linux/Documentation/video4linux/v4l2-controls.txt
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   1Introduction
   2============
   3
   4The V4L2 control API seems simple enough, but quickly becomes very hard to
   5implement correctly in drivers. But much of the code needed to handle controls
   6is actually not driver specific and can be moved to the V4L core framework.
   7
   8After all, the only part that a driver developer is interested in is:
   9
  101) How do I add a control?
  112) How do I set the control's value? (i.e. s_ctrl)
  12
  13And occasionally:
  14
  153) How do I get the control's value? (i.e. g_volatile_ctrl)
  164) How do I validate the user's proposed control value? (i.e. try_ctrl)
  17
  18All the rest is something that can be done centrally.
  19
  20The control framework was created in order to implement all the rules of the
  21V4L2 specification with respect to controls in a central place. And to make
  22life as easy as possible for the driver developer.
  23
  24Note that the control framework relies on the presence of a struct v4l2_device
  25for V4L2 drivers and struct v4l2_subdev for sub-device drivers.
  26
  27
  28Objects in the framework
  29========================
  30
  31There are two main objects:
  32
  33The v4l2_ctrl object describes the control properties and keeps track of the
  34control's value (both the current value and the proposed new value).
  35
  36v4l2_ctrl_handler is the object that keeps track of controls. It maintains a
  37list of v4l2_ctrl objects that it owns and another list of references to
  38controls, possibly to controls owned by other handlers.
  39
  40
  41Basic usage for V4L2 and sub-device drivers
  42===========================================
  43
  441) Prepare the driver:
  45
  461.1) Add the handler to your driver's top-level struct:
  47
  48        struct foo_dev {
  49                ...
  50                struct v4l2_ctrl_handler ctrl_handler;
  51                ...
  52        };
  53
  54        struct foo_dev *foo;
  55
  561.2) Initialize the handler:
  57
  58        v4l2_ctrl_handler_init(&foo->ctrl_handler, nr_of_controls);
  59
  60  The second argument is a hint telling the function how many controls this
  61  handler is expected to handle. It will allocate a hashtable based on this
  62  information. It is a hint only.
  63
  641.3) Hook the control handler into the driver:
  65
  661.3.1) For V4L2 drivers do this:
  67
  68        struct foo_dev {
  69                ...
  70                struct v4l2_device v4l2_dev;
  71                ...
  72                struct v4l2_ctrl_handler ctrl_handler;
  73                ...
  74        };
  75
  76        foo->v4l2_dev.ctrl_handler = &foo->ctrl_handler;
  77
  78  Where foo->v4l2_dev is of type struct v4l2_device.
  79
  80  Finally, remove all control functions from your v4l2_ioctl_ops:
  81  vidioc_queryctrl, vidioc_querymenu, vidioc_g_ctrl, vidioc_s_ctrl,
  82  vidioc_g_ext_ctrls, vidioc_try_ext_ctrls and vidioc_s_ext_ctrls.
  83  Those are now no longer needed.
  84
  851.3.2) For sub-device drivers do this:
  86
  87        struct foo_dev {
  88                ...
  89                struct v4l2_subdev sd;
  90                ...
  91                struct v4l2_ctrl_handler ctrl_handler;
  92                ...
  93        };
  94
  95        foo->sd.ctrl_handler = &foo->ctrl_handler;
  96
  97  Where foo->sd is of type struct v4l2_subdev.
  98
  99  And set all core control ops in your struct v4l2_subdev_core_ops to these
 100  helpers:
 101
 102        .queryctrl = v4l2_subdev_queryctrl,
 103        .querymenu = v4l2_subdev_querymenu,
 104        .g_ctrl = v4l2_subdev_g_ctrl,
 105        .s_ctrl = v4l2_subdev_s_ctrl,
 106        .g_ext_ctrls = v4l2_subdev_g_ext_ctrls,
 107        .try_ext_ctrls = v4l2_subdev_try_ext_ctrls,
 108        .s_ext_ctrls = v4l2_subdev_s_ext_ctrls,
 109
 110  Note: this is a temporary solution only. Once all V4L2 drivers that depend
 111  on subdev drivers are converted to the control framework these helpers will
 112  no longer be needed.
