v4l2_subdev using v4l2_set_subdevdata(). That makes it easy to go
189from a v4l2_subdev to the actual low-level bus-specific device data.
190
191You also need a way to go from the low-level struct to v4l2_subdev. For the
192common i2c_client struct the i2c_set_clientdata() call is used to store a
193v4l2_subdev pointer, for other busses you may have to use other methods.
194
195From the bridge driver perspective you load the sub-device module and somehow
196obtain the v4l2_subdev pointer. For i2c devices this is easy: you call
197i2c_get_clientdata(). For other busses something similar needs to be done.
198Helper functions exists for sub-devices on an I2C bus that do most of this
199tricky work for you.
200
201Each v4l2_subdev contains function pointers that sub-device drivers can
202implement (or leave NULL if it is not applicable). Since sub-devices can do
203so many different things and you do not want to end up with a huge ops struct
204of which only a handful of ops are commonly implemented, the function pointers
205are sorted according to category and each category has its own ops struct.
206
207The top-level ops struct contains pointers to the category ops structs, which
208may be NULL if the subdev driver does not support anything from that category.
209
210It looks like this:
211
212struct v4l2_subdev_core_ops {
213    int (*g_chip_ident)(struct v4l2_subdev *sd, struct v4l2_dbg_chip_ident *chip);
214    int (*log_status)(struct v4l2_subdev *sd);
215    int (*init)(struct v4l2_subdev *sd, u32 val);
216    ...
217};
218
219struct v4l2_subdev_tuner_ops {
220    ...
221};
222
223struct v4l2_subdev_audio_ops {
224    ...
225};
226
227struct v4l2_subdev_video_ops {
228    ...
229};
230
231struct v4l2_subdev_ops {
232    const struct v4l2_subdev_core_ops *core;
233    const struct v4l2_subdev_tuner_ops *tuner;
234    const struct v4l2_subdev_audio_ops *audio;
235    const struct v4l2_subdev_video_ops *video;
236};
237
238The core ops are common to all subdevs, the other categories are implemented
239depending on the sub-device. E.g. a video device is unlikely to support the
240audio ops and vice versa.
241
242This setup limits the number of function pointers while still making it easy
243to add new ops and categories.
244
245A sub-device driver initializes the v4l2_subdev struct using:
246
247    v4l2_subdev_init(sd, &ops);
248
249Afterwards you need to initialize subdev->name with a unique name and set the
250module owner. This is done for you if you use the i2c helper functions.
251
252A device (bridge) driver needs to register the v4l2_subdev with the
253v4l2_device:
254
255    int err = v4l2_device_register_subdev(v4l2_dev, sd);
256
257This can fail if the subdev module disappeared before it could be registered.
258After this function was called successfully the subdev->dev field points to
259the v4l2_device.
260
261You can unregister a sub-device using:
262
263    v4l2_device_unregister_subdev(sd);
264
265Afterwards the subdev module can be unloaded and sd->dev == NULL.
266
267You can call an ops function either directly:
268
269    err = sd->ops->core->g_chip_ident(sd, &chip);
270
271but it is better and easier to use this macro:
272
273    err = v4l2_subdev_call(sd, core, g_chip_ident, &chip);
274
275The macro will to the right NULL pointer checks and returns -ENODEV if subdev
276is NULL, -ENOIOCTLCMD if either subdev->core or subdev->core->g_chip_ident is
277NULL, or the actual result of the subdev->ops->core->g_chip_ident ops.
278
279It is also possible to call all or a subset of the sub-devices:
280
281    v4l2_device_call_all(v4l2_dev, 0, core, g_chip_ident, &chip);
282
283Any subdev that does not support this ops is skipped and error results are
284ignored. If you want to check for errors use this:
285
286    err = v4l2_device_call_until_err(v4l2_dev, 0, core, g_chip_ident, &chip);
287
288Any error except -ENOIOCTLCMD will exit the loop with that error. If no
289errors (except -ENOIOCTLCMD) occured, then 0 is returned.
