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+  <title>Input/Output</title>
+  <para>The V4L2 API defines several different methods to read from or
+write to a device. All drivers exchanging data with applications must
+support at least one of them.</para>
+  <para>The classic I/O method using the <function>read()</function>
+and <function>write()</function> function is automatically selected
+after opening a V4L2 device. When the driver does not support this
+method attempts to read or write will fail at any time.</para>
+  <para>Other methods must be negotiated. To select the streaming I/O
+method with memory mapped or user buffers applications call the
+&VIDIOC-REQBUFS; ioctl. The asynchronous I/O method is not defined
+yet.</para>
+  <para>Video overlay can be considered another I/O method, although
+the application does not directly receive the image data. It is
+selected by initiating video overlay with the &VIDIOC-S-FMT; ioctl.
+For more information see <xref linkend="overlay" />.</para>
+  <para>Generally exactly one I/O method, including overlay, is
+associated with each file descriptor. The only exceptions are
+applications not exchanging data with a driver ("panel applications",
+see <xref linkend="open" />) and drivers permitting simultaneous video capturing
+and overlay using the same file descriptor, for compatibility with V4L
+and earlier versions of V4L2.</para>
+  <para><constant>VIDIOC_S_FMT</constant> and
+<constant>VIDIOC_REQBUFS</constant> would permit this to some degree,
+but for simplicity drivers need not support switching the I/O method
+(after first switching away from read/write) other than by closing
+and reopening the device.</para>
+  <para>The following sections describe the various I/O methods in
+more detail.</para>
+  <section id="rw">
+    <title>Read/Write</title>
+    <para>Input and output devices support the
+<function>read()</function> and <function>write()</function> function,
+respectively, when the <constant>V4L2_CAP_READWRITE</constant> flag in
+the <structfield>capabilities</structfield> field of &v4l2-capability;
+returned by the &VIDIOC-QUERYCAP; ioctl is set.</para>
+    <para>Drivers may need the CPU to copy the data, but they may also
+support DMA to or from user memory, so this I/O method is not
+necessarily less efficient than other methods merely exchanging buffer
+pointers. It is considered inferior though because no meta-information
+like frame counters or timestamps are passed. This information is
+necessary to recognize frame dropping and to synchronize with other
+data streams. However this is also the simplest I/O method, requiring
+little or no setup to exchange data. It permits command line stunts
+like this (the <application>vidctrl</application> tool is
+fictitious):</para>
+    <informalexample>
+      <screen>
+&gt; vidctrl /dev/video --input=0 --format=YUYV --size=352x288
+&gt; dd if=/dev/video of=myimage.422 bs=202752 count=1
+</screen>
+    </informalexample>
+    <para>To read from the device applications use the
+&func-read; function, to write the &func-write; function.
+Drivers must implement one I/O method if they
+exchange data with applications, but it need not be this.<footnote>
+        <para>It would be desirable if applications could depend on
+drivers supporting all I/O interfaces, but as much as the complex
+memory mapping I/O can be inadequate for some devices we have no
+reason to require this interface, which is most useful for simple
+applications capturing still images.</para>
+      </footnote> When reading or writing is supported, the driver
+must also support the &func-select; and &func-poll;
+function.<footnote>
+        <para>At the driver level <function>select()</function> and
+<function>poll()</function> are the same, and
+<function>select()</function> is too important to be optional.</para>
+      </footnote></para>
+  </section>
+  <section id="mmap">
+    <title>Streaming I/O (Memory Mapping)</title>
+    <para>Input and output devices support this I/O method when the
+<constant>V4L2_CAP_STREAMING</constant> flag in the
+<structfield>capabilities</structfield> field of &v4l2-capability;
+returned by the &VIDIOC-QUERYCAP; ioctl is set. There are two
+streaming methods, to determine if the memory mapping flavor is
+supported applications must call the &VIDIOC-REQBUFS; ioctl.</para>
+    <para>Streaming is an I/O method where only pointers to buffers
+are exchanged between application and driver, the data itself is not
+copied. Memory mapping is primarily intended to map buffers in device
+memory into the application's address space. Device memory can be for
+example the video memory on a graphics card with a video capture
+add-on. However, being the most efficient I/O method available for a
+long time, many other drivers support streaming as well, allocating
+buffers in DMA-able main memory.</para>
+    <para>A driver can support many sets of buffers. Each set is
+identified by a unique buffer type value. The sets are independent and
+each set can hold a different type of data. To access different sets
+at the same time different file descriptors must be used.<footnote>
+        <para>One could use one file descriptor and set the buffer
+type field accordingly when calling &VIDIOC-QBUF; etc., but it makes
+the <function>select()</function> function ambiguous. We also like the
+clean approach of one file descriptor per logical stream. Video
+overlay for example is also a logical stream, although the CPU is not
+needed for continuous operation.</para>
+      </footnote></para>
+    <para>To allocate device buffers applications call the
+&VIDIOC-REQBUFS; ioctl with the desired number of buffers and buffer
+type, for example <constant>V4L2_BUF_TYPE_VIDEO_CAPTURE</constant>.
+This ioctl can also be used to change the number of buffers or to free
+the allocated memory, provided none of the buffers are still
+mapped.</para>
+    <para>Before applications can access the buffers they must map
+them into their address space with the &func-mmap; function. The
+location of the buffers in device memory can be determined with the
+&VIDIOC-QUERYBUF; ioctl. In the single-planar API case, the
+<structfield>m.offset</structfield> and <structfield>length</structfield>
+returned in a &v4l2-buffer; are passed as sixth and second parameter to the
+<function>mmap()</function> function. When using the multi-planar API,
+struct &v4l2-buffer; contains an array of &v4l2-plane; structures, each
+containing its own <structfield>m.offset</structfield> and
+<structfield>length</structfield>. When using the multi-planar API, every
+plane of every buffer has to be mapped separately, so the number of
+calls to &func-mmap; should be equal to number of buffers times number of
+planes in each buffer. The offset and length values must not be modified.