 113
 1141.4) Clean up the handler at the end:
 115
 116        v4l2_ctrl_handler_free(&foo->ctrl_handler);
 117
 118
 1192) Add controls:
 120
 121You add non-menu controls by calling v4l2_ctrl_new_std:
 122
 123        struct v4l2_ctrl *v4l2_ctrl_new_std(struct v4l2_ctrl_handler *hdl,
 124                        const struct v4l2_ctrl_ops *ops,
 125                        u32 id, s32 min, s32 max, u32 step, s32 def);
 126
 127Menu controls are added by calling v4l2_ctrl_new_std_menu:
 128
 129        struct v4l2_ctrl *v4l2_ctrl_new_std_menu(struct v4l2_ctrl_handler *hdl,
 130                        const struct v4l2_ctrl_ops *ops,
 131                        u32 id, s32 max, s32 skip_mask, s32 def);
 132
 133Or alternatively for integer menu controls, by calling v4l2_ctrl_new_int_menu:
 134
 135        struct v4l2_ctrl *v4l2_ctrl_new_int_menu(struct v4l2_ctrl_handler *hdl,
 136                        const struct v4l2_ctrl_ops *ops,
 137                        u32 id, s32 max, s32 def, const s64 *qmenu_int);
 138
 139Standard menu controls with a driver specific menu are added by calling
 140v4l2_ctrl_new_std_menu_items:
 141
 142        struct v4l2_ctrl *v4l2_ctrl_new_std_menu_items(
 143                struct v4l2_ctrl_handler *hdl,
 144                const struct v4l2_ctrl_ops *ops, u32 id, s32 max,
 145                s32 skip_mask, s32 def, const char * const *qmenu);
 146
 147These functions are typically called right after the v4l2_ctrl_handler_init:
 148
 149        static const s64 exp_bias_qmenu[] = {
 150               -2, -1, 0, 1, 2
 151        };
 152        static const char * const test_pattern[] = {
 153                "Disabled",
 154                "Vertical Bars",
 155                "Solid Black",
 156                "Solid White",
 157        };
 158
 159        v4l2_ctrl_handler_init(&foo->ctrl_handler, nr_of_controls);
 160        v4l2_ctrl_new_std(&foo->ctrl_handler, &foo_ctrl_ops,
 161                        V4L2_CID_BRIGHTNESS, 0, 255, 1, 128);
 162        v4l2_ctrl_new_std(&foo->ctrl_handler, &foo_ctrl_ops,
 163                        V4L2_CID_CONTRAST, 0, 255, 1, 128);
 164        v4l2_ctrl_new_std_menu(&foo->ctrl_handler, &foo_ctrl_ops,
 165                        V4L2_CID_POWER_LINE_FREQUENCY,
 166                        V4L2_CID_POWER_LINE_FREQUENCY_60HZ, 0,
 167                        V4L2_CID_POWER_LINE_FREQUENCY_DISABLED);
 168        v4l2_ctrl_new_int_menu(&foo->ctrl_handler, &foo_ctrl_ops,
 169                        V4L2_CID_EXPOSURE_BIAS,
 170                        ARRAY_SIZE(exp_bias_qmenu) - 1,
 171                        ARRAY_SIZE(exp_bias_qmenu) / 2 - 1,
 172                        exp_bias_qmenu);
 173        v4l2_ctrl_new_std_menu_items(&foo->ctrl_handler, &foo_ctrl_ops,
 174                        V4L2_CID_TEST_PATTERN, ARRAY_SIZE(test_pattern) - 1, 0,
 175                        0, test_pattern);
 176        ...
 177        if (foo->ctrl_handler.error) {
 178                int err = foo->ctrl_handler.error;
 179
 180                v4l2_ctrl_handler_free(&foo->ctrl_handler);
 181                return err;
 182        }
 183
 184The v4l2_ctrl_new_std function returns the v4l2_ctrl pointer to the new
 185control, but if you do not need to access the pointer outside the control ops,
 186then there is no need to store it.
