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V4L2 exposes parameters that influence buffers sizes through the format ioctls (VIDIOC_G_FMT, VIDIOC_TRY_FMT, VIDIOC_S_FMT, and possibly VIDIOC_G_SELECTION and VIDIOC_S_SELECTION). Other parameters not part of the format structure may also influence buffer sizes or buffer layout in general. One existing such parameter is rotation, which is implemented by the V4L2_CID_ROTATE control and thus exposed through the V4L2 control ioctls. The interaction between those parameters and buffers is currently only partially specified by the V4L2 API. In particular interactions between controls and buffers isn't specified at all. The behaviour of the VIDIOC_S_FMT and VIDIOC_S_SELECTION ioctls when buffers are allocated is also not fully specified. This patch clearly defines and documents the interactions between formats, selections, controls and buffers. The preparatory discussions for the documentation change considered completely disallowing controls that change the buffer size or layout, in favour of extending the format API with a new ioctl that would bundle those controls with format information. The idea has been rejected, as this would essentially be a restricted version of the upcoming request API that wouldn't bring any additional value. Another option we have considered was to mandate the use of the request API to modify controls that influence buffer size or layout. This has also been rejected on the grounds that requiring the request API to change rotation even when streaming is stopped would significantly complicate implementation of drivers and usage of the V4L2 API for applications. Applications will however be required to use the upcoming request API to change at runtime formats or controls that influence the buffer size or layout, because of the need to synchronize buffers with the formats and controls. Otherwise there would be no way to interpret the content of a buffer correctly. Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com> Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com> Acked-by: Hans Verkuil <hans.verkuil@cisco.com> Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com> |
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| .. | ||
| dvb-drivers | ||
| kapi | ||
| uapi | ||
| v4l-drivers | ||
| .gitignore | ||
| audio.h.rst.exceptions | ||
| ca.h.rst.exceptions | ||
| cec.h.rst.exceptions | ||
| conf_nitpick.py | ||
| conf.py | ||
| dmx.h.rst.exceptions | ||
| frontend.h.rst.exceptions | ||
| index.rst | ||
| intro.rst | ||
| lirc.h.rst.exceptions | ||
| Makefile | ||
| media_kapi.rst | ||
| media_uapi.rst | ||
| media.h.rst.exceptions | ||
| net.h.rst.exceptions | ||
| typical_media_device.svg | ||
| video.h.rst.exceptions | ||
| videodev2.h.rst.exceptions | ||
