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Based on kernel version 3.13. Page generated on 2014-01-20 22:02 EST.

1	  <title>Input/Output</title>
2	
3	  <para>The V4L2 API defines several different methods to read from or
4	write to a device. All drivers exchanging data with applications must
5	support at least one of them.</para>
6	
7	  <para>The classic I/O method using the <function>read()</function>
8	and <function>write()</function> function is automatically selected
9	after opening a V4L2 device. When the driver does not support this
10	method attempts to read or write will fail at any time.</para>
11	
12	  <para>Other methods must be negotiated. To select the streaming I/O
13	method with memory mapped or user buffers applications call the
14	&VIDIOC-REQBUFS; ioctl. The asynchronous I/O method is not defined
15	yet.</para>
16	
17	  <para>Video overlay can be considered another I/O method, although
18	the application does not directly receive the image data. It is
19	selected by initiating video overlay with the &VIDIOC-S-FMT; ioctl.
20	For more information see <xref linkend="overlay" />.</para>
21	
22	  <para>Generally exactly one I/O method, including overlay, is
23	associated with each file descriptor. The only exceptions are
24	applications not exchanging data with a driver ("panel applications",
25	see <xref linkend="open" />) and drivers permitting simultaneous video capturing
26	and overlay using the same file descriptor, for compatibility with V4L
27	and earlier versions of V4L2.</para>
28	
29	  <para><constant>VIDIOC_S_FMT</constant> and
30	<constant>VIDIOC_REQBUFS</constant> would permit this to some degree,
31	but for simplicity drivers need not support switching the I/O method
32	(after first switching away from read/write) other than by closing
33	and reopening the device.</para>
34	
35	  <para>The following sections describe the various I/O methods in
36	more detail.</para>
37	
38	  <section id="rw">
39	    <title>Read/Write</title>
40	
41	    <para>Input and output devices support the
42	<function>read()</function> and <function>write()</function> function,
43	respectively, when the <constant>V4L2_CAP_READWRITE</constant> flag in
44	the <structfield>capabilities</structfield> field of &v4l2-capability;
45	returned by the &VIDIOC-QUERYCAP; ioctl is set.</para>
46	
47	    <para>Drivers may need the CPU to copy the data, but they may also
48	support DMA to or from user memory, so this I/O method is not
49	necessarily less efficient than other methods merely exchanging buffer
50	pointers. It is considered inferior though because no meta-information
51	like frame counters or timestamps are passed. This information is
52	necessary to recognize frame dropping and to synchronize with other
53	data streams. However this is also the simplest I/O method, requiring
54	little or no setup to exchange data. It permits command line stunts
55	like this (the <application>vidctrl</application> tool is
56	fictitious):</para>
57	
58	    <informalexample>
59	      <screen>
60	&gt; vidctrl /dev/video --input=0 --format=YUYV --size=352x288
61	&gt; dd if=/dev/video of=myimage.422 bs=202752 count=1
62	</screen>
63	    </informalexample>
64	
65	    <para>To read from the device applications use the
66	&func-read; function, to write the &func-write; function.
