V4L2 File handles

.. SPDX-License-Identifier: GPL-2.0

struct v4l2_fh provides a way to easily keep file handle specific data that is used by the V4L2 framework. Its usage is mandatory in all drivers.

struct v4l2_fh is allocated in the driver's open() file operation handler. It is typically embedded in a larger driver-specific structure. The :c:type:v4l2_fh must be initialized with a call to :c:func:v4l2_fh_init, and added to the video device with :c:func:v4l2_fh_add. This associates the :c:type:v4l2_fh with the :c:type:file by setting file->private_data to point to the :c:type:v4l2_fh.

Similarly, the struct v4l2_fh is freed in the driver's release() file operation handler. It must be removed from the video device with :c:func:v4l2_fh_del and cleaned up with :c:func:v4l2_fh_exit before being freed.

Drivers must not access file->private_data directly. They can retrieve the :c:type:v4l2_fh associated with a :c:type:file by calling :c:func:file_to_v4l2_fh. Drivers can extract their own file handle structure by using the container_of macro.

Example:

.. code-block:: c

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);

	...

	v4l2_fh_add(&my_fh->fh, file);
	return 0;
}

int my_release(struct file *file)
{
	struct v4l2_fh *fh = file_to_v4l2_fh(file);
	struct my_fh *my_fh = container_of(fh, struct my_fh, fh);

	...
	v4l2_fh_del(&my_fh->fh, file);
	v4l2_fh_exit(&my_fh->fh);
	kfree(my_fh);
	return 0;
}

Below is a short description of the :c:type:v4l2_fh functions used:

:c:func:v4l2_fh_init <v4l2_fh_init> (:c:type:fh <v4l2_fh>, :c:type:vdev <video_device>)

Initialise the file handle. This MUST be performed in the driver's :c:type:v4l2_file_operations->open() handler.

:c:func:v4l2_fh_add <v4l2_fh_add> (:c:type:fh <v4l2_fh>, struct file *filp)

Add a :c:type:v4l2_fh to :c:type:video_device file handle list. Must be called once the file handle is completely initialized.

:c:func:v4l2_fh_del <v4l2_fh_del> (:c:type:fh <v4l2_fh>, struct file *filp)

Unassociate the file handle from :c:type:video_device. The file handle exit function may now be called.

:c:func:v4l2_fh_exit <v4l2_fh_exit> (:c:type:fh <v4l2_fh>)

Uninitialise the file handle. After uninitialisation the :c:type:v4l2_fh memory can be freed.

:c:func:file_to_v4l2_fh <file_to_v4l2_fh> (struct file *filp)

Retrieve the :c:type:v4l2_fh instance associated with a :c:type:file.

If struct v4l2_fh is not embedded, then you can use these helper functions:

:c:func:v4l2_fh_open <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.

:c:func:v4l2_fh_release <v4l2_fh_release> (struct file *filp)

This deletes it from the struct video_device associated with the file struct, uninitialised the :c:type: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 :c:type:v4l2_fh struct is the only open filehandle of the associated device node:

:c:func:v4l2_fh_is_singular <v4l2_fh_is_singular> (:c:type:fh <v4l2_fh>)

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

:c:func:v4l2_fh_is_singular_file <v4l2_fh_is_singular_file> (struct file *filp)

Same, but it calls v4l2_fh_is_singular with filp->private_data.

V4L2 fh functions and data structures ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

.. kernel-doc:: include/media/v4l2-fh.h