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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>
197 lines
6.9 KiB
XML
197 lines
6.9 KiB
XML
<refentry id="vidioc-g-fmt">
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<refmeta>
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<refentrytitle>ioctl VIDIOC_G_FMT, VIDIOC_S_FMT,
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VIDIOC_TRY_FMT</refentrytitle>
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&manvol;
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</refmeta>
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<refnamediv>
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<refname>VIDIOC_G_FMT</refname>
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<refname>VIDIOC_S_FMT</refname>
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<refname>VIDIOC_TRY_FMT</refname>
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<refpurpose>Get or set the data format, try a format</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>struct v4l2_format
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*<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_FMT, VIDIOC_S_FMT, VIDIOC_TRY_FMT</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>These ioctls are used to negotiate the format of data
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(typically image format) exchanged between driver and
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application.</para>
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<para>To query the current parameters applications set the
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<structfield>type</structfield> field of a struct
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<structname>v4l2_format</structname> to the respective buffer (stream)
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type. For example video capture devices use
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<constant>V4L2_BUF_TYPE_VIDEO_CAPTURE</constant> or
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<constant>V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE</constant>. When the application
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calls the <constant>VIDIOC_G_FMT</constant> ioctl with a pointer to
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this structure the driver fills the respective member of the
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<structfield>fmt</structfield> union. In case of video capture devices
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that is either the &v4l2-pix-format; <structfield>pix</structfield> or
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the &v4l2-pix-format-mplane; <structfield>pix_mp</structfield> member.
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When the requested buffer type is not supported drivers return an
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&EINVAL;.</para>
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<para>To change the current format parameters applications
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initialize the <structfield>type</structfield> field and all
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fields of the respective <structfield>fmt</structfield>
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union member. For details see the documentation of the various devices
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types in <xref linkend="devices" />. Good practice is to query the
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current parameters first, and to
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modify only those parameters not suitable for the application. When
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the application calls the <constant>VIDIOC_S_FMT</constant> ioctl
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with a pointer to a <structname>v4l2_format</structname> structure
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the driver checks
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and adjusts the parameters against hardware abilities. Drivers
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should not return an error code unless the input is ambiguous, this is
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a mechanism to fathom device capabilities and to approach parameters
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acceptable for both the application and driver. On success the driver
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may program the hardware, allocate resources and generally prepare for
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data exchange.
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Finally the <constant>VIDIOC_S_FMT</constant> ioctl returns the
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current format parameters as <constant>VIDIOC_G_FMT</constant> does.
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Very simple, inflexible devices may even ignore all input and always
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return the default parameters. However all V4L2 devices exchanging
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data with the application must implement the
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<constant>VIDIOC_G_FMT</constant> and
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<constant>VIDIOC_S_FMT</constant> ioctl. When the requested buffer
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type is not supported drivers return an &EINVAL; on a
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<constant>VIDIOC_S_FMT</constant> attempt. When I/O is already in
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progress or the resource is not available for other reasons drivers
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return the &EBUSY;.</para>
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<para>The <constant>VIDIOC_TRY_FMT</constant> ioctl is equivalent
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to <constant>VIDIOC_S_FMT</constant> with one exception: it does not
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change driver state. It can also be called at any time, never
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returning <errorcode>EBUSY</errorcode>. This function is provided to
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negotiate parameters, to learn about hardware limitations, without
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disabling I/O or possibly time consuming hardware preparations.
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Although strongly recommended drivers are not required to implement
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this ioctl.</para>
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<table pgwide="1" frame="none" id="v4l2-format">
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<title>struct <structname>v4l2_format</structname></title>
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<tgroup cols="4">
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<colspec colname="c1" />
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<colspec colname="c2" />
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<colspec colname="c3" />
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<colspec colname="c4" />
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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>Type of the data stream, 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>fmt</structfield></entry>
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</row>
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<row>
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<entry></entry>
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<entry>&v4l2-pix-format;</entry>
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<entry><structfield>pix</structfield></entry>
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<entry>Definition of an image format, see <xref
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linkend="pixfmt" />, used by video capture and output
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devices.</entry>
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</row>
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<row>
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<entry></entry>
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<entry>&v4l2-pix-format-mplane;</entry>
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<entry><structfield>pix_mp</structfield></entry>
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<entry>Definition of an image format, see <xref
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linkend="pixfmt" />, used by video capture and output
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devices that support the <link linkend="planar-apis">multi-planar
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version of the API</link>.</entry>
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</row>
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<row>
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<entry></entry>
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<entry>&v4l2-window;</entry>
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<entry><structfield>win</structfield></entry>
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<entry>Definition of an overlaid image, see <xref
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linkend="overlay" />, used by video overlay devices.</entry>
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</row>
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<row>
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<entry></entry>
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<entry>&v4l2-vbi-format;</entry>
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<entry><structfield>vbi</structfield></entry>
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<entry>Raw VBI capture or output parameters. This is
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discussed in more detail in <xref linkend="raw-vbi" />. Used by raw VBI
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capture and output devices.</entry>
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</row>
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<row>
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<entry></entry>
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<entry>&v4l2-sliced-vbi-format;</entry>
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<entry><structfield>sliced</structfield></entry>
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<entry>Sliced VBI capture or output parameters. See
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<xref linkend="sliced" /> for details. Used by sliced VBI
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capture and output devices.</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>Place holder for future extensions and custom
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(driver defined) formats with <structfield>type</structfield>
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<constant>V4L2_BUF_TYPE_PRIVATE</constant> and 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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</refsect1>
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<refsect1>
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&return-value;
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<variablelist>
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<varlistentry>
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<term><errorcode>EINVAL</errorcode></term>
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<listitem>
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<para>The &v4l2-format; <structfield>type</structfield>
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field is invalid, the requested buffer type not supported, or the
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format is not supported with this buffer type.</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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</refentry>
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