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Based on kernel version 4.7.2. Page generated on 2016-08-22 22:48 EST.

1				Soc-Camera Subsystem
2				====================
4	Terminology
5	-----------
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.
18	Purpose of the soc-camera subsystem
19	-----------------------------------
21	The soc-camera subsystem initially provided a unified API between camera host
22	drivers and camera sensor drivers. Later the soc-camera sensor API has been
23	replaced with the V4L2 standard subdev API. This also made camera driver re-use
24	with non-soc-camera hosts possible. The camera host API to the soc-camera core
25	has been preserved.
27	Soc-camera implements a V4L2 interface to the user, currently only the "mmap"
28	method is supported by host drivers. However, the soc-camera core also provides
29	support for the "read" method.
31	The subsystem has been designed to support multiple camera host interfaces and
32	multiple cameras per interface, although most applications have only one camera
33	sensor.
35	Existing drivers
36	----------------
38	As of 3.7 there are seven host drivers in the mainline: atmel-isi.c,
39	mx1_camera.c (broken, scheduled for removal), mx2_camera.c, mx3_camera.c,
40	omap1_camera.c, pxa_camera.c, sh_mobile_ceu_camera.c, and multiple sensor
41	drivers under drivers/media/i2c/soc_camera/.
43	Camera host API
44	---------------
46	A host camera driver is registered using the
48	soc_camera_host_register(struct soc_camera_host *);
50	function. The host object can be initialized as follows:
52		struct soc_camera_host	*ici;
53		ici->drv_name		= DRV_NAME;
54		ici->ops		= &camera_host_ops;
55		ici->priv		= pcdev;
56		ici->v4l2_dev.dev	= &pdev->dev;
57		ici->nr			= pdev->id;
59	All camera host methods are passed in a struct soc_camera_host_ops:
61	static struct soc_camera_host_ops camera_host_ops = {
62		.owner		= THIS_MODULE,
63		.add		= camera_add_device,
64		.remove		= camera_remove_device,
65		.set_fmt	= camera_set_fmt_cap,
66		.try_fmt	= camera_try_fmt_cap,
67		.init_videobuf2	= camera_init_videobuf2,
68		.poll		= camera_poll,
69		.querycap	= camera_querycap,
70		.set_bus_param	= camera_set_bus_param,
71		/* The rest of host operations are optional */
72	};
74	.add and .remove methods are called when a sensor is attached to or detached
75	from the host. .set_bus_param is used to configure physical connection
76	parameters between the host and the sensor. .init_videobuf2 is called by
77	soc-camera core when a video-device is opened, the host driver would typically
78	call vb2_queue_init() in this method. Further video-buffer management is
79	implemented completely by the specific camera host driver. If the host driver
80	supports non-standard pixel format conversion, it should implement a
81	.get_formats and, possibly, a .put_formats operations. See below for more
82	details about format conversion. The rest of the methods are called from
83	respective V4L2 operations.
85	Camera API
86	----------
88	Sensor drivers can use struct soc_camera_link, typically provided by the
89	platform, and used to specify to which camera host bus the sensor is connected,
90	and optionally provide platform .power and .reset methods for the camera. This
91	struct is provided to the camera driver via the I2C client device platform data
92	and can be obtained, using the soc_camera_i2c_to_link() macro. Care should be
93	taken, when using soc_camera_vdev_to_subdev() and when accessing struct
94	soc_camera_device, using v4l2_get_subdev_hostdata(): both only work, when
95	running on an soc-camera host. The actual camera driver operation is implemented
96	using the V4L2 subdev API. Additionally soc-camera camera drivers can use
97	auxiliary soc-camera helper functions like soc_camera_power_on() and
98	soc_camera_power_off(), which switch regulators, provided by the platform and call
99	board-specific power switching methods. soc_camera_apply_board_flags() takes
100	camera bus configuration capability flags and applies any board transformations,
101	e.g. signal polarity inversion. soc_mbus_get_fmtdesc() can be used to obtain a
102	pixel format descriptor, corresponding to a certain media-bus pixel format code.
103	soc_camera_limit_side() can be used to restrict beginning and length of a frame
104	side, based on camera capabilities.
106	VIDIOC_S_CROP and VIDIOC_S_FMT behaviour
107	----------------------------------------
109	Above user ioctls modify image geometry as follows:
111	VIDIOC_S_CROP: sets location and sizes of the sensor window. Unit is one sensor
112	pixel. Changing sensor window sizes preserves any scaling factors, therefore
113	user window sizes change as well.
115	VIDIOC_S_FMT: sets user window. Should preserve previously set sensor window as
116	much as possible by modifying scaling factors. If the sensor window cannot be
117	preserved precisely, it may be changed too.
119	In soc-camera there are two locations, where scaling and cropping can take
120	place: in the camera driver and in the host driver. User ioctls are first passed
121	to the host driver, which then generally passes them down to the camera driver.
122	It is more efficient to perform scaling and cropping in the camera driver to
123	save camera bus bandwidth and maximise the framerate. However, if the camera
124	driver failed to set the required parameters with sufficient precision, the host
125	driver may decide to also use its own scaling and cropping to fulfill the user's
126	request.
128	Camera drivers are interfaced to the soc-camera core and to host drivers over
129	the v4l2-subdev API, which is completely functional, it doesn't pass any data.
130	Therefore all camera drivers shall reply to .g_fmt() requests with their current
131	output geometry. This is necessary to correctly configure the camera bus.
132	.s_fmt() and .try_fmt() have to be implemented too. Sensor window and scaling
133	factors have to be maintained by camera drivers internally. According to the
134	V4L2 API all capture drivers must support the VIDIOC_CROPCAP ioctl, hence we
135	rely on camera drivers implementing .cropcap(). If the camera driver does not
136	support cropping, it may choose to not implement .s_crop(), but to enable
137	cropping support by the camera host driver at least the .g_crop method must be
138	implemented.
140	User window geometry is kept in .user_width and .user_height fields in struct
141	soc_camera_device and used by the soc-camera core and host drivers. The core
142	updates these fields upon successful completion of a .s_fmt() call, but if these
143	fields change elsewhere, e.g. during .s_crop() processing, the host driver is
144	responsible for updating them.
146	Format conversion
147	-----------------
149	V4L2 distinguishes between pixel formats, as they are stored in memory, and as
150	they are transferred over a media bus. Soc-camera provides support to
151	conveniently manage these formats. A table of standard transformations is
152	maintained by soc-camera core, which describes, what FOURCC pixel format will
153	be obtained, if a media-bus pixel format is stored in memory according to
154	certain rules. E.g. if MEDIA_BUS_FMT_YUYV8_2X8 data is sampled with 8 bits per
155	sample and stored in memory in the little-endian order with no gaps between
156	bytes, data in memory will represent the V4L2_PIX_FMT_YUYV FOURCC format. These
157	standard transformations will be used by soc-camera or by camera host drivers to
158	configure camera drivers to produce the FOURCC format, requested by the user,
159	using the VIDIOC_S_FMT ioctl(). Apart from those standard format conversions,
160	host drivers can also provide their own conversion rules by implementing a
161	.get_formats and, if required, a .put_formats methods.
163	--
164	Author: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
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