 187
 188The v4l2_ctrl_new_std function will fill in most fields based on the control
 189ID except for the min, max, step and default values. These are passed in the
 190last four arguments. These values are driver specific while control attributes
 191like type, name, flags are all global. The control's current value will be set
 192to the default value.
 193
 194The v4l2_ctrl_new_std_menu function is very similar but it is used for menu
 195controls. There is no min argument since that is always 0 for menu controls,
 196and instead of a step there is a skip_mask argument: if bit X is 1, then menu
 197item X is skipped.
 198
 199The v4l2_ctrl_new_int_menu function creates a new standard integer menu
 200control with driver-specific items in the menu. It differs from
 201v4l2_ctrl_new_std_menu in that it doesn't have the mask argument and takes
 202as the last argument an array of signed 64-bit integers that form an exact
 203menu item list.
 204
 205The v4l2_ctrl_new_std_menu_items function is very similar to
 206v4l2_ctrl_new_std_menu but takes an extra parameter qmenu, which is the driver
 207specific menu for an otherwise standard menu control. A good example for this
 208control is the test pattern control for capture/display/sensors devices that
 209have the capability to generate test patterns. These test patterns are hardware
 210specific, so the contents of the menu will vary from device to device.
 211
 212Note that if something fails, the function will return NULL or an error and
 213set ctrl_handler->error to the error code. If ctrl_handler->error was already
 214set, then it will just return and do nothing. This is also true for
 215v4l2_ctrl_handler_init if it cannot allocate the internal data structure.
 216
 217This makes it easy to init the handler and just add all controls and only check
 218the error code at the end. Saves a lot of repetitive error checking.
 219
 220It is recommended to add controls in ascending control ID order: it will be
 221a bit faster that way.
 222
 2233) Optionally force initial control setup:
 224
 225        v4l2_ctrl_handler_setup(&foo->ctrl_handler);
 226
 227This will call s_ctrl for all controls unconditionally. Effectively this
 228initializes the hardware to the default control values. It is recommended
 229that you do this as this ensures that both the internal data structures and
 230the hardware are in sync.
 231
 2324) Finally: implement the v4l2_ctrl_ops
 233
 234        static const struct v4l2_ctrl_ops foo_ctrl_ops = {
 235                .s_ctrl = foo_s_ctrl,
 236        };
 237
 238Usually all you need is s_ctrl:
 239
 240        static int foo_s_ctrl(struct v4l2_ctrl *ctrl)
 241        {
 242                struct foo *state = container_of(ctrl->handler, struct foo, ctrl_handler);
 243
 244                switch (ctrl->id) {
 245                case V4L2_CID_BRIGHTNESS:
 246                        write_reg(0x123, ctrl->val);
 247                        break;
 248                case V4L2_CID_CONTRAST:
 249                        write_reg(0x456, ctrl->val);
 250                        break;
 251                }
 252                return 0;
 253        }
 254
 255The control ops are called with the v4l2_ctrl pointer as argument.
 256The new control value has already been validated, so all you need to do is
 257to actually update the hardware registers.
 258
 259You're done! And this is sufficient for most of the drivers we have. No need
 260to do any validation of control values, or implement QUERYCTRL/QUERYMENU. And
 261G/S_CTRL as well as G/TRY/S_EXT_CTRLS are automatically supported.
 262
 263
 264==============================================================================
 265
 266The remainder of this document deals with more advanced topics and scenarios.
 267In practice the basic usage as described above is sufficient for most drivers.
 268
 269===============================================================================
 270
 271
 272Inheriting Controls
 273===================
 274
 275When a sub-device is registered with a V4L2 driver by calling
 276v4l2_device_register_subdev() and the ctrl_handler fields of both v4l2_subdev
 277and v4l2_device are set, then the controls of the subdev will become
 278automatically available in the V4L2 driver as well. If the subdev driver
 279contains controls that already exist in the V4L2 driver, then those will be
 280skipped (so a V4L2 driver can always override a subdev control).
