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Based on kernel version 3.16. Page generated on 2014-08-06 21:38 EST.

1	  <title>V4L2 Driver Programming</title>
2	
3	  <!-- This part defines the interface between the "videodev"
4	    module and individual drivers. -->
5	
6	  <para>to do</para>
7	<!--
8	  <para>V4L2 is a two-layer driver system. The top layer is the "videodev"
9	kernel module. When videodev initializes it registers as character device
10	with major number 81, and it registers a set of file operations. All V4L2
11	drivers are really clients of videodev, which calls V4L2 drivers through
12	driver method functions. V4L2 drivers are also written as kernel modules.
13	After probing the hardware they register one or more devices with
14	videodev.</para>
15	
16	  <section id="driver-modules">
17	    <title>Driver Modules</title>
18	
19	    <para>V4L2 driver modules must have an initialization function which is
20	called after the module was loaded into kernel, an exit function whis is
21	called before the module is removed.  When the driver is compiled into the
22	kernel these functions called at system boot and shutdown time.</para>
23	
24	    <informalexample>
25	      <programlisting>
26	#include &lt;linux/module.h&gt;
27	
28	/* Export information about this module. For details and other useful
29	   macros see <filename>linux/module.h</filename>. */
30	MODULE_DESCRIPTION("my - driver for my hardware");
31	MODULE_AUTHOR("Your name here");
32	MODULE_LICENSE("GPL");
33	
34	static void
35	my_module_exit (void)
36	{
37		/* Free all resources allocated by my_module_init(). */
38	}
39	
40	static int
41	my_module_init (void)
42	{
43		/* Bind the driver to the supported hardware, see
44		   <link linkend="driver-pci"> and
45		   <link linkend="driver-usb"> for examples. */
46	
47		return 0; /* a negative value on error, 0 on success. */
48	}
49	
50	/* Export module functions. */
51	module_init (my_module_init);
52	module_exit (my_module_exit);
53	</programlisting>
54	    </informalexample>
55	
56	    <para>Users can add parameters when kernel modules are inserted:</para>
57	
58	    <informalexample>
59	      <programlisting>
60	include &lt;linux/moduleparam.h&gt;
61	
62	static int my_option = 123;
63	static int my_option_array[47];
64	
65	/* Export the symbol, an int, with access permissions 0664.
66	   See <filename>linux/moduleparam.h</filename> for other types. */
67	module_param (my_option, int, 0644);
68	module_param_array (my_option_array, int, NULL, 0644);
69	
70	MODULE_PARM_DESC (my_option, "Does magic things, default 123");
71	</programlisting>
72	    </informalexample>
73	
74	    <para>One parameter should be supported by all V4L2 drivers, the minor
75	number of the device it will register. Purpose is to predictably link V4L2
76	drivers to device nodes if more than one video device is installed. Use the
77	name of the device node followed by a "_nr" suffix, for example "video_nr"
78	for <filename>/dev/video</filename>.</para>
79	
80	    <informalexample>
81	      <programlisting>
82	/* Minor number of the device, -1 to allocate the first unused. */
83	static int video_nr = -1;
84	
85	module_param (video_nr, int, 0444);
86	</programlisting>
87	    </informalexample>
88	  </section>
89	
90	  <section id="driver-pci">
91	    <title>PCI Devices</title>
92	
93	    <para>PCI devices are initialized like this:</para>
94	
95	    <informalexample>
96	      <programlisting>
97	typedef struct {
98		/* State of one physical device. */
99	} my_device;
100	
101	static int
102	my_resume               (struct pci_dev *               pci_dev)
103	{
104		/* Restore the suspended device to working state. */
105	}
106	
107	static int
108	my_suspend              (struct pci_dev *               pci_dev,
109				 pm_message_t                   state)
110	{
111		/* This function is called before the system goes to sleep.
112		   Stop all DMAs and disable interrupts, then put the device
113		   into a low power state. For details see the kernel
114		   sources under <filename>Documentation/power</filename>. */
115	
116		return 0; /* a negative value on error, 0 on success. */
117	}
118	
119	static void
120	my_remove               (struct pci_dev *               pci_dev)
121	{
122		my_device *my = pci_get_drvdata (pci_dev);
123	
124		/* Describe me. */
125	}
126	
127	static int
128	my_probe                (struct pci_dev *               pci_dev,
129				 const struct pci_device_id *   pci_id)
130	{
131		my_device *my;
132	
133		/* Describe me. */
134	
135		/* You can allocate per-device data here and store a pointer
136		   to it in the pci_dev structure. */
137		my = ...;
138		pci_set_drvdata (pci_dev, my);
139	
140		return 0; /* a negative value on error, 0 on success. */
141	}
142	
143	/* A list of supported PCI devices. */
144	static struct pci_device_id
145	my_pci_device_ids [] = {
146		{ PCI_VENDOR_ID_FOO, PCI_DEVICE_ID_BAR,
147		  PCI_ANY_ID, PCI_ANY_ID, 0, 0, 0 },
148		{ 0 } /* end of list */
149	};
150	
151	/* Load our module if supported PCI devices are installed. */
152	MODULE_DEVICE_TABLE (pci, my_pci_device_ids);
153	
154	static struct pci_driver
155	my_pci_driver = {
156		.name     = "my",
157		.id_table = my_pci_device_ids,
158	
159		.probe    = my_probe,
160		.remove   = my_remove,
161	
162		/* Power management functions. */
163		.suspend  = my_suspend,
164		.resume   = my_resume,
165	};
166	
167	static void
168	my_module_exit          (void)
169	{
170		pci_unregister_driver (&my_pci_driver);
171	}
172	
173	static int
174	my_module_init          (void)
175	{
176		return pci_register_driver (&my_pci_driver);
177	}
178	</programlisting>
179	    </informalexample>
180	  </section>
181	
182	  <section id="driver-usb">
183	    <title>USB Devices</title>
184	    <para>to do</para>
185	  </section>
186	  <section id="driver-registering">
187	    <title>Registering V4L2 Drivers</title>
188	
189	    <para>After a V4L2 driver probed the hardware it registers one or more
190	devices with the videodev module.</para>
191	  </section>
192	  <section id="driver-file-ops">
193	    <title>File Operations</title>
194	    <para>to do</para>
195	  </section>
196	  <section id="driver-internal-api">
197	    <title>Internal API</title>
198	    <para>to do</para>
199	  </section>
200	-->
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