[media] v4l2-fh.rst: add fh contents from v4l2-framework.rst

[deliverable/linux.git] / Documentation / media / kapi / v4l2-fh.rst

V4L2 File handlers
------------------

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 File Handler kAPI
^^^^^^^^^^^^^^^^^^^^^^
.. kernel-doc:: include/media/v4l2-fh.h