67	Drivers must implement one I/O method if they
68	exchange data with applications, but it need not be this.<footnote>
69		<para>It would be desirable if applications could depend on
70	drivers supporting all I/O interfaces, but as much as the complex
71	memory mapping I/O can be inadequate for some devices we have no
72	reason to require this interface, which is most useful for simple
73	applications capturing still images.</para>
74	      </footnote> When reading or writing is supported, the driver
75	must also support the &func-select; and &func-poll;
76	function.<footnote>
77		<para>At the driver level <function>select()</function> and
78	<function>poll()</function> are the same, and
79	<function>select()</function> is too important to be optional.</para>
80	      </footnote></para>
81	  </section>
82	
83	  <section id="mmap">
84	    <title>Streaming I/O (Memory Mapping)</title>
85	
86	    <para>Input and output devices support this I/O method when the
87	<constant>V4L2_CAP_STREAMING</constant> flag in the
88	<structfield>capabilities</structfield> field of &v4l2-capability;
89	returned by the &VIDIOC-QUERYCAP; ioctl is set. There are two
90	streaming methods, to determine if the memory mapping flavor is
91	supported applications must call the &VIDIOC-REQBUFS; ioctl.</para>
92	
93	    <para>Streaming is an I/O method where only pointers to buffers
94	are exchanged between application and driver, the data itself is not
95	copied. Memory mapping is primarily intended to map buffers in device
96	memory into the application's address space. Device memory can be for
97	example the video memory on a graphics card with a video capture
98	add-on. However, being the most efficient I/O method available for a
99	long time, many other drivers support streaming as well, allocating
100	buffers in DMA-able main memory.</para>
101	
102	    <para>A driver can support many sets of buffers. Each set is
103	identified by a unique buffer type value. The sets are independent and
104	each set can hold a different type of data. To access different sets
105	at the same time different file descriptors must be used.<footnote>
106		<para>One could use one file descriptor and set the buffer
107	type field accordingly when calling &VIDIOC-QBUF; etc., but it makes
108	the <function>select()</function> function ambiguous. We also like the
109	clean approach of one file descriptor per logical stream. Video
110	overlay for example is also a logical stream, although the CPU is not
111	needed for continuous operation.</para>
112	      </footnote></para>
113	
114	    <para>To allocate device buffers applications call the
115	&VIDIOC-REQBUFS; ioctl with the desired number of buffers and buffer
116	type, for example <constant>V4L2_BUF_TYPE_VIDEO_CAPTURE</constant>.
117	This ioctl can also be used to change the number of buffers or to free
118	the allocated memory, provided none of the buffers are still
119	mapped.</para>
120	
121	    <para>Before applications can access the buffers they must map
122	them into their address space with the &func-mmap; function. The
123	location of the buffers in device memory can be determined with the
124	&VIDIOC-QUERYBUF; ioctl. In the single-planar API case, the
125	<structfield>m.offset</structfield> and <structfield>length</structfield>
126	returned in a &v4l2-buffer; are passed as sixth and second parameter to the
127	<function>mmap()</function> function. When using the multi-planar API,
128	struct &v4l2-buffer; contains an array of &v4l2-plane; structures, each
129	containing its own <structfield>m.offset</structfield> and
130	<structfield>length</structfield>. When using the multi-planar API, every
131	plane of every buffer has to be mapped separately, so the number of
132	calls to &func-mmap; should be equal to number of buffers times number of
133	planes in each buffer. The offset and length values must not be modified.
134	Remember, the buffers are allocated in physical memory, as opposed to virtual
135	memory, which can be swapped out to disk. Applications should free the buffers
136	as soon as possible with the &func-munmap; function.</para>
137	
138	    <example>
139	      <title>Mapping buffers in the single-planar API</title>
140	      <programlisting>
141	&v4l2-requestbuffers; reqbuf;
142	struct {
143		void *start;
144		size_t length;
145	} *buffers;
146	unsigned int i;
147	
148	memset(&amp;reqbuf, 0, sizeof(reqbuf));
149	reqbuf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