 281
 282What happens here is that v4l2_device_register_subdev() calls
 283v4l2_ctrl_add_handler() adding the controls of the subdev to the controls
 284of v4l2_device.
 285
 286
 287Accessing Control Values
 288========================
 289
 290The v4l2_ctrl struct contains these two unions:
 291
 292        /* The current control value. */
 293        union {
 294                s32 val;
 295                s64 val64;
 296                char *string;
 297        } cur;
 298
 299        /* The new control value. */
 300        union {
 301                s32 val;
 302                s64 val64;
 303                char *string;
 304        };
 305
 306Within the control ops you can freely use these. The val and val64 speak for
 307themselves. The string pointers point to character buffers of length
 308ctrl->maximum + 1, and are always 0-terminated.
 309
 310In most cases 'cur' contains the current cached control value. When you create
 311a new control this value is made identical to the default value. After calling
 312v4l2_ctrl_handler_setup() this value is passed to the hardware. It is generally
 313a good idea to call this function.
 314
 315Whenever a new value is set that new value is automatically cached. This means
 316that most drivers do not need to implement the g_volatile_ctrl() op. The
 317exception is for controls that return a volatile register such as a signal
 318strength read-out that changes continuously. In that case you will need to
 319implement g_volatile_ctrl like this:
 320
 321        static int foo_g_volatile_ctrl(struct v4l2_ctrl *ctrl)
 322        {
 323                switch (ctrl->id) {
 324                case V4L2_CID_BRIGHTNESS:
 325                        ctrl->val = read_reg(0x123);
 326                        break;
 327                }
 328        }
 329
 330Note that you use the 'new value' union as well in g_volatile_ctrl. In general
 331controls that need to implement g_volatile_ctrl are read-only controls.
 332
 333To mark a control as volatile you have to set V4L2_CTRL_FLAG_VOLATILE:
 334
 335        ctrl = v4l2_ctrl_new_std(&sd->ctrl_handler, ...);
 336        if (ctrl)
 337                ctrl->flags |= V4L2_CTRL_FLAG_VOLATILE;
 338
 339For try/s_ctrl the new values (i.e. as passed by the user) are filled in and
 340you can modify them in try_ctrl or set them in s_ctrl. The 'cur' union
 341contains the current value, which you can use (but not change!) as well.
 342
 343If s_ctrl returns 0 (OK), then the control framework will copy the new final
 344values to the 'cur' union.
 345
 346While in g_volatile/s/try_ctrl you can access the value of all controls owned
 347by the same handler since the handler's lock is held. If you need to access
 348the value of controls owned by other handlers, then you have to be very careful
 349not to introduce deadlocks.
 350
 351Outside of the control ops you have to go through to helper functions to get
 352or set a single control value safely in your driver:
 353
 354        s32 v4l2_ctrl_g_ctrl(struct v4l2_ctrl *ctrl);
 355        int v4l2_ctrl_s_ctrl(struct v4l2_ctrl *ctrl, s32 val);
 356
 357These functions go through the control framework just as VIDIOC_G/S_CTRL ioctls
 358do. Don't use these inside the control ops g_volatile/s/try_ctrl, though, that
 359will result in a deadlock since these helpers lock the handler as well.
 360
 361You can also take the handler lock yourself:
 362
 363        mutex_lock(&state->ctrl_handler.lock);
 364        printk(KERN_INFO "String value is '%s'\n", ctrl1->cur.string);
 365        printk(KERN_INFO "Integer value is '%s'\n", ctrl2->cur.val);
 366        mutex_unlock(&state->ctrl_handler.lock);
 367
 368
 369Menu Controls
 370=============
 371
 372The v4l2_ctrl struct contains this union:
 373
 374        union {
 375                u32 step;
 376                u32 menu_skip_mask;
 377        };
 378
 379For menu controls menu_skip_mask is used. What it does is that it allows you
 380to easily exclude certain menu items. This is used in the VIDIOC_QUERYMENU
 381implementation where you can return -EINVAL if a certain menu item is not
 382present. Note that VIDIOC_QUERYCTRL always returns a step value of 1 for
 383menu controls.