290
291The second argument to both calls is a group ID. If 0, then all subdevs are
292called. If non-zero, then only those whose group ID match that value will
293be called. Before a bridge driver registers a subdev it can set sd->grp_id
294to whatever value it wants (it's 0 by default). This value is owned by the
295bridge driver and the sub-device driver will never modify or use it.
296
297The group ID gives the bridge driver more control how callbacks are called.
298For example, there may be multiple audio chips on a board, each capable of
299changing the volume. But usually only one will actually be used when the
300user want to change the volume. You can set the group ID for that subdev to
301e.g. AUDIO_CONTROLLER and specify that as the group ID value when calling
302v4l2_device_call_all(). That ensures that it will only go to the subdev
303that needs it.
304
305If the sub-device needs to notify its v4l2_device parent of an event, then
306it can call v4l2_subdev_notify(sd, notification, arg). This macro checks
307whether there is a notify() callback defined and returns -ENODEV if not.
308Otherwise the result of the notify() call is returned.
309
310The advantage of using v4l2_subdev is that it is a generic struct and does
311not contain any knowledge about the underlying hardware. So a driver might
312contain several subdevs that use an I2C bus, but also a subdev that is
313controlled through GPIO pins. This distinction is only relevant when setting
314up the device, but once the subdev is registered it is completely transparent.
315
316
317I2C sub-device drivers
318----------------------
319
320Since these drivers are so common, special helper functions are available to
321ease the use of these drivers (v4l2-common.h).
322
323The recommended method of adding v4l2_subdev support to an I2C driver is to
324embed the v4l2_subdev struct into the state struct that is created for each
325I2C device instance. Very simple devices have no state struct and in that case
326you can just create a v4l2_subdev directly.
327
328A typical state struct would look like this (where 'chipname' is replaced by
329the name of the chip):
330
331struct chipname_state {
332    struct v4l2_subdev sd;
333    ... /* additional state fields */
334};
335
336Initialize the v4l2_subdev struct as follows:
337
338    v4l2_i2c_subdev_init(&state->sd, client, subdev_ops);
339
340This function will fill in all the fields of v4l2_subdev and ensure that the
341v4l2_subdev and i2c_client both point to one another.
342
343You should also add a helper inline function to go from a v4l2_subdev pointer
344to a chipname_state struct:
345
346static inline struct chipname_state *to_state(struct v4l2_subdev *sd)
347{
348    return container_of(sd, struct chipname_state, sd);
349}
350
351Use this to go from the v4l2_subdev struct to the i2c_client struct:
352
353    struct i2c_client *client = v4l2_get_subdevdata(sd);
354
355And this to go from an i2c_client to a v4l2_subdev struct:
356
357    struct v4l2_subdev *sd = i2c_get_clientdata(client);
358
359Make sure to call v4l2_device_unregister_subdev(sd) when the remove() callback
360is called. This will unregister the sub-device from the bridge driver. It is
361safe to call this even if the sub-device was never registered.
362
363You need to do this because when the bridge driver destroys the i2c adapter
364the remove() callbacks are called of the i2c devices on that adapter.
365After that the corresponding v4l2_subdev structures are invalid, so they
366have to be unregistered first. Calling v4l2_device_unregister_subdev(sd)
367from the remove() callback ensures that this is always done correctly.
368
369
370The bridge driver also has some helper functions it can use:
371
372struct v4l2_subdev *sd = v4l2_i2c_new_subdev(v4l2_dev, adapter,
373           "module_foo", "chipid", 0x36, NULL);
374
375This loads the given module (can be NULL if no module needs to be loaded) and
376calls i2c_new_device() with the given i2c_adapter and chip/address arguments.
377If all goes well, then it registers the subdev with the v4l2_device.
378
379You can also use the last argument of v4l2_i2c_new_subdev() to pass an array
380of possible I2C addresses that it should probe. These probe addresses are
381only used if the previous argument is 0. A non-zero argument means that you
382know the exact i2c address so in that case no probing will take place.
383
384Both functions return NULL if something went wrong.
385
386Note that the chipid you pass to v4l2_i2c_new_subdev() is usually
387the same as the module name. It allows you to specify a chip variant, e.g.