+Remember, the buffers are allocated in physical memory, as opposed to virtual
+memory, which can be swapped out to disk. Applications should free the buffers
+as soon as possible with the &func-munmap; function.</para>
+    <example>
+      <title>Mapping buffers in the single-planar API</title>
+      <programlisting>
+&v4l2-requestbuffers; reqbuf;
+struct {
+        void *start;
+        size_t length;
+} *buffers;
+unsigned int i;
+memset(&amp;reqbuf, 0, sizeof(reqbuf));
+reqbuf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
+reqbuf.memory = V4L2_MEMORY_MMAP;
+reqbuf.count = 20;
+if (-1 == ioctl (fd, &VIDIOC-REQBUFS;, &amp;reqbuf)) {
+        if (errno == EINVAL)
+                printf("Video capturing or mmap-streaming is not supported\n");
+        else
+                perror("VIDIOC_REQBUFS");
+        exit(EXIT_FAILURE);
+}
+/* We want at least five buffers. */
+if (reqbuf.count &lt; 5) {
+        /* You may need to free the buffers here. */
+        printf("Not enough buffer memory\n");
+        exit(EXIT_FAILURE);
+}
+buffers = calloc(reqbuf.count, sizeof(*buffers));
+assert(buffers != NULL);
+for (i = 0; i &lt; reqbuf.count; i++) {
+        &v4l2-buffer; buffer;
+        memset(&amp;buffer, 0, sizeof(buffer));
+        buffer.type = reqbuf.type;
+        buffer.memory = V4L2_MEMORY_MMAP;
+        buffer.index = i;
+        if (-1 == ioctl (fd, &VIDIOC-QUERYBUF;, &amp;buffer)) {
+                perror("VIDIOC_QUERYBUF");
+                exit(EXIT_FAILURE);
+        }
+        buffers[i].length = buffer.length; /* remember for munmap() */
+        buffers[i].start = mmap(NULL, buffer.length,
+                                PROT_READ | PROT_WRITE, /* recommended */
+                                MAP_SHARED,             /* recommended */
+                                fd, buffer.m.offset);
+        if (MAP_FAILED == buffers[i].start) {
+                /* If you do not exit here you should unmap() and free()
+                   the buffers mapped so far. */
+                perror("mmap");
+                exit(EXIT_FAILURE);
+        }
+}
+/* Cleanup. */
+for (i = 0; i &lt; reqbuf.count; i++)
+        munmap(buffers[i].start, buffers[i].length);
+      </programlisting>
+    </example>
+    <example>
+      <title>Mapping buffers in the multi-planar API</title>
+      <programlisting>
+&v4l2-requestbuffers; reqbuf;
+/* Our current format uses 3 planes per buffer */
+#define FMT_NUM_PLANES = 3
+struct {
+        void *start[FMT_NUM_PLANES];
+        size_t length[FMT_NUM_PLANES];
+} *buffers;
+unsigned int i, j;
+memset(&amp;reqbuf, 0, sizeof(reqbuf));
+reqbuf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE;
+reqbuf.memory = V4L2_MEMORY_MMAP;
+reqbuf.count = 20;
+if (ioctl(fd, &VIDIOC-REQBUFS;, &amp;reqbuf) &lt; 0) {
+        if (errno == EINVAL)
+                printf("Video capturing or mmap-streaming is not supported\n");
+        else
+                perror("VIDIOC_REQBUFS");
+        exit(EXIT_FAILURE);
+}
+/* We want at least five buffers. */
+if (reqbuf.count &lt; 5) {
+        /* You may need to free the buffers here. */
+        printf("Not enough buffer memory\n");
+        exit(EXIT_FAILURE);
+}
+buffers = calloc(reqbuf.count, sizeof(*buffers));
+assert(buffers != NULL);
+for (i = 0; i &lt; reqbuf.count; i++) {
+        &v4l2-buffer; buffer;
+        &v4l2-plane; planes[FMT_NUM_PLANES];
+        memset(&amp;buffer, 0, sizeof(buffer));
+        buffer.type = reqbuf.type;
+        buffer.memory = V4L2_MEMORY_MMAP;
+        buffer.index = i;
+        /* length in struct v4l2_buffer in multi-planar API stores the size
+         * of planes array. */
+        buffer.length = FMT_NUM_PLANES;
+        buffer.m.planes = planes;
+        if (ioctl(fd, &VIDIOC-QUERYBUF;, &amp;buffer) &lt; 0) {
+                perror("VIDIOC_QUERYBUF");
+                exit(EXIT_FAILURE);
+        }
+        /* Every plane has to be mapped separately */
+        for (j = 0; j &lt; FMT_NUM_PLANES; j++) {
+                buffers[i].length[j] = buffer.m.planes[j].length; /* remember for munmap() */
+                buffers[i].start[j] = mmap(NULL, buffer.m.planes[j].length,
+                                 PROT_READ | PROT_WRITE, /* recommended */
+                                 MAP_SHARED,             /* recommended */
+                                 fd, buffer.m.planes[j].m.offset);
+                if (MAP_FAILED == buffers[i].start[j]) {
+                        /* If you do not exit here you should unmap() and free()
+                           the buffers and planes mapped so far. */
+                        perror("mmap");
+                        exit(EXIT_FAILURE);
+                }
+        }
+}
+/* Cleanup. */
+for (i = 0; i &lt; reqbuf.count; i++)
+        for (j = 0; j &lt; FMT_NUM_PLANES; j++)
+                munmap(buffers[i].start[j], buffers[i].length[j]);
+      </programlisting>
+    </example>
+    <para>Conceptually streaming drivers maintain two buffer queues, an incoming
+and an outgoing queue. They separate the synchronous capture or output
+operation locked to a video clock from the application which is
+subject to random disk or network delays and preemption by
+other processes, thereby reducing the probability of data loss.
