1 .. SPDX-License-Identifier: GPL-2.0
2
3 V4L2 File handlers
4 ------------------
5
6 struct v4l2_fh provides a way to easily keep file handle specific
7 data that is used by the V4L2 framework.
8
9 .. attention::
10 New drivers must use struct v4l2_fh
11 since it is also used to implement priority handling
12 (:ref:`VIDIOC_G_PRIORITY`).
13
14 The users of :c:type:`v4l2_fh` (in the V4L2 framework, not the driver) know
15 whether a driver uses :c:type:`v4l2_fh` as its ``file->private_data`` pointer
16 by testing the ``V4L2_FL_USES_V4L2_FH`` bit in :c:type:`video_device`->flags.
17 This bit is set whenever :c:func:`v4l2_fh_init` is called.
18
19 struct v4l2_fh is allocated as a part of the driver's own file handle
20 structure and ``file->private_data`` is set to it in the driver's ``open()``
21 function by the driver.
22
23 In many cases the struct v4l2_fh will be embedded in a larger
24 structure. In that case you should call:
25
26 #) :c:func:`v4l2_fh_init` and :c:func:`v4l2_fh_add` in ``open()``
27 #) :c:func:`v4l2_fh_del` and :c:func:`v4l2_fh_exit` in ``release()``
28
29 Drivers can extract their own file handle structure by using the container_of
30 macro.
31
32 Example:
33
34 .. code-block:: c
35
36 struct my_fh {
37 int blah;
38 struct v4l2_fh fh;
39 };
40
41 ...
42
43 int my_open(struct file *file)
44 {
45 struct my_fh *my_fh;
46 struct video_device *vfd;
47 int ret;
48
49 ...
50
51 my_fh = kzalloc(sizeof(*my_fh), GFP_KERNEL);
52
53 ...
54
55 v4l2_fh_init(&my_fh->fh, vfd);
56
57 ...
58
59 file->private_data = &my_fh->fh;
60 v4l2_fh_add(&my_fh->fh);
61 return 0;
62 }
63
64 int my_release(struct file *file)
65 {
66 struct v4l2_fh *fh = file->private_data;
67 struct my_fh *my_fh = container_of(fh, struct my_fh, fh);
68
69 ...
70 v4l2_fh_del(&my_fh->fh);
71 v4l2_fh_exit(&my_fh->fh);
72 kfree(my_fh);
73 return 0;
74 }
75
76 Below is a short description of the :c:type:`v4l2_fh` functions used:
77
78 :c:func:`v4l2_fh_init <v4l2_fh_init>`
79 (:c:type:`fh <v4l2_fh>`, :c:type:`vdev <video_device>`)
80
81
82 - Initialise the file handle. This **MUST** be performed in the driver's
83 :c:type:`v4l2_file_operations`->open() handler.
84
85
86 :c:func:`v4l2_fh_add <v4l2_fh_add>`
87 (:c:type:`fh <v4l2_fh>`)
88
89 - Add a :c:type:`v4l2_fh` to :c:type:`video_device` file handle list.
90 Must be called once the file handle is completely initialized.
91
92 :c:func:`v4l2_fh_del <v4l2_fh_del>`
93 (:c:type:`fh <v4l2_fh>`)
94
95 - Unassociate the file handle from :c:type:`video_device`. The file handle
96 exit function may now be called.
97
98 :c:func:`v4l2_fh_exit <v4l2_fh_exit>`
99 (:c:type:`fh <v4l2_fh>`)
100
101 - Uninitialise the file handle. After uninitialisation the :c:type:`v4l2_fh`
102 memory can be freed.
103
104
105 If struct v4l2_fh is not embedded, then you can use these helper functions:
106
107 :c:func:`v4l2_fh_open <v4l2_fh_open>`
108 (struct file \*filp)
109
110 - This allocates a struct v4l2_fh, initializes it and adds it to
111 the struct video_device associated with the file struct.
112
113 :c:func:`v4l2_fh_release <v4l2_fh_release>`
114 (struct file \*filp)
115
116 - This deletes it from the struct video_device associated with the
117 file struct, uninitialised the :c:type:`v4l2_fh` and frees it.
118
119 These two functions can be plugged into the v4l2_file_operation's ``open()``
120 and ``release()`` ops.
121
122 Several drivers need to do something when the first file handle is opened and
123 when the last file handle closes. Two helper functions were added to check
124 whether the :c:type:`v4l2_fh` struct is the only open filehandle of the
125 associated device node:
126
127 :c:func:`v4l2_fh_is_singular <v4l2_fh_is_singular>`
128 (:c:type:`fh <v4l2_fh>`)
129
130 - Returns 1 if the file handle is the only open file handle, else 0.
131
132 :c:func:`v4l2_fh_is_singular_file <v4l2_fh_is_singular_file>`
133 (struct file \*filp)
134
135 - Same, but it calls v4l2_fh_is_singular with filp->private_data.
136
137
138 V4L2 fh functions and data structures
139 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
140
141 .. kernel-doc:: include/media/v4l2-fh.h
Linux® is a registered trademark of Linus Torvalds in the United States and other countries.
TOMOYO® is a registered trademark of NTT DATA CORPORATION.