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-sdr.rst

@@ -0,0 +1,114 @@
+.. 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
+
+.. _sdr:
+
+**************************************
+Software Defined Radio Interface (SDR)
+**************************************
+
+SDR is an abbreviation of Software Defined Radio, the radio device which
+uses application software for modulation or demodulation. This interface
+is intended for controlling and data streaming of such devices.
+
+SDR devices are accessed through character device special files named
+``/dev/swradio0`` to ``/dev/swradio255`` with major number 81 and
+dynamically allocated minor numbers 0 to 255.
+
+
+Querying Capabilities
+=====================
+
+Devices supporting the SDR receiver interface set the
+``V4L2_CAP_SDR_CAPTURE`` and ``V4L2_CAP_TUNER`` flag in the
+``capabilities`` field of struct
+:c:type:`v4l2_capability` returned by the
+:ref:`VIDIOC_QUERYCAP` ioctl. That flag means the
+device has an Analog to Digital Converter (ADC), which is a mandatory
+element for the SDR receiver.
+
+Devices supporting the SDR transmitter interface set the
+``V4L2_CAP_SDR_OUTPUT`` and ``V4L2_CAP_MODULATOR`` flag in the
+``capabilities`` field of struct
+:c:type:`v4l2_capability` returned by the
+:ref:`VIDIOC_QUERYCAP` ioctl. That flag means the
+device has an Digital to Analog Converter (DAC), which is a mandatory
+element for the SDR transmitter.
+
+At least one of the read/write, streaming or asynchronous I/O methods
+must be supported.
+
+
+Supplemental Functions
+======================
+
+SDR devices can support :ref:`controls <control>`, and must support
+the :ref:`tuner` ioctls. Tuner ioctls are used for setting the
+ADC/DAC sampling rate (sampling frequency) and the possible radio
+frequency (RF).
+
+The ``V4L2_TUNER_SDR`` tuner type is used for setting SDR device ADC/DAC
+frequency, and the ``V4L2_TUNER_RF`` tuner type is used for setting
+radio frequency. The tuner index of the RF tuner (if any) must always
+follow the SDR tuner index. Normally the SDR tuner is #0 and the RF
+tuner is #1.
+
+The :ref:`VIDIOC_S_HW_FREQ_SEEK` ioctl is
+not supported.
+
+
+Data Format Negotiation
+=======================
+
+The SDR device uses the :ref:`format` ioctls to select the
+capture and output format. Both the sampling resolution and the data
+streaming format are bound to that selectable format. In addition to the
+basic :ref:`format` ioctls, the
+:ref:`VIDIOC_ENUM_FMT` ioctl must be supported as
+well.
+
+To use the :ref:`format` ioctls applications set the ``type``
+field of a struct :c:type:`v4l2_format` to
+``V4L2_BUF_TYPE_SDR_CAPTURE`` or ``V4L2_BUF_TYPE_SDR_OUTPUT`` and use
+the struct :c:type:`v4l2_sdr_format` ``sdr`` member
+of the ``fmt`` union as needed per the desired operation. Currently
+there is two fields, ``pixelformat`` and ``buffersize``, of struct
+struct :c:type:`v4l2_sdr_format` which are used.
+Content of the ``pixelformat`` is V4L2 FourCC code of the data format.
+The ``buffersize`` field is maximum buffer size in bytes required for
+data transfer, set by the driver in order to inform application.
+
+
+.. c:type:: v4l2_sdr_format
+
+.. tabularcolumns:: |p{4.4cm}|p{4.4cm}|p{8.7cm}|
+
+.. flat-table:: struct v4l2_sdr_format
+    :header-rows:  0
+    :stub-columns: 0
+    :widths:       1 1 2
+
+    * - __u32
+      - ``pixelformat``
+      - The data format or type of compression, set by the application.
+	This is a little endian
+	:ref:`four character code <v4l2-fourcc>`. V4L2 defines SDR
+	formats in :ref:`sdr-formats`.
+    * - __u32
+      - ``buffersize``
+      - Maximum size in bytes required for data. Value is set by the
+	driver.
+    * - __u8
+      - ``reserved[24]``
+      - This array is reserved for future extensions. Drivers and
+	applications must set it to zero.
+
+
+An SDR device may support :ref:`read/write <rw>` and/or streaming
+(:ref:`memory mapping <mmap>` or :ref:`user pointer <userp>`) I/O.