+The queues are organized as FIFOs, buffers will be
+output in the order enqueued in the incoming FIFO, and were
+captured in the order dequeued from the outgoing FIFO.</para>
+    <para>The driver may require a minimum number of buffers enqueued
+at all times to function, apart of this no limit exists on the number
+of buffers applications can enqueue in advance, or dequeue and
+process. They can also enqueue in a different order than buffers have
+been dequeued, and the driver can <emphasis>fill</emphasis> enqueued
+<emphasis>empty</emphasis> buffers in any order. <footnote>
+        <para>Random enqueue order permits applications processing
+images out of order (such as video codecs) to return buffers earlier,
+reducing the probability of data loss. Random fill order allows
+drivers to reuse buffers on a LIFO-basis, taking advantage of caches
+holding scatter-gather lists and the like.</para>
+      </footnote> The index number of a buffer (&v4l2-buffer;
+<structfield>index</structfield>) plays no role here, it only
+identifies the buffer.</para>
+    <para>Initially all mapped buffers are in dequeued state,
+inaccessible by the driver. For capturing applications it is customary
+to first enqueue all mapped buffers, then to start capturing and enter
+the read loop. Here the application waits until a filled buffer can be
+dequeued, and re-enqueues the buffer when the data is no longer
+needed. Output applications fill and enqueue buffers, when enough
+buffers are stacked up the output is started with
+<constant>VIDIOC_STREAMON</constant>. In the write loop, when
+the application runs out of free buffers, it must wait until an empty
+buffer can be dequeued and reused.</para>
+    <para>To enqueue and dequeue a buffer applications use the
+&VIDIOC-QBUF; and &VIDIOC-DQBUF; ioctl. The status of a buffer being
+mapped, enqueued, full or empty can be determined at any time using the
+&VIDIOC-QUERYBUF; ioctl. Two methods exist to suspend execution of the
+application until one or more buffers can be dequeued. By default
+<constant>VIDIOC_DQBUF</constant> blocks when no buffer is in the
+outgoing queue. When the <constant>O_NONBLOCK</constant> flag was
+given to the &func-open; function, <constant>VIDIOC_DQBUF</constant>
+returns immediately with an &EAGAIN; when no buffer is available. The
+&func-select; or &func-poll; function are always available.</para>
+    <para>To start and stop capturing or output applications call the
+&VIDIOC-STREAMON; and &VIDIOC-STREAMOFF; ioctl. Note
+<constant>VIDIOC_STREAMOFF</constant> removes all buffers from both
+queues as a side effect. Since there is no notion of doing anything
+"now" on a multitasking system, if an application needs to synchronize
+with another event it should examine the &v4l2-buffer;
+<structfield>timestamp</structfield> of captured buffers, or set the
+field before enqueuing buffers for output.</para>
+    <para>Drivers implementing memory mapping I/O must
+support the <constant>VIDIOC_REQBUFS</constant>,
+<constant>VIDIOC_QUERYBUF</constant>,
+<constant>VIDIOC_QBUF</constant>, <constant>VIDIOC_DQBUF</constant>,
+<constant>VIDIOC_STREAMON</constant> and
+<constant>VIDIOC_STREAMOFF</constant> ioctl, the
+<function>mmap()</function>, <function>munmap()</function>,
+<function>select()</function> and <function>poll()</function>
+function.<footnote>
+        <para>At the driver level <function>select()</function> and
+<function>poll()</function> are the same, and
+<function>select()</function> is too important to be optional. The
+rest should be evident.</para>
+      </footnote></para>
+    <para>[capture example]</para>
+  </section>
+  <section id="userp">
+    <title>Streaming I/O (User Pointers)</title>
+    <para>Input and output devices support this I/O method when the
+<constant>V4L2_CAP_STREAMING</constant> flag in the
+<structfield>capabilities</structfield> field of &v4l2-capability;
+returned by the &VIDIOC-QUERYCAP; ioctl is set. If the particular user
+pointer method (not only memory mapping) is supported must be
+determined by calling the &VIDIOC-REQBUFS; ioctl.</para>
+    <para>This I/O method combines advantages of the read/write and
+memory mapping methods. Buffers (planes) are allocated by the application
+itself, and can reside for example in virtual or shared memory. Only
+pointers to data are exchanged, these pointers and meta-information
+are passed in &v4l2-buffer; (or in &v4l2-plane; in the multi-planar API case).
+The driver must be switched into user pointer I/O mode by calling the
+&VIDIOC-REQBUFS; with the desired buffer type. No buffers (planes) are allocated
+beforehand, consequently they are not indexed and cannot be queried like mapped
+buffers with the <constant>VIDIOC_QUERYBUF</constant> ioctl.</para>
+    <example>
+      <title>Initiating streaming I/O with user pointers</title>
+      <programlisting>
+&v4l2-requestbuffers; reqbuf;
+memset (&amp;reqbuf, 0, sizeof (reqbuf));
+reqbuf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
+reqbuf.memory = V4L2_MEMORY_USERPTR;
+if (ioctl (fd, &VIDIOC-REQBUFS;, &amp;reqbuf) == -1) {
+        if (errno == EINVAL)
+                printf ("Video capturing or user pointer streaming is not supported\n");
+        else
+                perror ("VIDIOC_REQBUFS");
+        exit (EXIT_FAILURE);
+}
+      </programlisting>
+    </example>
+    <para>Buffer (plane) addresses and sizes are passed on the fly with the
+&VIDIOC-QBUF; ioctl. Although buffers are commonly cycled,
+applications can pass different addresses and sizes at each
+<constant>VIDIOC_QBUF</constant> call. If required by the hardware the
+driver swaps memory pages within physical memory to create a
+continuous area of memory. This happens transparently to the
+application in the virtual memory subsystem of the kernel. When buffer
+pages have been swapped out to disk they are brought back and finally
+locked in physical memory for DMA.<footnote>
+        <para>We expect that frequently used buffers are typically not
+swapped out. Anyway, the process of swapping, locking or generating
+scatter-gather lists may be time consuming. The delay can be masked by
+the depth of the incoming buffer queue, and perhaps by maintaining
+caches assuming a buffer will be soon enqueued again. On the other
+hand, to optimize memory usage drivers can limit the number of buffers
+locked in advance and recycle the most recently used buffers first. Of
+course, the pages of empty buffers in the incoming queue need not be
+saved to disk. Output buffers must be saved on the incoming and
+outgoing queue because an application may share them with other
+processes.</para>
+      </footnote></para>
+    <para>Filled or displayed buffers are dequeued with the
+&VIDIOC-DQBUF; ioctl. The driver can unlock the memory pages at any
+time between the completion of the DMA and this ioctl. The memory is
+also unlocked when &VIDIOC-STREAMOFF; is called, &VIDIOC-REQBUFS;, or
+when the device is closed. Applications must take care not to free
+buffers without dequeuing. For once, the buffers remain locked until
+further, wasting physical memory. Second the driver will not be
+notified when the memory is returned to the application's free list
+and subsequently reused for other purposes, possibly completing the
+requested DMA and overwriting valuable data.</para>
+    <para>For capturing applications it is customary to enqueue a
+number of empty buffers, to start capturing and enter the read loop.
+Here the application waits until a filled buffer can be dequeued, and
+re-enqueues the buffer when the data is no longer needed. Output
+applications fill and enqueue buffers, when enough buffers are stacked
+up output is started. In the write loop, when the application
+runs out of free buffers it must wait until an empty buffer can be
+dequeued and reused. Two methods exist to suspend execution of the
+application until one or more buffers can be dequeued. By default
+<constant>VIDIOC_DQBUF</constant> blocks when no buffer is in the
+outgoing queue. When the <constant>O_NONBLOCK</constant> flag was
+given to the &func-open; function, <constant>VIDIOC_DQBUF</constant>
+returns immediately with an &EAGAIN; when no buffer is available. The
+&func-select; or &func-poll; function are always available.</para>
+    <para>To start and stop capturing or output applications call the
+&VIDIOC-STREAMON; and &VIDIOC-STREAMOFF; ioctl. Note
+<constant>VIDIOC_STREAMOFF</constant> removes all buffers from both
+queues and unlocks all buffers as a side effect. Since there is no
+notion of doing anything "now" on a multitasking system, if an
+application needs to synchronize with another event it should examine
+the &v4l2-buffer; <structfield>timestamp</structfield> of captured
+buffers, or set the field before enqueuing buffers for output.</para>
+    <para>Drivers implementing user pointer I/O must
+support the <constant>VIDIOC_REQBUFS</constant>,
+<constant>VIDIOC_QBUF</constant>, <constant>VIDIOC_DQBUF</constant>,
+<constant>VIDIOC_STREAMON</constant> and
+<constant>VIDIOC_STREAMOFF</constant> ioctl, the
+<function>select()</function> and <function>poll()</function> function.<footnote>
+        <para>At the driver level <function>select()</function> and
+<function>poll()</function> are the same, and
+<function>select()</function> is too important to be optional. The
+rest should be evident.</para>
+      </footnote></para>
+  </section>
+  <section id="async">
+    <title>Asynchronous I/O</title>
+    <para>This method is not defined yet.</para>
+  </section>
+  <section id="buffer">
+    <title>Buffers</title>
+    <para>A buffer contains data exchanged by application and
+driver using one of the Streaming I/O methods. In the multi-planar API, the
+data is held in planes, while the buffer structure acts as a container
+for the planes. Only pointers to buffers (planes) are exchanged, the data
+itself is not copied. These pointers, together with meta-information like
+timestamps or field parity, are stored in a struct
+<structname>v4l2_buffer</structname>, argument to
+the &VIDIOC-QUERYBUF;, &VIDIOC-QBUF; and &VIDIOC-DQBUF; ioctl.
