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-			Soc-Camera Subsystem
-			====================
-
-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
-
-soc_camera_host_register(struct soc_camera_host *);
-
-function. The host object can be initialized as follows:
-
-	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:
-
-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.
-
---
-Author: Guennadi Liakhovetski <g.liakhovetski@gmx.de>