150	reqbuf.memory = V4L2_MEMORY_MMAP;
151	reqbuf.count = 20;
152	
153	if (-1 == ioctl (fd, &VIDIOC-REQBUFS;, &amp;reqbuf)) {
154		if (errno == EINVAL)
155			printf("Video capturing or mmap-streaming is not supported\n");
156		else
157			perror("VIDIOC_REQBUFS");
158	
159		exit(EXIT_FAILURE);
160	}
161	
162	/* We want at least five buffers. */
163	
164	if (reqbuf.count &lt; 5) {
165		/* You may need to free the buffers here. */
166		printf("Not enough buffer memory\n");
167		exit(EXIT_FAILURE);
168	}
169	
170	buffers = calloc(reqbuf.count, sizeof(*buffers));
171	assert(buffers != NULL);
172	
173	for (i = 0; i &lt; reqbuf.count; i++) {
174		&v4l2-buffer; buffer;
175	
176		memset(&amp;buffer, 0, sizeof(buffer));
177		buffer.type = reqbuf.type;
178		buffer.memory = V4L2_MEMORY_MMAP;
179		buffer.index = i;
180	
181		if (-1 == ioctl (fd, &VIDIOC-QUERYBUF;, &amp;buffer)) {
182			perror("VIDIOC_QUERYBUF");
183			exit(EXIT_FAILURE);
184		}
185	
186		buffers[i].length = buffer.length; /* remember for munmap() */
187	
188		buffers[i].start = mmap(NULL, buffer.length,
189					PROT_READ | PROT_WRITE, /* recommended */
190					MAP_SHARED,             /* recommended */
191					fd, buffer.m.offset);
192	
193		if (MAP_FAILED == buffers[i].start) {
194			/* If you do not exit here you should unmap() and free()
195			   the buffers mapped so far. */
196			perror("mmap");
197			exit(EXIT_FAILURE);
198		}
199	}
200	
201	/* Cleanup. */
202	
203	for (i = 0; i &lt; reqbuf.count; i++)
204		munmap(buffers[i].start, buffers[i].length);
205	      </programlisting>
206	    </example>
207	
208	    <example>
209	      <title>Mapping buffers in the multi-planar API</title>
210	      <programlisting>
211	&v4l2-requestbuffers; reqbuf;
212	/* Our current format uses 3 planes per buffer */
213	#define FMT_NUM_PLANES = 3
214	
215	struct {
216		void *start[FMT_NUM_PLANES];
217		size_t length[FMT_NUM_PLANES];
218	} *buffers;
219	unsigned int i, j;
220	
221	memset(&amp;reqbuf, 0, sizeof(reqbuf));
222	reqbuf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE;
223	reqbuf.memory = V4L2_MEMORY_MMAP;
224	reqbuf.count = 20;
225	
226	if (ioctl(fd, &VIDIOC-REQBUFS;, &amp;reqbuf) &lt; 0) {
227		if (errno == EINVAL)
228			printf("Video capturing or mmap-streaming is not supported\n");
229		else
230			perror("VIDIOC_REQBUFS");
231	
232		exit(EXIT_FAILURE);
233	}
234	
235	/* We want at least five buffers. */
236	
237	if (reqbuf.count &lt; 5) {
238		/* You may need to free the buffers here. */
239		printf("Not enough buffer memory\n");
240		exit(EXIT_FAILURE);
241	}
242	
243	buffers = calloc(reqbuf.count, sizeof(*buffers));
244	assert(buffers != NULL);
245	
246	for (i = 0; i &lt; reqbuf.count; i++) {
247		&v4l2-buffer; buffer;
248		&v4l2-plane; planes[FMT_NUM_PLANES];
249	
250		memset(&amp;buffer, 0, sizeof(buffer));
251		buffer.type = reqbuf.type;
252		buffer.memory = V4L2_MEMORY_MMAP;
253		buffer.index = i;
254		/* length in struct v4l2_buffer in multi-planar API stores the size
255		 * of planes array. */
256		buffer.length = FMT_NUM_PLANES;
257		buffer.m.planes = planes;
258	
259		if (ioctl(fd, &VIDIOC-QUERYBUF;, &amp;buffer) &lt; 0) {
260			perror("VIDIOC_QUERYBUF");
261			exit(EXIT_FAILURE);
262		}
263	
264		/* Every plane has to be mapped separately */
265		for (j = 0; j &lt; FMT_NUM_PLANES; j++) {
266			buffers[i].length[j] = buffer.m.planes[j].length; /* remember for munmap() */
267	
268			buffers[i].start[j] = mmap(NULL, buffer.m.planes[j].length,
269					 PROT_READ | PROT_WRITE, /* recommended */
270					 MAP_SHARED,             /* recommended */
271					 fd, buffer.m.planes[j].m.offset);
272	
273			if (MAP_FAILED == buffers[i].start[j]) {
274				/* If you do not exit here you should unmap() and free()
275				   the buffers and planes mapped so far. */
276				perror("mmap");
277				exit(EXIT_FAILURE);
278			}
279		}
280	}
281	
282	/* Cleanup. */
283	
284	for (i = 0; i &lt; reqbuf.count; i++)
285		for (j = 0; j &lt; FMT_NUM_PLANES; j++)
286			munmap(buffers[i].start[j], buffers[i].length[j]);
287	      </programlisting>
288	    </example>
289	
290	    <para>Conceptually streaming drivers maintain two buffer queues, an incoming
291	and an outgoing queue. They separate the synchronous capture or output
292	operation locked to a video clock from the application which is
293	subject to random disk or network delays and preemption by
294	other processes, thereby reducing the probability of data loss.