+In the multi-planar API, some plane-specific members of struct
+<structname>v4l2_buffer</structname>, such as pointers and sizes for each
+plane, are stored in struct <structname>v4l2_plane</structname> instead.
+In that case, struct <structname>v4l2_buffer</structname> contains an array of
+plane structures.</para>
+      <para>Nominally timestamps refer to the first data byte transmitted.
+In practice however the wide range of hardware covered by the V4L2 API
+limits timestamp accuracy. Often an interrupt routine will
+sample the system clock shortly after the field or frame was stored
+completely in memory. So applications must expect a constant
+difference up to one field or frame period plus a small (few scan
+lines) random error. The delay and error can be much
+larger due to compression or transmission over an external bus when
+the frames are not properly stamped by the sender. This is frequently
+the case with USB cameras. Here timestamps refer to the instant the
+field or frame was received by the driver, not the capture time. These
+devices identify by not enumerating any video standards, see <xref
+linkend="standard" />.</para>
+      <para>Similar limitations apply to output timestamps. Typically
+the video hardware locks to a clock controlling the video timing, the
+horizontal and vertical synchronization pulses. At some point in the
+line sequence, possibly the vertical blanking, an interrupt routine
+samples the system clock, compares against the timestamp and programs
+the hardware to repeat the previous field or frame, or to display the
+buffer contents.</para>
+      <para>Apart of limitations of the video device and natural
+inaccuracies of all clocks, it should be noted system time itself is
+not perfectly stable. It can be affected by power saving cycles,
+warped to insert leap seconds, or even turned back or forth by the
+system administrator affecting long term measurements. <footnote>
+          <para>Since no other Linux multimedia
+API supports unadjusted time it would be foolish to introduce here. We
+must use a universally supported clock to synchronize different media,
+hence time of day.</para>
+        </footnote></para>
+    <table frame="none" pgwide="1" id="v4l2-buffer">
+      <title>struct <structname>v4l2_buffer</structname></title>
+      <tgroup cols="4">
+        &cs-ustr;
+        <tbody valign="top">
+          <row>
+            <entry>__u32</entry>
+            <entry><structfield>index</structfield></entry>
+            <entry></entry>
+            <entry>Number of the buffer, set by the application. This
+field is only used for <link linkend="mmap">memory mapping</link> I/O
+and can range from zero to the number of buffers allocated
+with the &VIDIOC-REQBUFS; ioctl (&v4l2-requestbuffers; <structfield>count</structfield>) minus one.</entry>
+          </row>
+          <row>
+            <entry>&v4l2-buf-type;</entry>
+            <entry><structfield>type</structfield></entry>
+            <entry></entry>
+            <entry>Type of the buffer, same as &v4l2-format;
+<structfield>type</structfield> or &v4l2-requestbuffers;
+<structfield>type</structfield>, set by the application.</entry>
+          </row>
+          <row>
+            <entry>__u32</entry>
+            <entry><structfield>bytesused</structfield></entry>
+            <entry></entry>
+            <entry>The number of bytes occupied by the data in the
+buffer. It depends on the negotiated data format and may change with
+each buffer for compressed variable size data like JPEG images.
+Drivers must set this field when <structfield>type</structfield>
+refers to an input stream, applications when an output stream.</entry>
+          </row>
+          <row>
+            <entry>__u32</entry>
+            <entry><structfield>flags</structfield></entry>
+            <entry></entry>
+            <entry>Flags set by the application or driver, see <xref
+linkend="buffer-flags" />.</entry>
+          </row>
+          <row>
+            <entry>&v4l2-field;</entry>
+            <entry><structfield>field</structfield></entry>
+            <entry></entry>
+            <entry>Indicates the field order of the image in the
+buffer, see <xref linkend="v4l2-field" />. This field is not used when
+the buffer contains VBI data. Drivers must set it when
+<structfield>type</structfield> refers to an input stream,
+applications when an output stream.</entry>
+          </row>
+          <row>
+            <entry>struct timeval</entry>
+            <entry><structfield>timestamp</structfield></entry>
+            <entry></entry>
+            <entry><para>For input streams this is the
+system time (as returned by the <function>gettimeofday()</function>
+function) when the first data byte was captured. For output streams
+the data will not be displayed before this time, secondary to the
+nominal frame rate determined by the current video standard in
+enqueued order. Applications can for example zero this field to
+display frames as soon as possible. The driver stores the time at
+which the first data byte was actually sent out in the
+<structfield>timestamp</structfield> field. This permits
+applications to monitor the drift between the video and system
+clock.</para></entry>
+          </row>
+          <row>
+            <entry>&v4l2-timecode;</entry>
+            <entry><structfield>timecode</structfield></entry>
+            <entry></entry>
+            <entry>When <structfield>type</structfield> is
+<constant>V4L2_BUF_TYPE_VIDEO_CAPTURE</constant> and the
+<constant>V4L2_BUF_FLAG_TIMECODE</constant> flag is set in
+<structfield>flags</structfield>, this structure contains a frame
+timecode. In <link linkend="v4l2-field">V4L2_FIELD_ALTERNATE</link>
+mode the top and bottom field contain the same timecode.
