34. The Soc-Camera Drivers

Author: Guennadi Liakhovetski <g.liakhovetski@gmx.de>

34.1. 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, 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.

34.2. 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.

34.3. 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/.

34.4. 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.

34.5. 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.

34.6. 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.

34.7. 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.