295	The queues are organized as FIFOs, buffers will be
296	output in the order enqueued in the incoming FIFO, and were
297	captured in the order dequeued from the outgoing FIFO.</para>
298	
299	    <para>The driver may require a minimum number of buffers enqueued
300	at all times to function, apart of this no limit exists on the number
301	of buffers applications can enqueue in advance, or dequeue and
302	process. They can also enqueue in a different order than buffers have
303	been dequeued, and the driver can <emphasis>fill</emphasis> enqueued
304	<emphasis>empty</emphasis> buffers in any order. <footnote>
305		<para>Random enqueue order permits applications processing
306	images out of order (such as video codecs) to return buffers earlier,
307	reducing the probability of data loss. Random fill order allows
308	drivers to reuse buffers on a LIFO-basis, taking advantage of caches
309	holding scatter-gather lists and the like.</para>
310	      </footnote> The index number of a buffer (&v4l2-buffer;
311	<structfield>index</structfield>) plays no role here, it only
312	identifies the buffer.</para>
313	
314	    <para>Initially all mapped buffers are in dequeued state,
315	inaccessible by the driver. For capturing applications it is customary
316	to first enqueue all mapped buffers, then to start capturing and enter
317	the read loop. Here the application waits until a filled buffer can be
318	dequeued, and re-enqueues the buffer when the data is no longer
319	needed. Output applications fill and enqueue buffers, when enough
320	buffers are stacked up the output is started with
321	<constant>VIDIOC_STREAMON</constant>. In the write loop, when
322	the application runs out of free buffers, it must wait until an empty
323	buffer can be dequeued and reused.</para>
324	
325	    <para>To enqueue and dequeue a buffer applications use the
326	&VIDIOC-QBUF; and &VIDIOC-DQBUF; ioctl. The status of a buffer being
327	mapped, enqueued, full or empty can be determined at any time using the
328	&VIDIOC-QUERYBUF; ioctl. Two methods exist to suspend execution of the
329	application until one or more buffers can be dequeued. By default
330	<constant>VIDIOC_DQBUF</constant> blocks when no buffer is in the
331	outgoing queue. When the <constant>O_NONBLOCK</constant> flag was
332	given to the &func-open; function, <constant>VIDIOC_DQBUF</constant>
333	returns immediately with an &EAGAIN; when no buffer is available. The
334	&func-select; or &func-poll; functions are always available.</para>
335	
336	    <para>To start and stop capturing or output applications call the
337	&VIDIOC-STREAMON; and &VIDIOC-STREAMOFF; ioctl. Note
338	<constant>VIDIOC_STREAMOFF</constant> removes all buffers from both
339	queues as a side effect. Since there is no notion of doing anything
340	"now" on a multitasking system, if an application needs to synchronize
341	with another event it should examine the &v4l2-buffer;
342	<structfield>timestamp</structfield> of captured buffers, or set the
343	field before enqueuing buffers for output.</para>
344	
345	    <para>Drivers implementing memory mapping I/O must
346	support the <constant>VIDIOC_REQBUFS</constant>,
347	<constant>VIDIOC_QUERYBUF</constant>,
348	<constant>VIDIOC_QBUF</constant>, <constant>VIDIOC_DQBUF</constant>,
349	<constant>VIDIOC_STREAMON</constant> and
350	<constant>VIDIOC_STREAMOFF</constant> ioctl, the
351	<function>mmap()</function>, <function>munmap()</function>,
352	<function>select()</function> and <function>poll()</function>
353	function.<footnote>
354		<para>At the driver level <function>select()</function> and
355	<function>poll()</function> are the same, and
356	<function>select()</function> is too important to be optional. The
357	rest should be evident.</para>
358	      </footnote></para>
359	
360	    <para>[capture example]</para>
361	
362	  </section>
363	
364	  <section id="userp">
365	    <title>Streaming I/O (User Pointers)</title>
