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+ <title>Software Defined Radio Interface (SDR)</title>
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+
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+ <para>
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+SDR is an abbreviation of Software Defined Radio, the radio device
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+which uses application software for modulation or demodulation. This interface
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+is intended for controlling and data streaming of such devices.
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+ </para>
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+
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+ <para>
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+SDR devices are accessed through character device special files named
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+<filename>/dev/swradio0</filename> to <filename>/dev/swradio255</filename>
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+with major number 81 and dynamically allocated minor numbers 0 to 255.
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+ </para>
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+
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+ <section>
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+ <title>Querying Capabilities</title>
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+
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+ <para>
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+Devices supporting the SDR receiver interface set the
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+<constant>V4L2_CAP_SDR_CAPTURE</constant> and
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+<constant>V4L2_CAP_TUNER</constant> flag in the
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+<structfield>capabilities</structfield> field of &v4l2-capability;
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+returned by the &VIDIOC-QUERYCAP; ioctl. That flag means the device has an
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+Analog to Digital Converter (ADC), which is a mandatory element for the SDR receiver.
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+At least one of the read/write, streaming or asynchronous I/O methods must
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+be supported.
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+ </para>
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+ </section>
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+
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+ <section>
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+ <title>Supplemental Functions</title>
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+
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+ <para>
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+SDR devices can support <link linkend="control">controls</link>, and must
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+support the <link linkend="tuner">tuner</link> ioctls. Tuner ioctls are used
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+for setting the ADC sampling rate (sampling frequency) and the possible RF tuner
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+frequency.
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+ </para>
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+
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+ <para>
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+The <constant>V4L2_TUNER_ADC</constant> tuner type is used for ADC tuners, and
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+the <constant>V4L2_TUNER_RF</constant> tuner type is used for RF tuners. The
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+tuner index of the RF tuner (if any) must always follow the ADC tuner index.
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+Normally the ADC tuner is #0 and the RF tuner is #1.
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+ </para>
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+
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+ <para>
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+The &VIDIOC-S-HW-FREQ-SEEK; ioctl is not supported.
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+ </para>
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+ </section>
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+
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+ <section>
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+ <title>Data Format Negotiation</title>
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+
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+ <para>
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+The SDR capture device uses the <link linkend="format">format</link> ioctls to
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+select the capture format. Both the sampling resolution and the data streaming
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+format are bound to that selectable format. In addition to the basic
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+<link linkend="format">format</link> ioctls, the &VIDIOC-ENUM-FMT; ioctl
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+must be supported as well.
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+ </para>
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+
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+ <para>
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+To use the <link linkend="format">format</link> ioctls applications set the
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+<structfield>type</structfield> field of a &v4l2-format; to
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+<constant>V4L2_BUF_TYPE_SDR_CAPTURE</constant> and use the &v4l2-format-sdr;
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+<structfield>sdr</structfield> member of the <structfield>fmt</structfield>
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+union as needed per the desired operation.
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+Currently only the <structfield>pixelformat</structfield> field of
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+&v4l2-format-sdr; is used. The content of that field is the V4L2 fourcc code
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+of the data format.
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+ </para>
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+
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+ <table pgwide="1" frame="none" id="v4l2-format-sdr">
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+ <title>struct <structname>v4l2_format_sdr</structname></title>
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+ <tgroup cols="3">
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+ &cs-str;
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+ <tbody valign="top">
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+ <row>
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+ <entry>__u32</entry>
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+ <entry><structfield>pixelformat</structfield></entry>
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+ <entry>
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+The data format or type of compression, set by the application. This is a
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+little endian <link linkend="v4l2-fourcc">four character code</link>.
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+V4L2 defines SDR formats in <xref linkend="sdr-formats" />.
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+ </entry>
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+ </row>
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+ <row>
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+ <entry>__u8</entry>
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+ <entry><structfield>reserved[28]</structfield></entry>
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+ <entry>This array is reserved for future extensions.
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+Drivers and applications must set it to zero.</entry>
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+ </row>
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+ </tbody>
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+ </tgroup>
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+ </table>
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+
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+ <para>
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+An SDR device may support <link linkend="rw">read/write</link>
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+and/or streaming (<link linkend="mmap">memory mapping</link>
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+or <link linkend="userp">user pointer</link>) I/O.
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+ </para>
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+
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+ </section>
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Antti Palosaari