 384
 385A good example is the MPEG Audio Layer II Bitrate menu control where the
 386menu is a list of standardized possible bitrates. But in practice hardware
 387implementations will only support a subset of those. By setting the skip
 388mask you can tell the framework which menu items should be skipped. Setting
 389it to 0 means that all menu items are supported.
 390
 391You set this mask either through the v4l2_ctrl_config struct for a custom
 392control, or by calling v4l2_ctrl_new_std_menu().
 393
 394
 395Custom Controls
 396===============
 397
 398Driver specific controls can be created using v4l2_ctrl_new_custom():
 399
 400        static const struct v4l2_ctrl_config ctrl_filter = {
 401                .ops = &ctrl_custom_ops,
 402                .id = V4L2_CID_MPEG_CX2341X_VIDEO_SPATIAL_FILTER,
 403                .name = "Spatial Filter",
 404                .type = V4L2_CTRL_TYPE_INTEGER,
 405                .flags = V4L2_CTRL_FLAG_SLIDER,
 406                .max = 15,
 407                .step = 1,
 408        };
 409
 410        ctrl = v4l2_ctrl_new_custom(&foo->ctrl_handler, &ctrl_filter, NULL);
 411
 412The last argument is the priv pointer which can be set to driver-specific
 413private data.
 414
 415The v4l2_ctrl_config struct also has a field to set the is_private flag.
 416
 417If the name field is not set, then the framework will assume this is a standard
 418control and will fill in the name, type and flags fields accordingly.
 419
 420
 421Active and Grabbed Controls
 422===========================
 423
 424If you get more complex relationships between controls, then you may have to
 425activate and deactivate controls. For example, if the Chroma AGC control is
 426on, then the Chroma Gain control is inactive. That is, you may set it, but
 427the value will not be used by the hardware as long as the automatic gain
 428control is on. Typically user interfaces can disable such input fields.
 429
 430You can set the 'active' status using v4l2_ctrl_activate(). By default all
 431controls are active. Note that the framework does not check for this flag.
 432It is meant purely for GUIs. The function is typically called from within
 433s_ctrl.
 434
 435The other flag is the 'grabbed' flag. A grabbed control means that you cannot
 436change it because it is in use by some resource. Typical examples are MPEG
 437bitrate controls that cannot be changed while capturing is in progress.
 438
 439If a control is set to 'grabbed' using v4l2_ctrl_grab(), then the framework
 440will return -EBUSY if an attempt is made to set this control. The
 441v4l2_ctrl_grab() function is typically called from the driver when it
 442starts or stops streaming.
 443
 444
 445Control Clusters
 446================
 447
 448By default all controls are independent from the others. But in more
 449complex scenarios you can get dependencies from one control to another.
 450In that case you need to 'cluster' them:
 451
 452        struct foo {
 453                struct v4l2_ctrl_handler ctrl_handler;
 454#define AUDIO_CL_VOLUME (0)
 455#define AUDIO_CL_MUTE   (1)
 456                struct v4l2_ctrl *audio_cluster[2];
 457                ...
 458        };
 459
 460        state->audio_cluster[AUDIO_CL_VOLUME] =
 461                v4l2_ctrl_new_std(&state->ctrl_handler, ...);
 462        state->audio_cluster[AUDIO_CL_MUTE] =
 463                v4l2_ctrl_new_std(&state->ctrl_handler, ...);
 464        v4l2_ctrl_cluster(ARRAY_SIZE(state->audio_cluster), state->audio_cluster);
 465
 466From now on whenever one or more of the controls belonging to the same
 467cluster is set (or 'gotten', or 'tried'), only the control ops of the first
 468control ('volume' in this example) is called. You effectively create a new
 469composite control. Similar to how a 'struct' works in C.