366	
367	    <para>Input and output devices support this I/O method when the
368	<constant>V4L2_CAP_STREAMING</constant> flag in the
369	<structfield>capabilities</structfield> field of &v4l2-capability;
370	returned by the &VIDIOC-QUERYCAP; ioctl is set. If the particular user
371	pointer method (not only memory mapping) is supported must be
372	determined by calling the &VIDIOC-REQBUFS; ioctl.</para>
373	
374	    <para>This I/O method combines advantages of the read/write and
375	memory mapping methods. Buffers (planes) are allocated by the application
376	itself, and can reside for example in virtual or shared memory. Only
377	pointers to data are exchanged, these pointers and meta-information
378	are passed in &v4l2-buffer; (or in &v4l2-plane; in the multi-planar API case).
379	The driver must be switched into user pointer I/O mode by calling the
380	&VIDIOC-REQBUFS; with the desired buffer type. No buffers (planes) are allocated
381	beforehand, consequently they are not indexed and cannot be queried like mapped
382	buffers with the <constant>VIDIOC_QUERYBUF</constant> ioctl.</para>
383	
384	    <example>
385	      <title>Initiating streaming I/O with user pointers</title>
386	
387	      <programlisting>
388	&v4l2-requestbuffers; reqbuf;
389	
390	memset (&amp;reqbuf, 0, sizeof (reqbuf));
391	reqbuf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
392	reqbuf.memory = V4L2_MEMORY_USERPTR;
393	
394	if (ioctl (fd, &VIDIOC-REQBUFS;, &amp;reqbuf) == -1) {
395		if (errno == EINVAL)
396			printf ("Video capturing or user pointer streaming is not supported\n");
397		else
398			perror ("VIDIOC_REQBUFS");
399	
400		exit (EXIT_FAILURE);
401	}
402	      </programlisting>
403	    </example>
404	
405	    <para>Buffer (plane) addresses and sizes are passed on the fly with the
406	&VIDIOC-QBUF; ioctl. Although buffers are commonly cycled,
407	applications can pass different addresses and sizes at each
408	<constant>VIDIOC_QBUF</constant> call. If required by the hardware the
409	driver swaps memory pages within physical memory to create a
410	continuous area of memory. This happens transparently to the
411	application in the virtual memory subsystem of the kernel. When buffer
412	pages have been swapped out to disk they are brought back and finally
413	locked in physical memory for DMA.<footnote>
414		<para>We expect that frequently used buffers are typically not
415	swapped out. Anyway, the process of swapping, locking or generating
416	scatter-gather lists may be time consuming. The delay can be masked by
417	the depth of the incoming buffer queue, and perhaps by maintaining
418	caches assuming a buffer will be soon enqueued again. On the other
419	hand, to optimize memory usage drivers can limit the number of buffers
420	locked in advance and recycle the most recently used buffers first. Of
421	course, the pages of empty buffers in the incoming queue need not be
422	saved to disk. Output buffers must be saved on the incoming and
423	outgoing queue because an application may share them with other
424	processes.</para>
425	      </footnote></para>
426	
427	    <para>Filled or displayed buffers are dequeued with the
428	&VIDIOC-DQBUF; ioctl. The driver can unlock the memory pages at any
429	time between the completion of the DMA and this ioctl. The memory is
430	also unlocked when &VIDIOC-STREAMOFF; is called, &VIDIOC-REQBUFS;, or
431	when the device is closed. Applications must take care not to free
432	buffers without dequeuing. For once, the buffers remain locked until
433	further, wasting physical memory. Second the driver will not be
434	notified when the memory is returned to the application's free list
435	and subsequently reused for other purposes, possibly completing the
436	requested DMA and overwriting valuable data.</para>
437	
438	    <para>For capturing applications it is customary to enqueue a
439	number of empty buffers, to start capturing and enter the read loop.
