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