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Diffstat (limited to 'Documentation/userspace-api/media/v4l/streaming-par.rst')
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diff --git a/Documentation/userspace-api/media/v4l/streaming-par.rst b/Documentation/userspace-api/media/v4l/streaming-par.rst new file mode 100644 index 000000000000..6d1a1b93ac8b --- /dev/null +++ b/Documentation/userspace-api/media/v4l/streaming-par.rst @@ -0,0 +1,40 @@ +.. Permission is granted to copy, distribute and/or modify this +.. document under the terms of the GNU Free Documentation License, +.. Version 1.1 or any later version published by the Free Software +.. Foundation, with no Invariant Sections, no Front-Cover Texts +.. and no Back-Cover Texts. A copy of the license is included at +.. Documentation/userspace-api/media/fdl-appendix.rst. +.. +.. TODO: replace it to GFDL-1.1-or-later WITH no-invariant-sections + +.. _streaming-par: + +******************** +Streaming Parameters +******************** + +Streaming parameters are intended to optimize the video capture process +as well as I/O. Presently applications can request a high quality +capture mode with the :ref:`VIDIOC_S_PARM <VIDIOC_G_PARM>` ioctl. + +The current video standard determines a nominal number of frames per +second. If less than this number of frames is to be captured or output, +applications can request frame skipping or duplicating on the driver +side. This is especially useful when using the +:ref:`read() <func-read>` or :ref:`write() <func-write>`, which are +not augmented by timestamps or sequence counters, and to avoid +unnecessary data copying. + +Finally these ioctls can be used to determine the number of buffers used +internally by a driver in read/write mode. For implications see the +section discussing the :ref:`read() <func-read>` function. + +To get and set the streaming parameters applications call the +:ref:`VIDIOC_G_PARM <VIDIOC_G_PARM>` and +:ref:`VIDIOC_S_PARM <VIDIOC_G_PARM>` ioctl, respectively. They take +a pointer to a struct :c:type:`v4l2_streamparm`, which +contains a union holding separate parameters for input and output +devices. + +These ioctls are optional, drivers need not implement them. If so, they +return the ``EINVAL`` error code. |