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+.. SPDX-License-Identifier: GPL-2.0
+
+V4L2 File handlers
+------------------
+
+struct v4l2_fh provides a way to easily keep file handle specific
+data that is used by the V4L2 framework.
+
+.. attention::
+ New drivers must use struct v4l2_fh
+ since it is also used to implement priority handling
+ (:ref:`VIDIOC_G_PRIORITY`).
+
+The users of :c:type:`v4l2_fh` (in the V4L2 framework, not the driver) know
+whether a driver uses :c:type:`v4l2_fh` as its ``file->private_data`` pointer
+by testing the ``V4L2_FL_USES_V4L2_FH`` bit in :c:type:`video_device`->flags.
+This bit is set whenever :c:func:`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:
+
+#) :c:func:`v4l2_fh_init` and :c:func:`v4l2_fh_add` in ``open()``
+#) :c:func:`v4l2_fh_del` and :c:func:`v4l2_fh_exit` in ``release()``
+
+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);
+
+ ...
+
+ 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 :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>`)
+
+- 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>`)
+
+- 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.
+
+
+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