388"saa7114" or "saa7115". In general though the i2c driver autodetects this.
389The use of chipid is something that needs to be looked at more closely at a
390later date. It differs between i2c drivers and as such can be confusing.
391To see which chip variants are supported you can look in the i2c driver code
392for the i2c_device_id table. This lists all the possibilities.
393
394There are two more helper functions:
395
396v4l2_i2c_new_subdev_cfg: this function adds new irq and platform_data
397arguments and has both 'addr' and 'probed_addrs' arguments: if addr is not
3980 then that will be used (non-probing variant), otherwise the probed_addrs
399are probed.
400
401For example: this will probe for address 0x10:
402
403struct v4l2_subdev *sd = v4l2_i2c_new_subdev_cfg(v4l2_dev, adapter,
404           "module_foo", "chipid", 0, NULL, 0, I2C_ADDRS(0x10));
405
406v4l2_i2c_new_subdev_board uses an i2c_board_info struct which is passed
407to the i2c driver and replaces the irq, platform_data and addr arguments.
408
409If the subdev supports the s_config core ops, then that op is called with
410the irq and platform_data arguments after the subdev was setup. The older
411v4l2_i2c_new_(probed_)subdev functions will call s_config as well, but with
412irq set to 0 and platform_data set to NULL.
413
414struct video_device
415-------------------
416
417The actual device nodes in the /dev directory are created using the
418video_device struct (v4l2-dev.h). This struct can either be allocated
419dynamically or embedded in a larger struct.
420
421To allocate it dynamically use:
422
423    struct video_device *vdev = video_device_alloc();
424
425    if (vdev == NULL)
426        return -ENOMEM;
427
428    vdev->release = video_device_release;
429
430If you embed it in a larger struct, then you must set the release()
431callback to your own function:
432
433    struct video_device *vdev = &my_vdev->vdev;
434
435    vdev->release = my_vdev_release;
436
437The release callback must be set and it is called when the last user
438of the video device exits.
439
440The default video_device_release() callback just calls kfree to free the
441allocated memory.
442
443You should also set these fields:
444
445- v4l2_dev: set to the v4l2_device parent device.
446- name: set to something descriptive and unique.
447- fops: set to the v4l2_file_operations struct.
448- ioctl_ops: if you use the v4l2_ioctl_ops to simplify ioctl maintenance
449  (highly recommended to use this and it might become compulsory in the
450  future!), then set this to your v4l2_ioctl_ops struct.
451- parent: you only set this if v4l2_device was registered with NULL as
452  the parent device struct. This only happens in cases where one hardware
453  device has multiple PCI devices that all share the same v4l2_device core.
454
455  The cx88 driver is an example of this: one core v4l2_device struct, but
456  it is used by both an raw video PCI device (cx8800) and a MPEG PCI device
457  (cx8802). Since the v4l2_device cannot be associated with a particular
458  PCI device it is setup without a parent device. But when the struct
459  video_device is setup you do know which parent PCI device to use.
460
461If you use v4l2_ioctl_ops, then you should set either .unlocked_ioctl or
462.ioctl to video_ioctl2 in your v4l2_file_operations struct.
463
464The v4l2_file_operations struct is a subset of file_operations. The main
465difference is that the inode argument is omitted since it is never used.
466
467
468video_device registration
469-------------------------
470
471Next you register the video device: this will create the character device
472for you.
473
474    err = video_register_device(vdev, VFL_TYPE_GRABBER, -1);
475    if (err) {
476        video_device_release(vdev); /* or kfree(my_vdev); */
477        return err;
478    }
479
480Which device is registered depends on the type argument. The following
481types exist:
482
483VFL_TYPE_GRABBER: videoX for video input/output devices
484VFL_TYPE_VBI: vbiX for vertical blank data (i.e. closed captions, teletext)
485VFL_TYPE_RADIO: radioX for radio tuners
486VFL_TYPE_VTX: vtxX for teletext devices (deprecated, don't use)
487
488The last argument gives you a certain amount of control over the device
489device node number used (i.e. the X in videoX). Normally you will pass -1
490to let the v4l2 framework pick the first free number. But sometimes users
491want to select a specific node number. It is common that drivers allow
492the user to select a specific device node number through a driver module
493option. That number is then passed to this function and video_register_device
494will attempt to select that device node number. If that number was already
495in use, then the next free device node number will be selected and it
496will send a warning to the kernel log.
