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diff --git a/Documentation/video4linux/soc-camera.txt b/Documentation/video4linux/soc-camera.txt new file mode 100644 index 000000000..84f41cf1f --- /dev/null +++ b/Documentation/video4linux/soc-camera.txt @@ -0,0 +1,164 @@ + 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> |