1 .. SPDX-License-Identifier: GPL-2.0
2
3 V4L2 device instance
4 --------------------
5
6 Each device instance is represented by a struct v4l2_device.
7 Very simple devices can just allocate this struct, but most of the time you
8 would embed this struct inside a larger struct.
9
10 You must register the device instance by calling:
11
12 :c:func:`v4l2_device_register <v4l2_device_register>`
13 (dev, :c:type:`v4l2_dev <v4l2_device>`).
14
15 Registration will initialize the :c:type:`v4l2_device` struct. If the
16 dev->driver_data field is ``NULL``, it will be linked to
17 :c:type:`v4l2_dev <v4l2_device>` argument.
18
19 Drivers that want integration with the media device framework need to set
20 dev->driver_data manually to point to the driver-specific device structure
21 that embed the struct v4l2_device instance. This is achieved by a
22 ``dev_set_drvdata()`` call before registering the V4L2 device instance.
23 They must also set the struct v4l2_device mdev field to point to a
24 properly initialized and registered :c:type:`media_device` instance.
25
26 If :c:type:`v4l2_dev <v4l2_device>`\ ->name is empty then it will be set to a
27 value derived from dev (driver name followed by the bus_id, to be precise).
28 If you set it up before calling :c:func:`v4l2_device_register` then it will
29 be untouched. If dev is ``NULL``, then you **must** setup
30 :c:type:`v4l2_dev <v4l2_device>`\ ->name before calling
31 :c:func:`v4l2_device_register`.
32
33 You can use :c:func:`v4l2_device_set_name` to set the name based on a driver
34 name and a driver-global atomic_t instance. This will generate names like
35 ``ivtv0``, ``ivtv1``, etc. If the name ends with a digit, then it will insert
36 a dash: ``cx18-0``, ``cx18-1``, etc. This function returns the instance number.
37
38 The first ``dev`` argument is normally the ``struct device`` pointer of a
39 ``pci_dev``, ``usb_interface`` or ``platform_device``. It is rare for dev to
40 be ``NULL``, but it happens with ISA devices or when one device creates
41 multiple PCI devices, thus making it impossible to associate
42 :c:type:`v4l2_dev <v4l2_device>` with a particular parent.
43
44 You can also supply a ``notify()`` callback that can be called by sub-devices
45 to notify you of events. Whether you need to set this depends on the
46 sub-device. Any notifications a sub-device supports must be defined in a header
47 in ``include/media/subdevice.h``.
48
49 V4L2 devices are unregistered by calling:
50
51 :c:func:`v4l2_device_unregister`
52 (:c:type:`v4l2_dev <v4l2_device>`).
53
54 If the dev->driver_data field points to :c:type:`v4l2_dev <v4l2_device>`,
55 it will be reset to ``NULL``. Unregistering will also automatically unregister
56 all subdevs from the device.
57
58 If you have a hotpluggable device (e.g. a USB device), then when a disconnect
59 happens the parent device becomes invalid. Since :c:type:`v4l2_device` has a
60 pointer to that parent device it has to be cleared as well to mark that the
61 parent is gone. To do this call:
62
63 :c:func:`v4l2_device_disconnect`
64 (:c:type:`v4l2_dev <v4l2_device>`).
65
66 This does *not* unregister the subdevs, so you still need to call the
67 :c:func:`v4l2_device_unregister` function for that. If your driver is not
68 hotpluggable, then there is no need to call :c:func:`v4l2_device_disconnect`.
69
70 Sometimes you need to iterate over all devices registered by a specific
71 driver. This is usually the case if multiple device drivers use the same
72 hardware. E.g. the ivtvfb driver is a framebuffer driver that uses the ivtv
73 hardware. The same is true for alsa drivers for example.
74
75 You can iterate over all registered devices as follows:
76
77 .. code-block:: c
78
79 static int callback(struct device *dev, void *p)
80 {
81 struct v4l2_device *v4l2_dev = dev_get_drvdata(dev);
82
83 /* test if this device was inited */
84 if (v4l2_dev == NULL)
85 return 0;
86 ...
87 return 0;
88 }
89
90 int iterate(void *p)
91 {
92 struct device_driver *drv;
93 int err;
94
95 /* Find driver 'ivtv' on the PCI bus.
96 pci_bus_type is a global. For USB buses use usb_bus_type. */
97 drv = driver_find("ivtv", &pci_bus_type);
98 /* iterate over all ivtv device instances */
99 err = driver_for_each_device(drv, NULL, p, callback);
100 put_driver(drv);
101 return err;
102 }
103
104 Sometimes you need to keep a running counter of the device instance. This is
105 commonly used to map a device instance to an index of a module option array.
106
107 The recommended approach is as follows:
108
109 .. code-block:: c
110
111 static atomic_t drv_instance = ATOMIC_INIT(0);
112
113 static int drv_probe(struct pci_dev *pdev, const struct pci_device_id *pci_id)
114 {
115 ...
116 state->instance = atomic_inc_return(&drv_instance) - 1;
117 }
118
119 If you have multiple device nodes then it can be difficult to know when it is
120 safe to unregister :c:type:`v4l2_device` for hotpluggable devices. For this
121 purpose :c:type:`v4l2_device` has refcounting support. The refcount is
122 increased whenever :c:func:`video_register_device` is called and it is
123 decreased whenever that device node is released. When the refcount reaches
124 zero, then the :c:type:`v4l2_device` release() callback is called. You can
125 do your final cleanup there.
126
127 If other device nodes (e.g. ALSA) are created, then you can increase and
128 decrease the refcount manually as well by calling:
129
130 :c:func:`v4l2_device_get`
131 (:c:type:`v4l2_dev <v4l2_device>`).
132
133 or:
134
135 :c:func:`v4l2_device_put`
136 (:c:type:`v4l2_dev <v4l2_device>`).
137
138 Since the initial refcount is 1 you also need to call
139 :c:func:`v4l2_device_put` in the ``disconnect()`` callback (for USB devices)
140 or in the ``remove()`` callback (for e.g. PCI devices), otherwise the refcount
141 will never reach 0.
142
143 v4l2_device functions and data structures
144 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
145
146 .. kernel-doc:: include/media/v4l2-device.h
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