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Diffstat (limited to 'linux/Documentation/video4linux/soc-camera.txt')
| -rw-r--r-- | linux/Documentation/video4linux/soc-camera.txt | 40 |
1 files changed, 40 insertions, 0 deletions
diff --git a/linux/Documentation/video4linux/soc-camera.txt b/linux/Documentation/video4linux/soc-camera.txt index 178ef3c5e..3f87c7da4 100644 --- a/linux/Documentation/video4linux/soc-camera.txt +++ b/linux/Documentation/video4linux/soc-camera.txt @@ -116,5 +116,45 @@ functionality. struct soc_camera_device also links to an array of struct soc_camera_data_format, listing pixel formats, supported by the camera. +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 taks +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. + -- Author: Guennadi Liakhovetski <g.liakhovetski@gmx.de> |