+Timecodes are intended to help video editing and are typically recorded on
+video tapes, but also embedded in compressed formats like MPEG. This
+field is independent of the <structfield>timestamp</structfield> and
+<structfield>sequence</structfield> fields.</entry>
+          </row>
+          <row>
+            <entry>__u32</entry>
+            <entry><structfield>sequence</structfield></entry>
+            <entry></entry>
+            <entry>Set by the driver, counting the frames in the
+sequence.</entry>
+          </row>
+          <row>
+            <entry spanname="hspan"><para>In <link
+linkend="v4l2-field">V4L2_FIELD_ALTERNATE</link> mode the top and
+bottom field have the same sequence number. The count starts at zero
+and includes dropped or repeated frames. A dropped frame was received
+by an input device but could not be stored due to lack of free buffer
+space. A repeated frame was displayed again by an output device
+because the application did not pass new data in
+time.</para><para>Note this may count the frames received
+e.g. over USB, without taking into account the frames dropped by the
+remote hardware due to limited compression throughput or bus
+bandwidth. These devices identify by not enumerating any video
+standards, see <xref linkend="standard" />.</para></entry>
+          </row>
+          <row>
+            <entry>&v4l2-memory;</entry>
+            <entry><structfield>memory</structfield></entry>
+            <entry></entry>
+            <entry>This field must be set by applications and/or drivers
+in accordance with the selected I/O method.</entry>
+          </row>
+          <row>
+            <entry>union</entry>
+            <entry><structfield>m</structfield></entry>
+          </row>
+          <row>
+            <entry></entry>
+            <entry>__u32</entry>
+            <entry><structfield>offset</structfield></entry>
+            <entry>For the single-planar API and when
+<structfield>memory</structfield> is <constant>V4L2_MEMORY_MMAP</constant> this
+is the offset of the buffer from the start of the device memory. The value is
+returned by the driver and apart of serving as parameter to the &func-mmap;
+function not useful for applications. See <xref linkend="mmap" /> for details
+          </entry>
+          </row>
+          <row>
+            <entry></entry>
+            <entry>unsigned long</entry>
+            <entry><structfield>userptr</structfield></entry>
+            <entry>For the single-planar API and when
+<structfield>memory</structfield> is <constant>V4L2_MEMORY_USERPTR</constant>
+this is a pointer to the buffer (casted to unsigned long type) in virtual
+memory, set by the application. See <xref linkend="userp" /> for details.
+            </entry>
+          </row>
+          <row>
+            <entry></entry>
+            <entry>struct v4l2_plane</entry>
+            <entry><structfield>*planes</structfield></entry>
+            <entry>When using the multi-planar API, contains a userspace pointer
+            to an array of &v4l2-plane;. The size of the array should be put
+            in the <structfield>length</structfield> field of this
+            <structname>v4l2_buffer</structname> structure.</entry>
+          </row>
+          <row>
+            <entry>__u32</entry>
+            <entry><structfield>length</structfield></entry>
+            <entry></entry>
+            <entry>Size of the buffer (not the payload) in bytes for the
+            single-planar API. For the multi-planar API should contain the
+            number of elements in the <structfield>planes</structfield> array.
+            </entry>
+          </row>
+          <row>
+            <entry>__u32</entry>
+            <entry><structfield>input</structfield></entry>
+            <entry></entry>
+            <entry>Some video capture drivers support rapid and
+synchronous video input changes, a function useful for example in
+video surveillance applications. For this purpose applications set the
+<constant>V4L2_BUF_FLAG_INPUT</constant> flag, and this field to the
+number of a video input as in &v4l2-input; field
+<structfield>index</structfield>.</entry>
+          </row>
+          <row>
+            <entry>__u32</entry>
+            <entry><structfield>reserved</structfield></entry>
+            <entry></entry>
+            <entry>A place holder for future extensions and custom
+(driver defined) buffer types
+<constant>V4L2_BUF_TYPE_PRIVATE</constant> and higher. Applications
+should set this to 0.</entry>
+          </row>
+        </tbody>
+      </tgroup>
+    </table>
+    <table frame="none" pgwide="1" id="v4l2-plane">
+      <title>struct <structname>v4l2_plane</structname></title>
+      <tgroup cols="4">
+        &cs-ustr;
+        <tbody valign="top">
+          <row>
+            <entry>__u32</entry>
+            <entry><structfield>bytesused</structfield></entry>
+            <entry></entry>
+            <entry>The number of bytes occupied by data in the plane
+            (its payload).</entry>
+          </row>
+          <row>
+            <entry>__u32</entry>
+            <entry><structfield>length</structfield></entry>
+            <entry></entry>
+            <entry>Size in bytes of the plane (not its payload).</entry>
+          </row>
+          <row>
+            <entry>union</entry>
+            <entry><structfield>m</structfield></entry>
+            <entry></entry>
+            <entry></entry>
+          </row>
+          <row>
+            <entry></entry>
+            <entry>__u32</entry>
+            <entry><structfield>mem_offset</structfield></entry>
+            <entry>When the memory type in the containing &v4l2-buffer; is
+              <constant>V4L2_MEMORY_MMAP</constant>, this is the value that
+              should be passed to &func-mmap;, similar to the
+              <structfield>offset</structfield> field in &v4l2-buffer;.</entry>
+          </row>
+          <row>
+            <entry></entry>
+            <entry>__unsigned long</entry>
+            <entry><structfield>userptr</structfield></entry>
+            <entry>When the memory type in the containing &v4l2-buffer; is
+              <constant>V4L2_MEMORY_USERPTR</constant>, this is a userspace
+              pointer to the memory allocated for this plane by an application.
+              </entry>
+          </row>
+          <row>
+            <entry>__u32</entry>
+            <entry><structfield>data_offset</structfield></entry>
+            <entry></entry>
+            <entry>Offset in bytes to video data in the plane, if applicable.
