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V4L2 uses the enum type in IOCTL arguments in IOCTLs that were defined until the use of enum was considered less than ideal. Recently Rémi Denis-Courmont brought up the issue by proposing a patch to convert the enums to unsigned: <URL:http://www.spinics.net/lists/linux-media/msg46167.html> This sparked a long discussion where another solution to the issue was proposed: two sets of IOCTL structures, one with __u32 and the other with enums, and conversion code between the two: <URL:http://www.spinics.net/lists/linux-media/msg47168.html> Both approaches implement a complete solution that resolves the problem. The first one is simple but requires assuming enums and __u32 are the same in size (so we won't break the ABI) while the second one is more complex and less clean but does not require making that assumption. The issue boils down to whether enums are fundamentally different from __u32 or not, and can the former be substituted by the latter. During the discussion it was concluded that the __u32 has the same size as enums on all archs Linux is supported: it has not been shown that replacing those enums in IOCTL arguments would break neither source or binary compatibility. If no such reason is found, just replacing the enums with __u32s is the way to go. This is what this patch does. This patch is slightly different from Remi's first RFC (link above): it uses __u32 instead of unsigned and also changes the arguments of VIDIOC_G_PRIORITY and VIDIOC_S_PRIORITY. Signed-off-by: Rémi Denis-Courmont <remi@remlab.net> Signed-off-by: Sakari Ailus <sakari.ailus@iki.fi> Acked-by: Hans Verkuil <hans.verkuil@cisco.com> Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>
316 lines
11 KiB
XML
316 lines
11 KiB
XML
<refentry id="vidioc-g-parm">
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<refmeta>
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<refentrytitle>ioctl VIDIOC_G_PARM, VIDIOC_S_PARM</refentrytitle>
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&manvol;
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</refmeta>
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<refnamediv>
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<refname>VIDIOC_G_PARM</refname>
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<refname>VIDIOC_S_PARM</refname>
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<refpurpose>Get or set streaming parameters</refpurpose>
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</refnamediv>
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<refsynopsisdiv>
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<funcsynopsis>
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<funcprototype>
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<funcdef>int <function>ioctl</function></funcdef>
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<paramdef>int <parameter>fd</parameter></paramdef>
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<paramdef>int <parameter>request</parameter></paramdef>
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<paramdef>v4l2_streamparm *<parameter>argp</parameter></paramdef>
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</funcprototype>
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</funcsynopsis>
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</refsynopsisdiv>
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<refsect1>
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<title>Arguments</title>
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<variablelist>
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<varlistentry>
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<term><parameter>fd</parameter></term>
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<listitem>
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<para>&fd;</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><parameter>request</parameter></term>
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<listitem>
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<para>VIDIOC_G_PARM, VIDIOC_S_PARM</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><parameter>argp</parameter></term>
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<listitem>
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<para></para>
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</listitem>
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</varlistentry>
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</variablelist>
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</refsect1>
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<refsect1>
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<title>Description</title>
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<para>The current video standard determines a nominal number of
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frames per second. If less than this number of frames is to be
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captured or output, applications can request frame skipping or
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duplicating on the driver side. This is especially useful when using
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the <function>read()</function> or <function>write()</function>, which
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are not augmented by timestamps or sequence counters, and to avoid
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unnecessary data copying.</para>
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<para>Further these ioctls can be used to determine the number of
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buffers used internally by a driver in read/write mode. For
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implications see the section discussing the &func-read;
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function.</para>
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<para>To get and set the streaming parameters applications call
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the <constant>VIDIOC_G_PARM</constant> and
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<constant>VIDIOC_S_PARM</constant> ioctl, respectively. They take a
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pointer to a struct <structname>v4l2_streamparm</structname> which
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contains a union holding separate parameters for input and output
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devices.</para>
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<table pgwide="1" frame="none" id="v4l2-streamparm">
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<title>struct <structname>v4l2_streamparm</structname></title>
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<tgroup cols="4">
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&cs-ustr;
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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>type</structfield></entry>
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<entry></entry>
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<entry>The buffer (stream) type, same as &v4l2-format;
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<structfield>type</structfield>, set by the application. See <xref
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linkend="v4l2-buf-type" /></entry>
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</row>
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<row>
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<entry>union</entry>
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<entry><structfield>parm</structfield></entry>
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<entry></entry>
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<entry></entry>
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</row>
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<row>
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<entry></entry>
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<entry>&v4l2-captureparm;</entry>
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<entry><structfield>capture</structfield></entry>
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<entry>Parameters for capture devices, used when
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<structfield>type</structfield> is
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<constant>V4L2_BUF_TYPE_VIDEO_CAPTURE</constant>.</entry>
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</row>
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<row>
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<entry></entry>
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<entry>&v4l2-outputparm;</entry>
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<entry><structfield>output</structfield></entry>
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<entry>Parameters for output devices, used when
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<structfield>type</structfield> is
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<constant>V4L2_BUF_TYPE_VIDEO_OUTPUT</constant>.</entry>
