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164 lines
7.7 KiB
164 lines
7.7 KiB
Soc-Camera Subsystem |
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==================== |
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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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AVR32, 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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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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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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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 V4L2_MBUS_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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-- |
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Author: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
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