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Based on kernel version 3.2. Page generated on 2012-01-05 23:29 EST.

1				Soc-Camera Subsystem
2				====================
3	
4	Terminology
5	-----------
6	
7	The following terms are used in this document:
8	 - camera / camera device / camera sensor - a video-camera sensor chip, capable
9	   of connecting to a variety of systems and interfaces, typically uses i2c for
10	   control and configuration, and a parallel or a serial bus for data.
11	 - camera host - an interface, to which a camera is connected. Typically a
12	   specialised interface, present on many SoCs, e.g., PXA27x and PXA3xx, SuperH,
13	   AVR32, i.MX27, i.MX31.
14	 - camera host bus - a connection between a camera host and a camera. Can be
15	   parallel or serial, consists of data and control lines, e.g., clock, vertical
16	   and horizontal synchronization signals.
17	
18	Purpose of the soc-camera subsystem
19	-----------------------------------
20	
21	The soc-camera subsystem provides a unified API between camera host drivers and
22	camera sensor drivers. It implements a V4L2 interface to the user, currently
23	only the mmap method is supported.
24	
25	This subsystem has been written to connect drivers for System-on-Chip (SoC)
26	video capture interfaces with drivers for CMOS camera sensor chips to enable
27	the reuse of sensor drivers with various hosts. The subsystem has been designed
28	to support multiple camera host interfaces and multiple cameras per interface,
29	although most applications have only one camera sensor.
30	
31	Existing drivers
32	----------------
33	
34	As of 2.6.27-rc4 there are two host drivers in the mainline: pxa_camera.c for
35	PXA27x SoCs and sh_mobile_ceu_camera.c for SuperH SoCs, and four sensor drivers:
36	mt9m001.c, mt9m111.c, mt9v022.c and a generic soc_camera_platform.c driver. This
37	list is not supposed to be updated, look for more examples in your tree.
38	
39	Camera host API
40	---------------
41	
42	A host camera driver is registered using the
43	
44	soc_camera_host_register(struct soc_camera_host *);
45	
46	function. The host object can be initialized as follows:
47	
48	static struct soc_camera_host pxa_soc_camera_host = {
49		.drv_name	= PXA_CAM_DRV_NAME,
50		.ops		= &pxa_soc_camera_host_ops,
51	};
52	
53	All camera host methods are passed in a struct soc_camera_host_ops:
54	
55	static struct soc_camera_host_ops pxa_soc_camera_host_ops = {
56		.owner		= THIS_MODULE,
57		.add		= pxa_camera_add_device,
58		.remove		= pxa_camera_remove_device,
59		.suspend	= pxa_camera_suspend,
60		.resume		= pxa_camera_resume,
61		.set_fmt_cap	= pxa_camera_set_fmt_cap,
62		.try_fmt_cap	= pxa_camera_try_fmt_cap,
63		.init_videobuf	= pxa_camera_init_videobuf,
64		.reqbufs	= pxa_camera_reqbufs,
65		.poll		= pxa_camera_poll,
66		.querycap	= pxa_camera_querycap,
67		.try_bus_param	= pxa_camera_try_bus_param,
68		.set_bus_param	= pxa_camera_set_bus_param,
69	};
70	
71	.add and .remove methods are called when a sensor is attached to or detached
72	from the host, apart from performing host-internal tasks they shall also call
73	sensor driver's .init and .release methods respectively. .suspend and .resume
74	methods implement host's power-management functionality and its their
75	responsibility to call respective sensor's methods. .try_bus_param and
76	.set_bus_param are used to negotiate physical connection parameters between the
77	host and the sensor. .init_videobuf is called by soc-camera core when a
78	video-device is opened, further video-buffer management is implemented completely
79	by the specific camera host driver. The rest of the methods are called from
80	respective V4L2 operations.
81	
82	Camera API
83	----------
84	
85	Sensor drivers can use struct soc_camera_link, typically provided by the
86	platform, and used to specify to which camera host bus the sensor is connected,
87	and arbitrarily provide platform .power and .reset methods for the camera.
88	soc_camera_device_register() and soc_camera_device_unregister() functions are
89	used to add a sensor driver to or remove one from the system. The registration
90	function takes a pointer to struct soc_camera_device as the only parameter.
91	This struct can be initialized as follows:
92	
93		/* link to driver operations */
94		icd->ops	= &mt9m001_ops;
95		/* link to the underlying physical (e.g., i2c) device */
96		icd->control	= &client->dev;
97		/* window geometry */
98		icd->x_min	= 20;
99		icd->y_min	= 12;
100		icd->x_current	= 20;
101		icd->y_current	= 12;
102		icd->width_min	= 48;
103		icd->width_max	= 1280;
104		icd->height_min	= 32;
105		icd->height_max	= 1024;
106		icd->y_skip_top	= 1;
107		/* camera bus ID, typically obtained from platform data */
108		icd->iface	= icl->bus_id;
109	
110	struct soc_camera_ops provides .probe and .remove methods, which are called by
111	the soc-camera core, when a camera is matched against or removed from a camera
112	host bus, .init, .release, .suspend, and .resume are called from the camera host
113	driver as discussed above. Other members of this struct provide respective V4L2
114	functionality.
115	
116	struct soc_camera_device also links to an array of struct soc_camera_data_format,
117	listing pixel formats, supported by the camera.
118	
119	VIDIOC_S_CROP and VIDIOC_S_FMT behaviour
120	----------------------------------------
121	
122	Above user ioctls modify image geometry as follows:
123	
124	VIDIOC_S_CROP: sets location and sizes of the sensor window. Unit is one sensor
125	pixel. Changing sensor window sizes preserves any scaling factors, therefore
126	user window sizes change as well.
127	
128	VIDIOC_S_FMT: sets user window. Should preserve previously set sensor window as
129	much as possible by modifying scaling factors. If the sensor window cannot be
130	preserved precisely, it may be changed too.
131	
132	In soc-camera there are two locations, where scaling and cropping can taks
133	place: in the camera driver and in the host driver. User ioctls are first passed
134	to the host driver, which then generally passes them down to the camera driver.
135	It is more efficient to perform scaling and cropping in the camera driver to
136	save camera bus bandwidth and maximise the framerate. However, if the camera
137	driver failed to set the required parameters with sufficient precision, the host
138	driver may decide to also use its own scaling and cropping to fulfill the user's
139	request.
140	
141	Camera drivers are interfaced to the soc-camera core and to host drivers over
142	the v4l2-subdev API, which is completely functional, it doesn't pass any data.
143	Therefore all camera drivers shall reply to .g_fmt() requests with their current
144	output geometry. This is necessary to correctly configure the camera bus.
145	.s_fmt() and .try_fmt() have to be implemented too. Sensor window and scaling
146	factors have to be maintained by camera drivers internally. According to the
147	V4L2 API all capture drivers must support the VIDIOC_CROPCAP ioctl, hence we
148	rely on camera drivers implementing .cropcap(). If the camera driver does not
149	support cropping, it may choose to not implement .s_crop(), but to enable
150	cropping support by the camera host driver at least the .g_crop method must be
151	implemented.
152	
153	User window geometry is kept in .user_width and .user_height fields in struct
154	soc_camera_device and used by the soc-camera core and host drivers. The core
155	updates these fields upon successful completion of a .s_fmt() call, but if these
156	fields change elsewhere, e.g., during .s_crop() processing, the host driver is
157	responsible for updating them.
158	
159	--
160	Author: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
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