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V4L/DVB (12534): soc-camera: V4L2 API compliant scaling (S_FMT) and cropping (S_CROP)

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The initial soc-camera scaling and cropping implementation turned out to be
incompliant with the V4L2 API, e.g., it expected the user to specify cropping
in output window pixels, instead of input window pixels. This patch converts
the soc-camera core and all drivers to comply with the standard.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>
This commit is contained in:
Guennadi Liakhovetski
2009-08-25 11:50:46 -03:00
committed by Mauro Carvalho Chehab
parent 0166b74374
commit 6a6c878672
14 changed files with 1573 additions and 682 deletions
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40
Documentation/video4linux/soc-camera.txt
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@@ -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>