media: docs: move uAPI book to userspace-api/media

Since 2017, there is an space reserved for userspace API,
created by changeset 1d596dee38 ("docs: Create a user-space API guide").

As the media subsystem was one of the first subsystems to use
Sphinx, until this patch, we were keeping things on a separate
place.

Let's just use the new location, as having all uAPI altogether
will likely make things easier for developers.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

Documentation/userspace-api/media/v4l/dev-output.rst

@@ -0,0 +1,108 @@
+.. 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
+
+.. _output:
+
+**********************
+Video Output Interface
+**********************
+
+Video output devices encode stills or image sequences as analog video
+signal. With this interface applications can control the encoding
+process and move images from user space to the driver.
+
+Conventionally V4L2 video output devices are accessed through character
+device special files named ``/dev/video`` and ``/dev/video0`` to
+``/dev/video63`` with major number 81 and minor numbers 0 to 63.
+``/dev/video`` is typically a symbolic link to the preferred video
+device.
+
+.. note:: The same device file names are used also for video capture devices.
+
+
+Querying Capabilities
+=====================
+
+Devices supporting the video output interface set the
+``V4L2_CAP_VIDEO_OUTPUT`` or ``V4L2_CAP_VIDEO_OUTPUT_MPLANE`` flag in
+the ``capabilities`` field of struct
+:c:type:`v4l2_capability` returned by the
+:ref:`VIDIOC_QUERYCAP` ioctl. As secondary device
+functions they may also support the :ref:`raw VBI output <raw-vbi>`
+(``V4L2_CAP_VBI_OUTPUT``) interface. At least one of the read/write or
+streaming I/O methods must be supported. Modulators and audio outputs
+are optional.
+
+
+Supplemental Functions
+======================
+
+Video output devices shall support :ref:`audio output <audio>`,
+:ref:`modulator <tuner>`, :ref:`controls <control>`,
+:ref:`cropping and scaling <crop>` and
+:ref:`streaming parameter <streaming-par>` ioctls as needed. The
+:ref:`video output <video>` ioctls must be supported by all video
+output devices.
+
+
+Image Format Negotiation
+========================
+
+The output is determined by cropping and image format parameters. The
+former select an area of the video picture where the image will appear,
+the latter how images are stored in memory, i. e. in RGB or YUV format,
+the number of bits per pixel or width and height. Together they also
+define how images are scaled in the process.
+
+As usual these parameters are *not* reset at :ref:`open() <func-open>`
+time to permit Unix tool chains, programming a device and then writing
+to it as if it was a plain file. Well written V4L2 applications ensure
+they really get what they want, including cropping and scaling.
+
+Cropping initialization at minimum requires to reset the parameters to
+defaults. An example is given in :ref:`crop`.
+
+To query the current image format applications set the ``type`` field of
+a struct :c:type:`v4l2_format` to
+``V4L2_BUF_TYPE_VIDEO_OUTPUT`` or ``V4L2_BUF_TYPE_VIDEO_OUTPUT_MPLANE``
+and call the :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>` ioctl with a pointer
+to this structure. Drivers fill the struct
+:c:type:`v4l2_pix_format` ``pix`` or the struct
+:c:type:`v4l2_pix_format_mplane` ``pix_mp``
+member of the ``fmt`` union.
+
+To request different parameters applications set the ``type`` field of a
+struct :c:type:`v4l2_format` as above and initialize all
+fields of the struct :c:type:`v4l2_pix_format`
+``vbi`` member of the ``fmt`` union, or better just modify the results
+of :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>`, and call the :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>`
+ioctl with a pointer to this structure. Drivers may adjust the
+parameters and finally return the actual parameters as :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>`
+does.
+
+Like :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` the :ref:`VIDIOC_TRY_FMT <VIDIOC_G_FMT>` ioctl
+can be used to learn about hardware limitations without disabling I/O or
+possibly time consuming hardware preparations.
+
+The contents of struct :c:type:`v4l2_pix_format` and
+struct :c:type:`v4l2_pix_format_mplane` are
+discussed in :ref:`pixfmt`. See also the specification of the
+:ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>`, :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` and :ref:`VIDIOC_TRY_FMT <VIDIOC_G_FMT>` ioctls for
+details. Video output devices must implement both the :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>`
+and :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl, even if :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ignores all
+requests and always returns default parameters as :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>` does.
+:ref:`VIDIOC_TRY_FMT <VIDIOC_G_FMT>` is optional.
+
+
+Writing Images
+==============
+
+A video output device may support the :ref:`write() function <rw>`
+and/or streaming (:ref:`memory mapping <mmap>` or
+:ref:`user pointer <userp>`) I/O. See :ref:`io` for details.