 470
 471So when s_ctrl is called with V4L2_CID_AUDIO_VOLUME as argument, you should set
 472all two controls belonging to the audio_cluster:
 473
 474        static int foo_s_ctrl(struct v4l2_ctrl *ctrl)
 475        {
 476                struct foo *state = container_of(ctrl->handler, struct foo, ctrl_handler);
 477
 478                switch (ctrl->id) {
 479                case V4L2_CID_AUDIO_VOLUME: {
 480                        struct v4l2_ctrl *mute = ctrl->cluster[AUDIO_CL_MUTE];
 481
 482                        write_reg(0x123, mute->val ? 0 : ctrl->val);
 483                        break;
 484                }
 485                case V4L2_CID_CONTRAST:
 486                        write_reg(0x456, ctrl->val);
 487                        break;
 488                }
 489                return 0;
 490        }
 491
 492In the example above the following are equivalent for the VOLUME case:
 493
 494        ctrl == ctrl->cluster[AUDIO_CL_VOLUME] == state->audio_cluster[AUDIO_CL_VOLUME]
 495        ctrl->cluster[AUDIO_CL_MUTE] == state->audio_cluster[AUDIO_CL_MUTE]
 496
 497In practice using cluster arrays like this becomes very tiresome. So instead
 498the following equivalent method is used:
 499
 500        struct {
 501                /* audio cluster */
 502                struct v4l2_ctrl *volume;
 503                struct v4l2_ctrl *mute;
 504        };
 505
 506The anonymous struct is used to clearly 'cluster' these two control pointers,
 507but it serves no other purpose. The effect is the same as creating an
 508array with two control pointers. So you can just do:
 509
 510        state->volume = v4l2_ctrl_new_std(&state->ctrl_handler, ...);
 511        state->mute = v4l2_ctrl_new_std(&state->ctrl_handler, ...);
 512        v4l2_ctrl_cluster(2, &state->volume);
 513
 514And in foo_s_ctrl you can use these pointers directly: state->mute->val.
 515
 516Note that controls in a cluster may be NULL. For example, if for some
 517reason mute was never added (because the hardware doesn't support that
 518particular feature), then mute will be NULL. So in that case we have a
 519cluster of 2 controls, of which only 1 is actually instantiated. The
 520only restriction is that the first control of the cluster must always be
 521present, since that is the 'master' control of the cluster. The master
 522control is the one that identifies the cluster and that provides the
 523pointer to the v4l2_ctrl_ops struct that is used for that cluster.
 524
 525Obviously, all controls in the cluster array must be initialized to either
 526a valid control or to NULL.
 527
 528In rare cases you might want to know which controls of a cluster actually
 529were set explicitly by the user. For this you can check the 'is_new' flag of
 530each control. For example, in the case of a volume/mute cluster the 'is_new'
 531flag of the mute control would be set if the user called VIDIOC_S_CTRL for
 532mute only. If the user would call VIDIOC_S_EXT_CTRLS for both mute and volume
 533controls, then the 'is_new' flag would be 1 for both controls.
 534
 535The 'is_new' flag is always 1 when called from v4l2_ctrl_handler_setup().
 536
 537
 538Handling autogain/gain-type Controls with Auto Clusters
 539=======================================================
 540
 541A common type of control cluster is one that handles 'auto-foo/foo'-type
 542controls. Typical examples are autogain/gain, autoexposure/exposure,
 543autowhitebalance/red balance/blue balance. In all cases you have one control
 544that determines whether another control is handled automatically by the hardware,
 545or whether it is under manual control from the user.
 546
 547If the cluster is in automatic mode, then the manual controls should be
 548marked inactive and volatile. When the volatile controls are read the
 549g_volatile_ctrl operation should return the value that the hardware's automatic
 550mode set up automatically.
 551
 552If the cluster is put in manual mode, then the manual controls should become
 553active again and the volatile flag is cleared (so g_volatile_ctrl is no longer
 554called while in manual mode). In addition just before switching to manual mode
 555the current values as determined by the auto mode are copied as the new manual
 556values.