440	Here the application waits until a filled buffer can be dequeued, and
441	re-enqueues the buffer when the data is no longer needed. Output
442	applications fill and enqueue buffers, when enough buffers are stacked
443	up output is started. In the write loop, when the application
444	runs out of free buffers it must wait until an empty buffer can be
445	dequeued and reused. Two methods exist to suspend execution of the
446	application until one or more buffers can be dequeued. By default
447	<constant>VIDIOC_DQBUF</constant> blocks when no buffer is in the
448	outgoing queue. When the <constant>O_NONBLOCK</constant> flag was
449	given to the &func-open; function, <constant>VIDIOC_DQBUF</constant>
450	returns immediately with an &EAGAIN; when no buffer is available. The
451	&func-select; or &func-poll; function are always available.</para>
452	
453	    <para>To start and stop capturing or output applications call the
454	&VIDIOC-STREAMON; and &VIDIOC-STREAMOFF; ioctl. Note
455	<constant>VIDIOC_STREAMOFF</constant> removes all buffers from both
456	queues and unlocks all buffers as a side effect. Since there is no
457	notion of doing anything "now" on a multitasking system, if an
458	application needs to synchronize with another event it should examine
459	the &v4l2-buffer; <structfield>timestamp</structfield> of captured
460	buffers, or set the field before enqueuing buffers for output.</para>
461	
462	    <para>Drivers implementing user pointer I/O must
463	support the <constant>VIDIOC_REQBUFS</constant>,
464	<constant>VIDIOC_QBUF</constant>, <constant>VIDIOC_DQBUF</constant>,
465	<constant>VIDIOC_STREAMON</constant> and
466	<constant>VIDIOC_STREAMOFF</constant> ioctl, the
467	<function>select()</function> and <function>poll()</function> function.<footnote>
468		<para>At the driver level <function>select()</function> and
469	<function>poll()</function> are the same, and
470	<function>select()</function> is too important to be optional. The
471	rest should be evident.</para>
472	      </footnote></para>
473	  </section>
474	
475	  <section id="dmabuf">
476	    <title>Streaming I/O (DMA buffer importing)</title>
477	
478	    <note>
479	      <title>Experimental</title>
480	      <para>This is an <link linkend="experimental">experimental</link>
481	      interface and may change in the future.</para>
482	    </note>
483	
484	<para>The DMABUF framework provides a generic method for sharing buffers
485	between multiple devices. Device drivers that support DMABUF can export a DMA
486	buffer to userspace as a file descriptor (known as the exporter role), import a
487	DMA buffer from userspace using a file descriptor previously exported for a
488	different or the same device (known as the importer role), or both. This
489	section describes the DMABUF importer role API in V4L2.</para>
490	
491	    <para>Refer to <link linkend="vidioc-expbuf">DMABUF exporting</link> for
492	details about exporting V4L2 buffers as DMABUF file descriptors.</para>
493	
494	<para>Input and output devices support the streaming I/O method when the
495	<constant>V4L2_CAP_STREAMING</constant> flag in the
496	<structfield>capabilities</structfield> field of &v4l2-capability; returned by
497	the &VIDIOC-QUERYCAP; ioctl is set. Whether importing DMA buffers through
498	DMABUF file descriptors is supported is determined by calling the
499	&VIDIOC-REQBUFS; ioctl with the memory type set to
500	<constant>V4L2_MEMORY_DMABUF</constant>.</para>
501	
502	    <para>This I/O method is dedicated to sharing DMA buffers between different
503	devices, which may be V4L devices or other video-related devices (e.g. DRM).
