diff options
Diffstat (limited to 'Documentation/video4linux/v4l2-framework.txt')
| -rw-r--r-- | Documentation/video4linux/v4l2-framework.txt | 273 |
1 files changed, 216 insertions, 57 deletions
diff --git a/Documentation/video4linux/v4l2-framework.txt b/Documentation/video4linux/v4l2-framework.txt index 659b2ba12a4..667a4336170 100644 --- a/Documentation/video4linux/v4l2-framework.txt +++ b/Documentation/video4linux/v4l2-framework.txt @@ -34,6 +34,10 @@ 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. +A good example to look at as a reference is the v4l2-pci-skeleton.c +source that is available in this directory. It is a skeleton driver for +a PCI capture card, and demonstrates how to use the V4L2 driver +framework. It can be used as a template for real PCI video capture driver. Structure of a driver --------------------- @@ -68,8 +72,7 @@ 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). +and the v4l2_fh struct keeps track of filehandle instances. 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 @@ -174,19 +177,18 @@ 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) +static int 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 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. +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: @@ -197,6 +199,10 @@ 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 ------------------ @@ -244,7 +250,6 @@ 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); ... @@ -262,11 +267,16 @@ 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 @@ -303,8 +313,43 @@ Don't forget to cleanup the media entity before the sub-device is destroyed: media_entity_cleanup(&sd->entity); -A device (bridge) driver needs to register the v4l2_subdev with the -v4l2_device: +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. + +There are currently two ways to register subdevices with the V4L2 core. The +first (traditional) possibility is to have subdevices registered by bridge +drivers. This can be done when the bridge driver has the complete information +about subdevices connected to it and knows exactly when to register them. This +is typically the case for internal subdevices, like video data processing units +within SoCs or complex PCI(e) boards, camera sensors in USB cameras or connected +to SoCs, which pass information about them to bridge drivers, usually in their +platform data. + +There are however also situations where subdevices have to be registered +asynchronously to bridge devices. An example of such a configuration is a Device +Tree based system where information about subdevices is made available to the +system independently from the bridge devices, e.g. when subdevices are defined +in DT as I2C device nodes. The API used in this second case is described further +below. + +Using one or the other registration method only affects the probing process, the +run-time bridge-subdevice interaction is in both cases the same. + +In the synchronous case a device (bridge) driver needs to register the +v4l2_subdev with the v4l2_device: int err = v4l2_device_register_subdev(v4l2_dev, sd); @@ -323,24 +368,24 @@ 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); + err = sd->ops->core->g_std(sd, &norm); but it is better and easier to use this macro: - err = v4l2_subdev_call(sd, core, g_chip_ident, &chip); + err = v4l2_subdev_call(sd, core, g_std, &norm); 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. +is NULL, -ENOIOCTLCMD if either subdev->core or subdev->core->g_std is +NULL, or the actual result of the subdev->ops->core->g_std 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); + v4l2_device_call_all(v4l2_dev, 0, core, g_std, &norm); 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); + err = v4l2_device_call_until_err(v4l2_dev, 0, core, g_std, &norm); Any error except -ENOIOCTLCMD will exit the loop with that error. If no errors (except -ENOIOCTLCMD) occurred, then 0 is returned. @@ -371,6 +416,30 @@ controlled through GPIO pins. This distinction is only relevant when setting up the device, but once the subdev is registered it is completely transparent. +In the asynchronous case subdevice probing can be invoked independently of the +bridge driver availability. The subdevice driver then has to verify whether all +the requirements for a successful probing are satisfied. This can include a +check for a master clock availability. If any of the conditions aren't satisfied +the driver might decide to return -EPROBE_DEFER to request further reprobing +attempts. Once all conditions are met the subdevice shall be registered using +the v4l2_async_register_subdev() function. Unregistration is performed using +the v4l2_async_unregister_subdev() call. Subdevices registered this way are +stored in a global list of subdevices, ready to be picked up by bridge drivers. + +Bridge drivers in turn have to register a notifier object with an array of +subdevice descriptors that the bridge device needs for its operation. This is +performed using the v4l2_async_notifier_register() call. To unregister the +notifier the driver has to call v4l2_async_notifier_unregister(). The former of +the two functions takes two arguments: a pointer to struct v4l2_device and a +pointer to struct v4l2_async_notifier. The latter contains a pointer to an array +of pointers to subdevice descriptors of type struct v4l2_async_subdev type. The +V4L2 core will then use these descriptors to match asynchronously registered +subdevices to them. If a match is detected the .bound() notifier callback is +called. After all subdevices have been located the .complete() callback is +called. When a subdevice is removed from the system the .unbind() method is +called. All three callbacks are optional. + + V4L2 sub-device userspace API ----------------------------- @@ -552,31 +621,60 @@ of the video device exits. The default video_device_release() callback just calls kfree to free the allocated memory. +There is also a video_device_release_empty() function that does nothing +(is empty) and can be used if the struct is embedded and there is nothing +to do when it is released. + You should also set these fields: -- v4l2_dev: set to the v4l2_device parent device. +- v4l2_dev: must be set to the v4l2_device parent device. + - name: set to something descriptive and unique. + +- vfl_dir: set this to VFL_DIR_RX for capture devices (VFL_DIR_RX has value 0, + so this is normally already the default), set to VFL_DIR_TX for output + devices and VFL_DIR_M2M for mem2mem (codec) devices. + - 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. + future!), then set this to your v4l2_ioctl_ops struct. The vfl_type and + vfl_dir fields are used to disable ops that do not match the type/dir + combination. E.g. VBI ops are disabled for non-VBI nodes, and output ops + are disabled for a capture device. This makes it possible to provide + just one v4l2_ioctl_ops struct for both vbi and video nodes. + - 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. + Otherwise you give it a pointer to a struct mutex_lock and before the + unlocked_ioctl file operation is called this lock will be taken by the + core and released afterwards. See the next section for more details. + +- queue: a pointer to the struct vb2_queue associated with this device node. + If queue is non-NULL, and queue->lock is non-NULL, then queue->lock is + used for the queuing ioctls (VIDIOC_REQBUFS, CREATE_BUFS, QBUF, DQBUF, + QUERYBUF, PREPARE_BUF, STREAMON and STREAMOFF) instead of the lock above. + That way the vb2 queuing framework does not have to wait for other ioctls. + This queue pointer is also used by the vb2 helper functions to check for + queuing ownership (i.e. is the filehandle calling it allowed to do the + operation). + - 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 + +- dev_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. + it is used by both a raw video PCI device (cx8800) and a MPEG PCI device + (cx8802). Since the v4l2_device cannot be associated with two PCI devices + at the same time it is setup without a parent device. But when the struct + video_device is initialized you *do* know which parent PCI device to use and + so you set dev_device to the correct PCI device. + - 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 @@ -587,6 +685,16 @@ in your v4l2_file_operations struct. Do not use .ioctl! This is deprecated and will go away in the future. +In some cases you want to tell the core that a function you had specified in +your v4l2_ioctl_ops should be ignored. You can mark such ioctls by calling this +function before video_device_register is called: + +void v4l2_disable_ioctl(struct video_device *vdev, unsigned int cmd); + +This tends to be needed if based on external factors (e.g. which card is +being used) you want to turns off certain features in v4l2_ioctl_ops without +having to make a new struct. + 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. @@ -605,33 +713,43 @@ 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 --------------------------------- +ioctls and locking +------------------ + +The V4L core provides optional locking services. The main service is the +lock field in struct video_device, which is a pointer to a mutex. If you set +this pointer, then that will be used by unlocked_ioctl to serialize all ioctls. + +If you are using the videobuf2 framework, then there is a second lock that you +can set: video_device->queue->lock. If set, then this lock will be used instead +of video_device->lock to serialize all queuing ioctls (see the previous section +for the full list of those ioctls). -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. If you want -finer-grained locking then you have to set it to NULL and do you own locking. +The advantage of using a different lock for the queuing ioctls is that for some +drivers (particularly USB drivers) certain commands such as setting controls +can take a long time, so you want to use a separate lock for the buffer queuing +ioctls. That way your VIDIOC_DQBUF doesn't stall because the driver is busy +changing the e.g. exposure of the webcam. -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. +Of course, you can always do all the locking yourself by leaving both lock +pointers at NULL. -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. +If you use the old videobuf then you must pass the video_device 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. +wait_finish callbacks to unlock/lock if applicable. If you use the queue->lock +pointer, then you can use the helper functions vb2_ops_wait_prepare/finish. -The implementation of a hotplug disconnect should also take the lock before -calling v4l2_device_disconnect. +The implementation of a hotplug disconnect should also take the lock from +video_device before calling v4l2_device_disconnect. If you are also using +video_device->queue->lock, then you have to first lock video_device->queue->lock +followed by video_device->lock. That way you can be sure no ioctl is running +when you call v4l2_device_disconnect. video_device registration ------------------------- @@ -654,6 +772,7 @@ 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 +VFL_TYPE_SDR: swradioX for Software Defined 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 @@ -941,21 +1060,35 @@ fast. Useful functions: -- v4l2_event_queue() +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. -- v4l2_event_subscribe() +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 last 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). + v4l2_event_subscribe() to subscribe the event. -- v4l2_event_unsubscribe() + 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 @@ -964,7 +1097,7 @@ Useful functions: The special type V4L2_EVENT_ALL may be used to unsubscribe all events. The drivers may want to handle this in a special way. -- v4l2_event_pending() +int v4l2_event_pending(struct v4l2_fh *fh) Returns the number of pending events. Useful when implementing poll. @@ -979,4 +1112,30 @@ 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). +3 ISP driver (drivers/media/platform/omap3isp). + + +V4L2 clocks +----------- + +Many subdevices, like camera sensors, TV decoders and encoders, need a clock +signal to be supplied by the system. Often this clock is supplied by the +respective bridge device. The Linux kernel provides a Common Clock Framework for +this purpose. However, it is not (yet) available on all architectures. Besides, +the nature of the multi-functional (clock, data + synchronisation, I2C control) +connection of subdevices to the system might impose special requirements on the +clock API usage. E.g. V4L2 has to support clock provider driver unregistration +while a subdevice driver is holding a reference to the clock. For these reasons +a V4L2 clock helper API has been developed and is provided to bridge and +subdevice drivers. + +The API consists of two parts: two functions to register and unregister a V4L2 +clock source: v4l2_clk_register() and v4l2_clk_unregister() and calls to control +a clock object, similar to the respective generic clock API calls: +v4l2_clk_get(), v4l2_clk_put(), v4l2_clk_enable(), v4l2_clk_disable(), +v4l2_clk_get_rate(), and v4l2_clk_set_rate(). Clock suppliers have to provide +clock operations that will be called when clock users invoke respective API +methods. + +It is expected that once the CCF becomes available on all relevant +architectures this API will be removed. |