 557
 558Finally the V4L2_CTRL_FLAG_UPDATE should be set for the auto control since
 559changing that control affects the control flags of the manual controls.
 560
 561In order to simplify this a special variation of v4l2_ctrl_cluster was
 562introduced:
 563
 564void v4l2_ctrl_auto_cluster(unsigned ncontrols, struct v4l2_ctrl **controls,
 565                        u8 manual_val, bool set_volatile);
 566
 567The first two arguments are identical to v4l2_ctrl_cluster. The third argument
 568tells the framework which value switches the cluster into manual mode. The
 569last argument will optionally set V4L2_CTRL_FLAG_VOLATILE for the non-auto controls.
 570If it is false, then the manual controls are never volatile. You would typically
 571use that if the hardware does not give you the option to read back to values as
 572determined by the auto mode (e.g. if autogain is on, the hardware doesn't allow
 573you to obtain the current gain value).
 574
 575The first control of the cluster is assumed to be the 'auto' control.
 576
 577Using this function will ensure that you don't need to handle all the complex
 578flag and volatile handling.
 579
 580
 581VIDIOC_LOG_STATUS Support
 582=========================
 583
 584This ioctl allow you to dump the current status of a driver to the kernel log.
 585The v4l2_ctrl_handler_log_status(ctrl_handler, prefix) can be used to dump the
 586value of the controls owned by the given handler to the log. You can supply a
 587prefix as well. If the prefix didn't end with a space, then ': ' will be added
 588for you.
 589
 590
 591Different Handlers for Different Video Nodes
 592============================================
 593
 594Usually the V4L2 driver has just one control handler that is global for
 595all video nodes. But you can also specify different control handlers for
 596different video nodes. You can do that by manually setting the ctrl_handler
 597field of struct video_device.
 598
 599That is no problem if there are no subdevs involved but if there are, then
 600you need to block the automatic merging of subdev controls to the global
 601control handler. You do that by simply setting the ctrl_handler field in
 602struct v4l2_device to NULL. Now v4l2_device_register_subdev() will no longer
 603merge subdev controls.
 604
 605After each subdev was added, you will then have to call v4l2_ctrl_add_handler
 606manually to add the subdev's control handler (sd->ctrl_handler) to the desired
 607control handler. This control handler may be specific to the video_device or
 608for a subset of video_device's. For example: the radio device nodes only have
 609audio controls, while the video and vbi device nodes share the same control
 610handler for the audio and video controls.
 611
 612If you want to have one handler (e.g. for a radio device node) have a subset
 613of another handler (e.g. for a video device node), then you should first add
 614the controls to the first handler, add the other controls to the second
 615handler and finally add the first handler to the second. For example:
 616
 617        v4l2_ctrl_new_std(&radio_ctrl_handler, &radio_ops, V4L2_CID_AUDIO_VOLUME, ...);
 618        v4l2_ctrl_new_std(&radio_ctrl_handler, &radio_ops, V4L2_CID_AUDIO_MUTE, ...);
 619        v4l2_ctrl_new_std(&video_ctrl_handler, &video_ops, V4L2_CID_BRIGHTNESS, ...);
 620        v4l2_ctrl_new_std(&video_ctrl_handler, &video_ops, V4L2_CID_CONTRAST, ...);
 621        v4l2_ctrl_add_handler(&video_ctrl_handler, &radio_ctrl_handler, NULL);
 622
 623The last argument to v4l2_ctrl_add_handler() is a filter function that allows
 624you to filter which controls will be added. Set it to NULL if you want to add
 625all controls.
 626
 627Or you can add specific controls to a handler:
 628
 629        volume = v4l2_ctrl_new_std(&video_ctrl_handler, &ops, V4L2_CID_AUDIO_VOLUME, ...);
 630        v4l2_ctrl_new_std(&video_ctrl_handler, &ops, V4L2_CID_BRIGHTNESS, ...);
 631        v4l2_ctrl_new_std(&video_ctrl_handler, &ops, V4L2_CID_CONTRAST, ...);
 632        v4l2_ctrl_add_ctrl(&radio_ctrl_handler, volume);
 633
 634What you should not do is make two identical controls for two handlers.