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</row>
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<row>
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<entry></entry>
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<entry>__u8</entry>
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<entry><structfield>raw_data</structfield>[200]</entry>
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<entry>A place holder for future extensions and custom
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(driver defined) buffer types <constant>V4L2_BUF_TYPE_PRIVATE</constant> and
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higher.</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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<table pgwide="1" frame="none" id="v4l2-captureparm">
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<title>struct <structname>v4l2_captureparm</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>capability</structfield></entry>
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<entry>See <xref linkend="parm-caps" />.</entry>
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</row>
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<row>
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<entry>__u32</entry>
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<entry><structfield>capturemode</structfield></entry>
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<entry>Set by drivers and applications, see <xref linkend="parm-flags" />.</entry>
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</row>
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<row>
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<entry>&v4l2-fract;</entry>
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<entry><structfield>timeperframe</structfield></entry>
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<entry><para>This is is the desired period between
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successive frames captured by the driver, in seconds. The
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field is intended to skip frames on the driver side, saving I/O
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bandwidth.</para><para>Applications store here the desired frame
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period, drivers return the actual frame period, which must be greater
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or equal to the nominal frame period determined by the current video
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standard (&v4l2-standard; <structfield>frameperiod</structfield>
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field). Changing the video standard (also implicitly by switching the
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video input) may reset this parameter to the nominal frame period. To
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reset manually applications can just set this field to
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zero.</para><para>Drivers support this function only when they set the
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<constant>V4L2_CAP_TIMEPERFRAME</constant> flag in the
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<structfield>capability</structfield> field.</para></entry>
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</row>
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<row>
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<entry>__u32</entry>
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<entry><structfield>extendedmode</structfield></entry>
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<entry>Custom (driver specific) streaming parameters. When
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unused, applications and drivers must set this field to zero.
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Applications using this field should check the driver name and
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version, see <xref linkend="querycap" />.</entry>
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</row>
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<row>
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<entry>__u32</entry>
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<entry><structfield>readbuffers</structfield></entry>
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<entry>Applications set this field to the desired number
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of buffers used internally by the driver in &func-read; mode. Drivers
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return the actual number of buffers. When an application requests zero
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buffers, drivers should just return the current setting rather than
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the minimum or an error code. For details see <xref
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linkend="rw" />.</entry>
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</row>
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<row>
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<entry>__u32</entry>
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<entry><structfield>reserved</structfield>[4]</entry>
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<entry>Reserved for future extensions. Drivers and
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applications must set the array 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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<table pgwide="1" frame="none" id="v4l2-outputparm">
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<title>struct <structname>v4l2_outputparm</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>capability</structfield></entry>
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<entry>See <xref linkend="parm-caps" />.</entry>
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</row>
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<row>
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<entry>__u32</entry>
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<entry><structfield>outputmode</structfield></entry>
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<entry>Set by drivers and applications, see <xref
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linkend="parm-flags" />.</entry>
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</row>
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<row>
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<entry>&v4l2-fract;</entry>
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<entry><structfield>timeperframe</structfield></entry>
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<entry>This is is the desired period between
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successive frames output by the driver, in seconds.</entry>
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</row>
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<row>
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<entry spanname="hspan"><para>The field is intended to
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repeat frames on the driver side in &func-write; mode (in streaming
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mode timestamps can be used to throttle the output), saving I/O
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bandwidth.</para><para>Applications store here the desired frame
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period, drivers return the actual frame period, which must be greater
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or equal to the nominal frame period determined by the current video
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standard (&v4l2-standard; <structfield>frameperiod</structfield>
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field). Changing the video standard (also implicitly by switching the
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video output) may reset this parameter to the nominal frame period. To
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reset manually applications can just set this field to
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zero.</para><para>Drivers support this function only when they set the
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<constant>V4L2_CAP_TIMEPERFRAME</constant> flag in the
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<structfield>capability</structfield> field.</para></entry>
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</row>
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<row>
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<entry>__u32</entry>
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<entry><structfield>extendedmode</structfield></entry>
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<entry>Custom (driver specific) streaming parameters. When
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unused, applications and drivers must set this field to zero.
