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+The Soc-Camera Drivers
+======================
+
+Author: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
+
+Terminology
+-----------
+
+The following terms are used in this document:
+ - camera / camera device / camera sensor - a video-camera sensor chip, capable
+   of connecting to a variety of systems and interfaces, typically uses i2c for
+   control and configuration, and a parallel or a serial bus for data.
+ - camera host - an interface, to which a camera is connected. Typically a
+   specialised interface, present on many SoCs, e.g. PXA27x and PXA3xx, SuperH,
+   AVR32, i.MX27, i.MX31.
+ - camera host bus - a connection between a camera host and a camera. Can be
+   parallel or serial, consists of data and control lines, e.g. clock, vertical
+   and horizontal synchronization signals.
+
+Purpose of the soc-camera subsystem
+-----------------------------------
+
+The soc-camera subsystem initially provided a unified API between camera host
+drivers and camera sensor drivers. Later the soc-camera sensor API has been
+replaced with the V4L2 standard subdev API. This also made camera driver re-use
+with non-soc-camera hosts possible. The camera host API to the soc-camera core
+has been preserved.
+
+Soc-camera implements a V4L2 interface to the user, currently only the "mmap"
+method is supported by host drivers. However, the soc-camera core also provides
+support for the "read" method.
+
+The subsystem has been designed to support multiple camera host interfaces and
+multiple cameras per interface, although most applications have only one camera
+sensor.
+
+Existing drivers
+----------------
+
+As of 3.7 there are seven host drivers in the mainline: atmel-isi.c,
+mx1_camera.c (broken, scheduled for removal), mx2_camera.c, mx3_camera.c,
+omap1_camera.c, pxa_camera.c, sh_mobile_ceu_camera.c, and multiple sensor
+drivers under drivers/media/i2c/soc_camera/.
+
+Camera host API
+---------------
+
+A host camera driver is registered using the
+
+.. code-block:: none
+
+	soc_camera_host_register(struct soc_camera_host *);
+
+function. The host object can be initialized as follows:
+
+.. code-block:: none
+
+	struct soc_camera_host	*ici;
+	ici->drv_name		= DRV_NAME;
+	ici->ops		= &camera_host_ops;
+	ici->priv		= pcdev;
+	ici->v4l2_dev.dev	= &pdev->dev;
+	ici->nr			= pdev->id;
+
+All camera host methods are passed in a struct soc_camera_host_ops:
+
+.. code-block:: none
+
+	static struct soc_camera_host_ops camera_host_ops = {
+		.owner		= THIS_MODULE,
+		.add		= camera_add_device,
+		.remove		= camera_remove_device,
+		.set_fmt	= camera_set_fmt_cap,
+		.try_fmt	= camera_try_fmt_cap,
+		.init_videobuf2	= camera_init_videobuf2,
+		.poll		= camera_poll,
+		.querycap	= camera_querycap,
+		.set_bus_param	= camera_set_bus_param,
+		/* The rest of host operations are optional */
+	};
+
+.add and .remove methods are called when a sensor is attached to or detached
+from the host. .set_bus_param is used to configure physical connection
+parameters between the host and the sensor. .init_videobuf2 is called by
+soc-camera core when a video-device is opened, the host driver would typically
+call vb2_queue_init() in this method. Further video-buffer management is
+implemented completely by the specific camera host driver. If the host driver
+supports non-standard pixel format conversion, it should implement a
+.get_formats and, possibly, a .put_formats operations. See below for more
+details about format conversion. The rest of the methods are called from
+respective V4L2 operations.