 635For example:
 636
 637        v4l2_ctrl_new_std(&radio_ctrl_handler, &radio_ops, V4L2_CID_AUDIO_MUTE, ...);
 638        v4l2_ctrl_new_std(&video_ctrl_handler, &video_ops, V4L2_CID_AUDIO_MUTE, ...);
 639
 640This would be bad since muting the radio would not change the video mute
 641control. The rule is to have one control for each hardware 'knob' that you
 642can twiddle.
 643
 644
 645Finding Controls
 646================
 647
 648Normally you have created the controls yourself and you can store the struct
 649v4l2_ctrl pointer into your own struct.
 650
 651But sometimes you need to find a control from another handler that you do
 652not own. For example, if you have to find a volume control from a subdev.
 653
 654You can do that by calling v4l2_ctrl_find:
 655
 656        struct v4l2_ctrl *volume;
 657
 658        volume = v4l2_ctrl_find(sd->ctrl_handler, V4L2_CID_AUDIO_VOLUME);
 659
 660Since v4l2_ctrl_find will lock the handler you have to be careful where you
 661use it. For example, this is not a good idea:
 662
 663        struct v4l2_ctrl_handler ctrl_handler;
 664
 665        v4l2_ctrl_new_std(&ctrl_handler, &video_ops, V4L2_CID_BRIGHTNESS, ...);
 666        v4l2_ctrl_new_std(&ctrl_handler, &video_ops, V4L2_CID_CONTRAST, ...);
 667
 668...and in video_ops.s_ctrl:
 669
 670        case V4L2_CID_BRIGHTNESS:
 671                contrast = v4l2_find_ctrl(&ctrl_handler, V4L2_CID_CONTRAST);
 672                ...
 673
 674When s_ctrl is called by the framework the ctrl_handler.lock is already taken, so
 675attempting to find another control from the same handler will deadlock.
 676
 677It is recommended not to use this function from inside the control ops.
 678
 679
 680Inheriting Controls
 681===================
 682
 683When one control handler is added to another using v4l2_ctrl_add_handler, then
 684by default all controls from one are merged to the other. But a subdev might
 685have low-level controls that make sense for some advanced embedded system, but
 686not when it is used in consumer-level hardware. In that case you want to keep
 687those low-level controls local to the subdev. You can do this by simply
 688setting the 'is_private' flag of the control to 1:
 689
 690        static const struct v4l2_ctrl_config ctrl_private = {
 691                .ops = &ctrl_custom_ops,
 692                .id = V4L2_CID_...,
 693                .name = "Some Private Control",
 694                .type = V4L2_CTRL_TYPE_INTEGER,
 695                .max = 15,
 696                .step = 1,
 697                .is_private = 1,
 698        };
 699
 700        ctrl = v4l2_ctrl_new_custom(&foo->ctrl_handler, &ctrl_private, NULL);
 701
 702These controls will now be skipped when v4l2_ctrl_add_handler is called.
 703
 704
 705V4L2_CTRL_TYPE_CTRL_CLASS Controls
 706==================================
 707
 708Controls of this type can be used by GUIs to get the name of the control class.
 709A fully featured GUI can make a dialog with multiple tabs with each tab
 710containing the controls belonging to a particular control class. The name of
 711each tab can be found by querying a special control with ID <control class | 1>.
 712
 713Drivers do not have to care about this. The framework will automatically add
 714a control of this type whenever the first control belonging to a new control
 715class is added.
 716
 717
 718Proposals for Extensions
 719========================
 720
 721Some ideas for future extensions to the spec:
 722
 7231) Add a V4L2_CTRL_FLAG_HEX to have values shown as hexadecimal instead of
 724decimal. Useful for e.g. video_mute_yuv.
 725
 7262) It is possible to mark in the controls array which controls have been
 727successfully written and which failed by for example adding a bit to the
 728control ID. Not sure if it is worth the effort, though.
 729
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