497
498Another use-case is if a driver creates many devices. In that case it can
499be useful to place different video devices in separate ranges. For example,
500video capture devices start at 0, video output devices start at 16.
501So you can use the last argument to specify a minimum device node number
502and the v4l2 framework will try to pick the first free number that is equal
503or higher to what you passed. If that fails, then it will just pick the
504first free number.
505
506Since in this case you do not care about a warning about not being able
507to select the specified device node number, you can call the function
508video_register_device_no_warn() instead.
509
510Whenever a device node is created some attributes are also created for you.
511If you look in /sys/class/video4linux you see the devices. Go into e.g.
512video0 and you will see 'name' and 'index' attributes. The 'name' attribute
513is the 'name' field of the video_device struct.
514
515The 'index' attribute is the index of the device node: for each call to
516video_register_device() the index is just increased by 1. The first video
517device node you register always starts with index 0.
518
519Users can setup udev rules that utilize the index attribute to make fancy
520device names (e.g. 'mpegX' for MPEG video capture device nodes).
521
522After the device was successfully registered, then you can use these fields:
523
524- vfl_type: the device type passed to video_register_device.
525- minor: the assigned device minor number.
526- num: the device node number (i.e. the X in videoX).
527- index: the device index number.
528
529If the registration failed, then you need to call video_device_release()
530to free the allocated video_device struct, or free your own struct if the
531video_device was embedded in it. The vdev->release() callback will never
532be called if the registration failed, nor should you ever attempt to
533unregister the device if the registration failed.
534
535
536video_device cleanup
537--------------------
538
539When the video device nodes have to be removed, either during the unload
540of the driver or because the USB device was disconnected, then you should
541unregister them:
542
543    video_unregister_device(vdev);
544
545This will remove the device nodes from sysfs (causing udev to remove them
546from /dev).
547
548After video_unregister_device() returns no new opens can be done.
549
550However, in the case of USB devices some application might still have one
551of these device nodes open. You should block all new accesses to read,
552write, poll, etc. except possibly for certain ioctl operations like
553queueing buffers.
554
555When the last user of the video device node exits, then the vdev->release()
556callback is called and you can do the final cleanup there.
557
558
559video_device helper functions
560-----------------------------
561
562There are a few useful helper functions:
563
564- file/video_device private data
565
566You can set/get driver private data in the video_device struct using:
567
568void *video_get_drvdata(struct video_device *vdev);
569void video_set_drvdata(struct video_device *vdev, void *data);
570
571Note that you can safely call video_set_drvdata() before calling
572video_register_device().
573
574And this function:
575
576struct video_device *video_devdata(struct file *file);
577
578returns the video_device belonging to the file struct.
579
580The video_drvdata function combines video_get_drvdata with video_devdata:
581
582void *video_drvdata(struct file *file);
583
584You can go from a video_device struct to the v4l2_device struct using:
585
586struct v4l2_device *v4l2_dev = vdev->v4l2_dev;
587
588- Device node name
589
590The video_device node kernel name can be retrieved using
591
592const char *video_device_node_name(struct video_device *vdev);
593
594The name is used as a hint by userspace tools such as udev. The function
595should be used where possible instead of accessing the video_device::num and
596video_device::minor fields.
597
598
599video buffer helper functions
600-----------------------------
601
602The v4l2 core API provides a set of standard methods (called "videobuf")
603for dealing with video buffers. Those methods allow a driver to implement
604read(), mmap() and overlay() in a consistent way. There are currently
605methods for using video buffers on devices that supports DMA with
606scatter/gather method (videobuf-dma-sg), DMA with linear access
607(videobuf-dma-contig), and vmalloced buffers, mostly used on USB drivers
608(videobuf-vmalloc).
609
610Please see Documentation/video4linux/videobuf for more information on how
611to use the videobuf layer.
612

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