+            </entry>
+          </row>
+          <row>
+            <entry>__u32</entry>
+            <entry><structfield>reserved[11]</structfield></entry>
+            <entry></entry>
+            <entry>Reserved for future use. Should be zeroed by an
+            application.</entry>
+          </row>
+        </tbody>
+      </tgroup>
+    </table>
+    <table frame="none" pgwide="1" id="v4l2-buf-type">
+      <title>enum v4l2_buf_type</title>
+      <tgroup cols="3">
+        &cs-def;
+        <tbody valign="top">
+          <row>
+            <entry><constant>V4L2_BUF_TYPE_VIDEO_CAPTURE</constant></entry>
+            <entry>1</entry>
+            <entry>Buffer of a single-planar video capture stream, see <xref
+                linkend="capture" />.</entry>
+          </row>
+          <row>
+            <entry><constant>V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE</constant>
+            </entry>
+            <entry>9</entry>
+            <entry>Buffer of a multi-planar video capture stream, see <xref
+                linkend="capture" />.</entry>
+          </row>
+          <row>
+            <entry><constant>V4L2_BUF_TYPE_VIDEO_OUTPUT</constant></entry>
+            <entry>2</entry>
+            <entry>Buffer of a single-planar video output stream, see <xref
+                linkend="output" />.</entry>
+          </row>
+          <row>
+            <entry><constant>V4L2_BUF_TYPE_VIDEO_OUTPUT_MPLANE</constant>
+            </entry>
+            <entry>10</entry>
+            <entry>Buffer of a multi-planar video output stream, see <xref
+                linkend="output" />.</entry>
+          </row>
+          <row>
+            <entry><constant>V4L2_BUF_TYPE_VIDEO_OVERLAY</constant></entry>
+            <entry>3</entry>
+            <entry>Buffer for video overlay, see <xref linkend="overlay" />.</entry>
+          </row>
+          <row>
+            <entry><constant>V4L2_BUF_TYPE_VBI_CAPTURE</constant></entry>
+            <entry>4</entry>
+            <entry>Buffer of a raw VBI capture stream, see <xref
+                linkend="raw-vbi" />.</entry>
+          </row>
+          <row>
+            <entry><constant>V4L2_BUF_TYPE_VBI_OUTPUT</constant></entry>
+            <entry>5</entry>
+            <entry>Buffer of a raw VBI output stream, see <xref
+                linkend="raw-vbi" />.</entry>
+          </row>
+          <row>
+            <entry><constant>V4L2_BUF_TYPE_SLICED_VBI_CAPTURE</constant></entry>
+            <entry>6</entry>
+            <entry>Buffer of a sliced VBI capture stream, see <xref
+                linkend="sliced" />.</entry>
+          </row>
+          <row>
+            <entry><constant>V4L2_BUF_TYPE_SLICED_VBI_OUTPUT</constant></entry>
+            <entry>7</entry>
+            <entry>Buffer of a sliced VBI output stream, see <xref
+                linkend="sliced" />.</entry>
+          </row>
+          <row>
+            <entry><constant>V4L2_BUF_TYPE_VIDEO_OUTPUT_OVERLAY</constant></entry>
+            <entry>8</entry>
+            <entry>Buffer for video output overlay (OSD), see <xref
+                linkend="osd" />. Status: <link
+linkend="experimental">Experimental</link>.</entry>
+          </row>
+          <row>
+            <entry><constant>V4L2_BUF_TYPE_PRIVATE</constant></entry>
+            <entry>0x80</entry>
+          <entry>This and higher values are reserved for custom
+(driver defined) buffer types.</entry>
+          </row>
+        </tbody>
+      </tgroup>
+    </table>
+    <table frame="none" pgwide="1" id="buffer-flags">
+      <title>Buffer Flags</title>
+      <tgroup cols="3">
+        &cs-def;
+        <tbody valign="top">
+          <row>
+            <entry><constant>V4L2_BUF_FLAG_MAPPED</constant></entry>
+            <entry>0x0001</entry>
+            <entry>The buffer resides in device memory and has been mapped
+into the application's address space, see <xref linkend="mmap" /> for details.
+Drivers set or clear this flag when the
+<link linkend="vidioc-querybuf">VIDIOC_QUERYBUF</link>, <link
+          linkend="vidioc-qbuf">VIDIOC_QBUF</link> or <link
+          linkend="vidioc-qbuf">VIDIOC_DQBUF</link> ioctl is called. Set by the driver.</entry>
+          </row>
+          <row>
+            <entry><constant>V4L2_BUF_FLAG_QUEUED</constant></entry>
+            <entry>0x0002</entry>
+          <entry>Internally drivers maintain two buffer queues, an
+incoming and outgoing queue. When this flag is set, the buffer is
+currently on the incoming queue. It automatically moves to the
+outgoing queue after the buffer has been filled (capture devices) or
+displayed (output devices). Drivers set or clear this flag when the
+<constant>VIDIOC_QUERYBUF</constant> ioctl is called. After
+(successful) calling the <constant>VIDIOC_QBUF </constant>ioctl it is
+always set and after <constant>VIDIOC_DQBUF</constant> always
+cleared.</entry>
+          </row>
+          <row>
+            <entry><constant>V4L2_BUF_FLAG_DONE</constant></entry>
+            <entry>0x0004</entry>
+            <entry>When this flag is set, the buffer is currently on
+the outgoing queue, ready to be dequeued from the driver. Drivers set
+or clear this flag when the <constant>VIDIOC_QUERYBUF</constant> ioctl
+is called. After calling the <constant>VIDIOC_QBUF</constant> or
+<constant>VIDIOC_DQBUF</constant> it is always cleared. Of course a
+buffer cannot be on both queues at the same time, the
+<constant>V4L2_BUF_FLAG_QUEUED</constant> and
+<constant>V4L2_BUF_FLAG_DONE</constant> flag are mutually exclusive.
+They can be both cleared however, then the buffer is in "dequeued"
+state, in the application domain to say so.</entry>
+          </row>
+          <row>
+            <entry><constant>V4L2_BUF_FLAG_ERROR</constant></entry>
+            <entry>0x0040</entry>
+            <entry>When this flag is set, the buffer has been dequeued
+            successfully, although the data might have been corrupted.
+            This is recoverable, streaming may continue as normal and
+            the buffer may be reused normally.
+            Drivers set this flag when the <constant>VIDIOC_DQBUF</constant>
+            ioctl is called.</entry>
+          </row>
+          <row>
+            <entry><constant>V4L2_BUF_FLAG_KEYFRAME</constant></entry>
+            <entry>0x0008</entry>
+          <entry>Drivers set or clear this flag when calling the
+<constant>VIDIOC_DQBUF</constant> ioctl. It may be set by video
+capture devices when the buffer contains a compressed image which is a
+key frame (or field), &ie; can be decompressed on its own.</entry>
+          </row>
+          <row>
+            <entry><constant>V4L2_BUF_FLAG_PFRAME</constant></entry>
+            <entry>0x0010</entry>
+            <entry>Similar to <constant>V4L2_BUF_FLAG_KEYFRAME</constant>
+this flags predicted frames or fields which contain only differences to a
+previous key frame.</entry>
+          </row>
+          <row>
+            <entry><constant>V4L2_BUF_FLAG_BFRAME</constant></entry>
+            <entry>0x0020</entry>
+            <entry>Similar to <constant>V4L2_BUF_FLAG_PFRAME</constant>
+        this is a bidirectional predicted frame or field. [ooc tbd]</entry>
+          </row>
+          <row>
+            <entry><constant>V4L2_BUF_FLAG_TIMECODE</constant></entry>
+            <entry>0x0100</entry>
+            <entry>The <structfield>timecode</structfield> field is valid.
+Drivers set or clear this flag when the <constant>VIDIOC_DQBUF</constant>
+ioctl is called.</entry>
+          </row>
+          <row>
+            <entry><constant>V4L2_BUF_FLAG_INPUT</constant></entry>
+            <entry>0x0200</entry>
+            <entry>The <structfield>input</structfield> field is valid.