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Applications using this field should check the driver name and
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version, see <xref linkend="querycap" />.</entry>
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</row>
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<row>
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<entry>__u32</entry>
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<entry><structfield>writebuffers</structfield></entry>
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<entry>Applications set this field to the desired number
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of buffers used internally by the driver in
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<function>write()</function> mode. Drivers return the actual number of
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buffers. When an application requests zero buffers, drivers should
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just return the current setting rather than the minimum or an error
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code. For details see <xref linkend="rw" />.</entry>
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</row>
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<row>
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<entry>__u32</entry>
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<entry><structfield>reserved</structfield>[4]</entry>
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<entry>Reserved for future extensions. Drivers and
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applications must set the array 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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<table pgwide="1" frame="none" id="parm-caps">
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<title>Streaming Parameters Capabilites</title>
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<tgroup cols="3">
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&cs-def;
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<tbody valign="top">
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<row>
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<entry><constant>V4L2_CAP_TIMEPERFRAME</constant></entry>
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<entry>0x1000</entry>
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<entry>The frame skipping/repeating controlled by the
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<structfield>timeperframe</structfield> field is supported.</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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<table pgwide="1" frame="none" id="parm-flags">
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<title>Capture Parameters Flags</title>
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<tgroup cols="3">
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&cs-def;
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<tbody valign="top">
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<row>
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<entry><constant>V4L2_MODE_HIGHQUALITY</constant></entry>
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<entry>0x0001</entry>
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<entry><para>High quality imaging mode. High quality mode
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is intended for still imaging applications. The idea is to get the
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best possible image quality that the hardware can deliver. It is not
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defined how the driver writer may achieve that; it will depend on the
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hardware and the ingenuity of the driver writer. High quality mode is
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a different mode from the the regular motion video capture modes. In
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high quality mode:<itemizedlist>
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<listitem>
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<para>The driver may be able to capture higher
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resolutions than for motion capture.</para>
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</listitem>
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<listitem>
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<para>The driver may support fewer pixel formats
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than motion capture (eg; true color).</para>
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</listitem>
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<listitem>
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<para>The driver may capture and arithmetically
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combine multiple successive fields or frames to remove color edge
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artifacts and reduce the noise in the video data.
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</para>
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</listitem>
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<listitem>
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<para>The driver may capture images in slices like
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a scanner in order to handle larger format images than would otherwise
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be possible. </para>
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</listitem>
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<listitem>
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<para>An image capture operation may be
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significantly slower than motion capture. </para>
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</listitem>
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<listitem>
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<para>Moving objects in the image might have
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excessive motion blur. </para>
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</listitem>
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<listitem>
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<para>Capture might only work through the
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<function>read()</function> call.</para>
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</listitem>
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</itemizedlist></para></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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</refsect1>
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<refsect1>
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&return-value;
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</refsect1>
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</refentry>
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