504	Buffers (planes) are allocated by a driver on behalf of an application. Next,
505	these buffers are exported to the application as file descriptors using an API
506	which is specific for an allocator driver.  Only such file descriptor are
507	exchanged. The descriptors and meta-information are passed in &v4l2-buffer; (or
508	in &v4l2-plane; in the multi-planar API case).  The driver must be switched
509	into DMABUF I/O mode by calling the &VIDIOC-REQBUFS; with the desired buffer
510	type.</para>
511	
512	    <example>
513	      <title>Initiating streaming I/O with DMABUF file descriptors</title>
514	
515	      <programlisting>
516	&v4l2-requestbuffers; reqbuf;
517	
518	memset(&amp;reqbuf, 0, sizeof (reqbuf));
519	reqbuf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
520	reqbuf.memory = V4L2_MEMORY_DMABUF;
521	reqbuf.count = 1;
522	
523	if (ioctl(fd, &VIDIOC-REQBUFS;, &amp;reqbuf) == -1) {
524		if (errno == EINVAL)
525			printf("Video capturing or DMABUF streaming is not supported\n");
526		else
527			perror("VIDIOC_REQBUFS");
528	
529		exit(EXIT_FAILURE);
530	}
531	      </programlisting>
532	    </example>
533	
534	    <para>The buffer (plane) file descriptor is passed on the fly with the
535	&VIDIOC-QBUF; ioctl. In case of multiplanar buffers, every plane can be
536	associated with a different DMABUF descriptor. Although buffers are commonly
537	cycled, applications can pass a different DMABUF descriptor at each
538	<constant>VIDIOC_QBUF</constant> call.</para>
539	
540	    <example>
541	      <title>Queueing DMABUF using single plane API</title>
542	
543	      <programlisting>
544	int buffer_queue(int v4lfd, int index, int dmafd)
545	{
546		&v4l2-buffer; buf;
547	
548		memset(&amp;buf, 0, sizeof buf);
549		buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
550		buf.memory = V4L2_MEMORY_DMABUF;
551		buf.index = index;
552		buf.m.fd = dmafd;
553	
554		if (ioctl(v4lfd, &VIDIOC-QBUF;, &amp;buf) == -1) {
555			perror("VIDIOC_QBUF");
556			return -1;
557		}
558	
559		return 0;
560	}
561	      </programlisting>
562	    </example>
563	
564	    <example>
565	      <title>Queueing DMABUF using multi plane API</title>
566	
567	      <programlisting>
568	int buffer_queue_mp(int v4lfd, int index, int dmafd[], int n_planes)
569	{
570		&v4l2-buffer; buf;
571		&v4l2-plane; planes[VIDEO_MAX_PLANES];
572		int i;
573	
574		memset(&amp;buf, 0, sizeof buf);
575		buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE;
576		buf.memory = V4L2_MEMORY_DMABUF;
577		buf.index = index;
578		buf.m.planes = planes;
579		buf.length = n_planes;
580	
581		memset(&amp;planes, 0, sizeof planes);
582	
583		for (i = 0; i &lt; n_planes; ++i)
584			buf.m.planes[i].m.fd = dmafd[i];
585	
586		if (ioctl(v4lfd, &VIDIOC-QBUF;, &amp;buf) == -1) {
587			perror("VIDIOC_QBUF");
588			return -1;
589		}
590	
591		return 0;
592	}
593	      </programlisting>
594	    </example>
595	
596	    <para>Captured or displayed buffers are dequeued with the
597	&VIDIOC-DQBUF; ioctl. The driver can unlock the buffer at any
598	time between the completion of the DMA and this ioctl. The memory is
599	also unlocked when &VIDIOC-STREAMOFF; is called, &VIDIOC-REQBUFS;, or
600	when the device is closed.</para>
601	
602	    <para>For capturing applications it is customary to enqueue a
603	number of empty buffers, to start capturing and enter the read loop.