+
+Camera API
+----------
+
+Sensor drivers can use struct soc_camera_link, typically provided by the
+platform, and used to specify to which camera host bus the sensor is connected,
+and optionally provide platform .power and .reset methods for the camera. This
+struct is provided to the camera driver via the I2C client device platform data
+and can be obtained, using the soc_camera_i2c_to_link() macro. Care should be
+taken, when using soc_camera_vdev_to_subdev() and when accessing struct
+soc_camera_device, using v4l2_get_subdev_hostdata(): both only work, when
+running on an soc-camera host. The actual camera driver operation is implemented
+using the V4L2 subdev API. Additionally soc-camera camera drivers can use
+auxiliary soc-camera helper functions like soc_camera_power_on() and
+soc_camera_power_off(), which switch regulators, provided by the platform and call
+board-specific power switching methods. soc_camera_apply_board_flags() takes
+camera bus configuration capability flags and applies any board transformations,
+e.g. signal polarity inversion. soc_mbus_get_fmtdesc() can be used to obtain a
+pixel format descriptor, corresponding to a certain media-bus pixel format code.
+soc_camera_limit_side() can be used to restrict beginning and length of a frame
+side, based on camera capabilities.
+
+VIDIOC_S_CROP and VIDIOC_S_FMT behaviour
+----------------------------------------
+
+Above user ioctls modify image geometry as follows:
+
+VIDIOC_S_CROP: sets location and sizes of the sensor window. Unit is one sensor
+pixel. Changing sensor window sizes preserves any scaling factors, therefore
+user window sizes change as well.
+
+VIDIOC_S_FMT: sets user window. Should preserve previously set sensor window as
+much as possible by modifying scaling factors. If the sensor window cannot be
+preserved precisely, it may be changed too.
+
+In soc-camera there are two locations, where scaling and cropping can take
+place: in the camera driver and in the host driver. User ioctls are first passed
+to the host driver, which then generally passes them down to the camera driver.
+It is more efficient to perform scaling and cropping in the camera driver to
+save camera bus bandwidth and maximise the framerate. However, if the camera
+driver failed to set the required parameters with sufficient precision, the host
+driver may decide to also use its own scaling and cropping to fulfill the user's
+request.
+
+Camera drivers are interfaced to the soc-camera core and to host drivers over
+the v4l2-subdev API, which is completely functional, it doesn't pass any data.
+Therefore all camera drivers shall reply to .g_fmt() requests with their current
+output geometry. This is necessary to correctly configure the camera bus.
+.s_fmt() and .try_fmt() have to be implemented too. Sensor window and scaling
+factors have to be maintained by camera drivers internally. According to the
+V4L2 API all capture drivers must support the VIDIOC_CROPCAP ioctl, hence we
+rely on camera drivers implementing .cropcap(). If the camera driver does not
+support cropping, it may choose to not implement .s_crop(), but to enable
+cropping support by the camera host driver at least the .g_crop method must be
+implemented.
+
+User window geometry is kept in .user_width and .user_height fields in struct
+soc_camera_device and used by the soc-camera core and host drivers. The core
+updates these fields upon successful completion of a .s_fmt() call, but if these
+fields change elsewhere, e.g. during .s_crop() processing, the host driver is
+responsible for updating them.
+
+Format conversion
+-----------------
+
+V4L2 distinguishes between pixel formats, as they are stored in memory, and as
+they are transferred over a media bus. Soc-camera provides support to
+conveniently manage these formats. A table of standard transformations is
+maintained by soc-camera core, which describes, what FOURCC pixel format will
+be obtained, if a media-bus pixel format is stored in memory according to
+certain rules. E.g. if MEDIA_BUS_FMT_YUYV8_2X8 data is sampled with 8 bits per
+sample and stored in memory in the little-endian order with no gaps between
+bytes, data in memory will represent the V4L2_PIX_FMT_YUYV FOURCC format. These
+standard transformations will be used by soc-camera or by camera host drivers to
+configure camera drivers to produce the FOURCC format, requested by the user,
+using the VIDIOC_S_FMT ioctl(). Apart from those standard format conversions,
+host drivers can also provide their own conversion rules by implementing a
+.get_formats and, if required, a .put_formats methods.