+Applications set or clear this flag before calling the
+<constant>VIDIOC_QBUF</constant> ioctl.</entry>
+          </row>
+        </tbody>
+      </tgroup>
+    </table>
+    <table pgwide="1" frame="none" id="v4l2-memory">
+      <title>enum v4l2_memory</title>
+      <tgroup cols="3">
+        &cs-def;
+        <tbody valign="top">
+          <row>
+            <entry><constant>V4L2_MEMORY_MMAP</constant></entry>
+            <entry>1</entry>
+            <entry>The buffer is used for <link linkend="mmap">memory
+mapping</link> I/O.</entry>
+          </row>
+          <row>
+            <entry><constant>V4L2_MEMORY_USERPTR</constant></entry>
+            <entry>2</entry>
+            <entry>The buffer is used for <link linkend="userp">user
+pointer</link> I/O.</entry>
+          </row>
+          <row>
+            <entry><constant>V4L2_MEMORY_OVERLAY</constant></entry>
+            <entry>3</entry>
+            <entry>[to do]</entry>
+          </row>
+        </tbody>
+      </tgroup>
+    </table>
+    <section>
+      <title>Timecodes</title>
+      <para>The <structname>v4l2_timecode</structname> structure is
+designed to hold a <xref linkend="smpte12m" /> or similar timecode.
+(struct <structname>timeval</structname> timestamps are stored in
+&v4l2-buffer; field <structfield>timestamp</structfield>.)</para>
+      <table frame="none" pgwide="1" id="v4l2-timecode">
+        <title>struct <structname>v4l2_timecode</structname></title>
+        <tgroup cols="3">
+          &cs-str;
+          <tbody valign="top">
+            <row>
+              <entry>__u32</entry>
+              <entry><structfield>type</structfield></entry>
+              <entry>Frame rate the timecodes are based on, see <xref
+                  linkend="timecode-type" />.</entry>
+            </row>
+            <row>
+              <entry>__u32</entry>
+              <entry><structfield>flags</structfield></entry>
+              <entry>Timecode flags, see <xref linkend="timecode-flags" />.</entry>
+            </row>
+            <row>
+              <entry>__u8</entry>
+              <entry><structfield>frames</structfield></entry>
+              <entry>Frame count, 0 ... 23/24/29/49/59, depending on the
+            type of timecode.</entry>
+            </row>
+            <row>
+              <entry>__u8</entry>
+              <entry><structfield>seconds</structfield></entry>
+              <entry>Seconds count, 0 ... 59. This is a binary, not BCD number.</entry>
+            </row>
+            <row>
+              <entry>__u8</entry>
+              <entry><structfield>minutes</structfield></entry>
+              <entry>Minutes count, 0 ... 59. This is a binary, not BCD number.</entry>
+            </row>
+            <row>
+              <entry>__u8</entry>
+              <entry><structfield>hours</structfield></entry>
+              <entry>Hours count, 0 ... 29. This is a binary, not BCD number.</entry>
+            </row>
+            <row>
+              <entry>__u8</entry>
+              <entry><structfield>userbits</structfield>[4]</entry>
+              <entry>The "user group" bits from the timecode.</entry>
+            </row>
+          </tbody>
+        </tgroup>
+      </table>
+      <table frame="none" pgwide="1" id="timecode-type">
+        <title>Timecode Types</title>
+        <tgroup cols="3">
+        &cs-def;
+          <tbody valign="top">
+            <row>
+              <entry><constant>V4L2_TC_TYPE_24FPS</constant></entry>
+              <entry>1</entry>
+              <entry>24 frames per second, i.&nbsp;e. film.</entry>
+            </row>
+            <row>
+              <entry><constant>V4L2_TC_TYPE_25FPS</constant></entry>
+              <entry>2</entry>
+              <entry>25 frames per second, &ie; PAL or SECAM video.</entry>
+            </row>
+            <row>
+              <entry><constant>V4L2_TC_TYPE_30FPS</constant></entry>
+              <entry>3</entry>
+              <entry>30 frames per second, &ie; NTSC video.</entry>
+            </row>
+            <row>
+              <entry><constant>V4L2_TC_TYPE_50FPS</constant></entry>
+              <entry>4</entry>
+              <entry></entry>
+            </row>
+            <row>
+              <entry><constant>V4L2_TC_TYPE_60FPS</constant></entry>
+              <entry>5</entry>
+              <entry></entry>
+            </row>
+          </tbody>
+        </tgroup>
+      </table>
+      <table frame="none" pgwide="1" id="timecode-flags">
+        <title>Timecode Flags</title>
+        <tgroup cols="3">
+        &cs-def;
+          <tbody valign="top">
+            <row>
+              <entry><constant>V4L2_TC_FLAG_DROPFRAME</constant></entry>
+              <entry>0x0001</entry>
+              <entry>Indicates "drop frame" semantics for counting frames
+in 29.97 fps material. When set, frame numbers 0 and 1 at the start of
+each minute, except minutes 0, 10, 20, 30, 40, 50 are omitted from the
+count.</entry>
+            </row>
+            <row>
+              <entry><constant>V4L2_TC_FLAG_COLORFRAME</constant></entry>
+              <entry>0x0002</entry>
+              <entry>The "color frame" flag.</entry>
+            </row>
+            <row>
+              <entry><constant>V4L2_TC_USERBITS_field</constant></entry>
+              <entry>0x000C</entry>
+              <entry>Field mask for the "binary group flags".</entry>
+            </row>
+            <row>
+              <entry><constant>V4L2_TC_USERBITS_USERDEFINED</constant></entry>
+              <entry>0x0000</entry>
+              <entry>Unspecified format.</entry>
+            </row>
+            <row>
+              <entry><constant>V4L2_TC_USERBITS_8BITCHARS</constant></entry>
+              <entry>0x0008</entry>
+              <entry>8-bit ISO characters.</entry>
+            </row>
+          </tbody>
+        </tgroup>
+      </table>
+    </section>
+  </section>
+  <section id="field-order">
+    <title>Field Order</title>
+    <para>We have to distinguish between progressive and interlaced
+video. Progressive video transmits all lines of a video image
+sequentially. Interlaced video divides an image into two fields,
+containing only the odd and even lines of the image, respectively.
