android_kernel_asus_sm8350/Documentation/media/kapi/v4l2-framework.rst

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.

A good example to look at as a reference is the v4l2-pci-skeleton.c
source that is available in samples/v4l/. 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
---------------------

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:

.. code-block:: none

    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 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
will automatically appear in the media framework as entities.


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:

.. code-block:: none

	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:

.. code-block:: none

	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.

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: 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. 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 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.

- 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 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.

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.

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:

.. code-block:: none

	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.

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_pads_init():

.. code-block:: none

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

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

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.

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).

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.

Of course, you can always do all the locking yourself by leaving both lock
pointers at NULL.

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. 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 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
-------------------------

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

.. code-block:: none

	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
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
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', 'dev_debug' and 'index' attributes. The 'name'
attribute is the 'name' field of the video_device struct. The 'dev_debug' attribute
can be used to enable core debugging. See the next section for more detailed
information on this.

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 debugging
----------------------

The 'dev_debug' attribute that is created for each video, vbi, radio or swradio
device in /sys/class/video4linux/<devX>/ allows you to enable logging of
file operations.

It is a bitmask and the following bits can be set:

.. code-block:: none

	0x01: Log the ioctl name and error code. VIDIOC_(D)QBUF ioctls are only logged
	      if bit 0x08 is also set.
	0x02: Log the ioctl name arguments and error code. VIDIOC_(D)QBUF ioctls are
	      only logged if bit 0x08 is also set.
	0x04: Log the file operations open, release, read, write, mmap and
	      get_unmapped_area. The read and write operations are only logged if
	      bit 0x08 is also set.
	0x08: Log the read and write file operations and the VIDIOC_QBUF and
	      VIDIOC_DQBUF ioctls.
	0x10: Log the poll file operation.

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:

.. code-block:: none

	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:

.. code-block:: none

	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:

.. code-block:: none

	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:

.. code-block:: none

	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:

.. code-block:: none

	void *video_drvdata(struct file *file);

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

.. code-block:: none

	struct v4l2_device *v4l2_dev = vdev->v4l2_dev;

- Device node name

The video_device node kernel name can be retrieved using

.. code-block:: none

	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).

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:

.. code-block:: none

	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:

.. code-block:: none

	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.

.. code-block:: none

	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.

.. code-block:: none

	void v4l2_fh_del(struct v4l2_fh *fh)

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

.. code-block:: none

	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:

.. code-block:: none

	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.

.. code-block:: none

	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:

.. code-block:: none

	int v4l2_fh_is_singular(struct v4l2_fh *fh)

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

.. code-block:: none

	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:

.. code-block:: none

	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.

.. code-block:: none

	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.

.. code-block:: none

	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.

.. code-block:: none

	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/platform/omap3isp).

A subdev can directly send an event to the v4l2_device notify function with
V4L2_DEVICE_NOTIFY_EVENT. This allows the bridge to map the subdev that sends
the event to the video node(s) associated with the subdev that need to be
informed about such an event.

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.