+Alternating the so called odd and even field are transmitted, and due
+to a small delay between fields a cathode ray TV displays the lines
+interleaved, yielding the original frame. This curious technique was
+invented because at refresh rates similar to film the image would
+fade out too quickly. Transmitting fields reduces the flicker without
+the necessity of doubling the frame rate and with it the bandwidth
+required for each channel.</para>
+    <para>It is important to understand a video camera does not expose
+one frame at a time, merely transmitting the frames separated into
+fields. The fields are in fact captured at two different instances in
+time. An object on screen may well move between one field and the
+next. For applications analysing motion it is of paramount importance
+to recognize which field of a frame is older, the <emphasis>temporal
+order</emphasis>.</para>
+    <para>When the driver provides or accepts images field by field
+rather than interleaved, it is also important applications understand
+how the fields combine to frames. We distinguish between top (aka odd) and
+bottom (aka even) fields, the <emphasis>spatial order</emphasis>: The first line
+of the top field is the first line of an interlaced frame, the first
+line of the bottom field is the second line of that frame.</para>
+    <para>However because fields were captured one after the other,
+arguing whether a frame commences with the top or bottom field is
+pointless. Any two successive top and bottom, or bottom and top fields
+yield a valid frame. Only when the source was progressive to begin
+with, &eg; when transferring film to video, two fields may come from
+the same frame, creating a natural order.</para>
+    <para>Counter to intuition the top field is not necessarily the
+older field. Whether the older field contains the top or bottom lines
+is a convention determined by the video standard. Hence the
+distinction between temporal and spatial order of fields. The diagrams
+below should make this clearer.</para>
+    <para>All video capture and output devices must report the current
+field order. Some drivers may permit the selection of a different
+order, to this end applications initialize the
+<structfield>field</structfield> field of &v4l2-pix-format; before
+calling the &VIDIOC-S-FMT; ioctl. If this is not desired it should
+have the value <constant>V4L2_FIELD_ANY</constant> (0).</para>
+    <table frame="none" pgwide="1" id="v4l2-field">
+      <title>enum v4l2_field</title>
+      <tgroup cols="3">
+        &cs-def;
+        <tbody valign="top">
+          <row>
+            <entry><constant>V4L2_FIELD_ANY</constant></entry>
+            <entry>0</entry>
+            <entry>Applications request this field order when any
+one of the <constant>V4L2_FIELD_NONE</constant>,
+<constant>V4L2_FIELD_TOP</constant>,
+<constant>V4L2_FIELD_BOTTOM</constant>, or
+<constant>V4L2_FIELD_INTERLACED</constant> formats is acceptable.
+Drivers choose depending on hardware capabilities or e.&nbsp;g. the
+requested image size, and return the actual field order. &v4l2-buffer;
+<structfield>field</structfield> can never be
+<constant>V4L2_FIELD_ANY</constant>.</entry>
+          </row>
+          <row>
+            <entry><constant>V4L2_FIELD_NONE</constant></entry>
+            <entry>1</entry>
+            <entry>Images are in progressive format, not interlaced.
+The driver may also indicate this order when it cannot distinguish
+between <constant>V4L2_FIELD_TOP</constant> and
+<constant>V4L2_FIELD_BOTTOM</constant>.</entry>
+          </row>
+          <row>
+            <entry><constant>V4L2_FIELD_TOP</constant></entry>
+            <entry>2</entry>
+            <entry>Images consist of the top (aka odd) field only.</entry>
+          </row>
+          <row>
+            <entry><constant>V4L2_FIELD_BOTTOM</constant></entry>
+            <entry>3</entry>
+            <entry>Images consist of the bottom (aka even) field only.
+Applications may wish to prevent a device from capturing interlaced
+images because they will have "comb" or "feathering" artefacts around
+moving objects.</entry>
+          </row>
+          <row>
+            <entry><constant>V4L2_FIELD_INTERLACED</constant></entry>
+            <entry>4</entry>
+            <entry>Images contain both fields, interleaved line by
+line. The temporal order of the fields (whether the top or bottom
+field is first transmitted) depends on the current video standard.
+M/NTSC transmits the bottom field first, all other standards the top
+field first.</entry>
+          </row>
+          <row>
+            <entry><constant>V4L2_FIELD_SEQ_TB</constant></entry>
+            <entry>5</entry>
+            <entry>Images contain both fields, the top field lines
+are stored first in memory, immediately followed by the bottom field
+lines. Fields are always stored in temporal order, the older one first
+in memory. Image sizes refer to the frame, not fields.</entry>
+          </row>
+          <row>
+            <entry><constant>V4L2_FIELD_SEQ_BT</constant></entry>
+            <entry>6</entry>
+            <entry>Images contain both fields, the bottom field
+lines are stored first in memory, immediately followed by the top
+field lines. Fields are always stored in temporal order, the older one
+first in memory. Image sizes refer to the frame, not fields.</entry>
+          </row>
+          <row>
+            <entry><constant>V4L2_FIELD_ALTERNATE</constant></entry>
+            <entry>7</entry>
+            <entry>The two fields of a frame are passed in separate
+buffers, in temporal order, &ie; the older one first. To indicate the field
+parity (whether the current field is a top or bottom field) the driver
+or application, depending on data direction, must set &v4l2-buffer;
+<structfield>field</structfield> to
+<constant>V4L2_FIELD_TOP</constant> or
+<constant>V4L2_FIELD_BOTTOM</constant>. Any two successive fields pair
+to build a frame. If fields are successive, without any dropped fields
+between them (fields can drop individually), can be determined from
+the &v4l2-buffer; <structfield>sequence</structfield> field. Image
+sizes refer to the frame, not fields. This format cannot be selected
+when using the read/write I/O method.<!-- Where it's indistinguishable
+from V4L2_FIELD_SEQ_*. --></entry>
+          </row>
+          <row>
+            <entry><constant>V4L2_FIELD_INTERLACED_TB</constant></entry>
+            <entry>8</entry>
+            <entry>Images contain both fields, interleaved line by
+line, top field first. The top field is transmitted first.</entry>
+          </row>
+          <row>
+            <entry><constant>V4L2_FIELD_INTERLACED_BT</constant></entry>
+            <entry>9</entry>
+            <entry>Images contain both fields, interleaved line by
+line, top field first. The bottom field is transmitted first.</entry>
+          </row>
+        </tbody>
+      </tgroup>
+    </table>
+    <figure id="fieldseq-tb">
+        <title>Field Order, Top Field First Transmitted</title>
+        <mediaobject>
+          <imageobject>
+            <imagedata fileref="fieldseq_tb.pdf" format="PS" />
+          </imageobject>
+          <imageobject>
+            <imagedata fileref="fieldseq_tb.gif" format="GIF" />
+          </imageobject>
+        </mediaobject>
+    </figure>
+    <figure id="fieldseq-bt">
+        <title>Field Order, Bottom Field First Transmitted</title>
+        <mediaobject>
+          <imageobject>
+            <imagedata fileref="fieldseq_bt.pdf" format="PS" />
+          </imageobject>
+          <imageobject>
+            <imagedata fileref="fieldseq_bt.gif" format="GIF" />
+          </imageobject>
+        </mediaobject>
+    </figure>
+  </section>
+  <!--
+Local Variables:
+mode: sgml
+sgml-parent-document: "v4l2.sgml"
+indent-tabs-mode: nil
+End:
+  -->