604	Here the application waits until a filled buffer can be dequeued, and
605	re-enqueues the buffer when the data is no longer needed. Output
606	applications fill and enqueue buffers, when enough buffers are stacked
607	up output is started. In the write loop, when the application
608	runs out of free buffers it must wait until an empty buffer can be
609	dequeued and reused. Two methods exist to suspend execution of the
610	application until one or more buffers can be dequeued. By default
611	<constant>VIDIOC_DQBUF</constant> blocks when no buffer is in the
612	outgoing queue. When the <constant>O_NONBLOCK</constant> flag was
613	given to the &func-open; function, <constant>VIDIOC_DQBUF</constant>
614	returns immediately with an &EAGAIN; when no buffer is available. The
615	&func-select; and &func-poll; functions are always available.</para>
616	
617	    <para>To start and stop capturing or displaying applications call the
618	&VIDIOC-STREAMON; and &VIDIOC-STREAMOFF; ioctls. Note that
619	<constant>VIDIOC_STREAMOFF</constant> removes all buffers from both queues and
620	unlocks all buffers as a side effect. Since there is no notion of doing
621	anything "now" on a multitasking system, if an application needs to synchronize
622	with another event it should examine the &v4l2-buffer;
623	<structfield>timestamp</structfield> of captured buffers, or set the field
624	before enqueuing buffers for output.</para>
625	
626	    <para>Drivers implementing DMABUF importing I/O must support the
627	<constant>VIDIOC_REQBUFS</constant>, <constant>VIDIOC_QBUF</constant>,
628	<constant>VIDIOC_DQBUF</constant>, <constant>VIDIOC_STREAMON</constant> and
629	<constant>VIDIOC_STREAMOFF</constant> ioctls, and the
630	<function>select()</function> and <function>poll()</function> functions.</para>
631	
632	  </section>
633	
634	  <section id="async">
635	    <title>Asynchronous I/O</title>
636	
637	    <para>This method is not defined yet.</para>
638	  </section>
639	
640	  <section id="buffer">
641	    <title>Buffers</title>
642	
643	    <para>A buffer contains data exchanged by application and
644	driver using one of the Streaming I/O methods. In the multi-planar API, the
645	data is held in planes, while the buffer structure acts as a container
646	for the planes. Only pointers to buffers (planes) are exchanged, the data
647	itself is not copied. These pointers, together with meta-information like
648	timestamps or field parity, are stored in a struct
649	<structname>v4l2_buffer</structname>, argument to
650	the &VIDIOC-QUERYBUF;, &VIDIOC-QBUF; and &VIDIOC-DQBUF; ioctl.
651	In the multi-planar API, some plane-specific members of struct
652	<structname>v4l2_buffer</structname>, such as pointers and sizes for each
653	plane, are stored in struct <structname>v4l2_plane</structname> instead.
654	In that case, struct <structname>v4l2_buffer</structname> contains an array of
655	plane structures.</para>
656	
657	      <para>Nominally timestamps refer to the first data byte transmitted.
658	In practice however the wide range of hardware covered by the V4L2 API
659	limits timestamp accuracy. Often an interrupt routine will
660	sample the system clock shortly after the field or frame was stored
661	completely in memory. So applications must expect a constant
662	difference up to one field or frame period plus a small (few scan
663	lines) random error. The delay and error can be much
664	larger due to compression or transmission over an external bus when
665	the frames are not properly stamped by the sender. This is frequently
666	the case with USB cameras. Here timestamps refer to the instant the
667	field or frame was received by the driver, not the capture time. These
668	devices identify by not enumerating any video standards, see <xref
669	linkend="standard" />.</para>
670	
671	      <para>Similar limitations apply to output timestamps. Typically
672	the video hardware locks to a clock controlling the video timing, the
673	horizontal and vertical synchronization pulses. At some point in the
674	line sequence, possibly the vertical blanking, an interrupt routine
675	samples the system clock, compares against the timestamp and programs
676	the hardware to repeat the previous field or frame, or to display the
677	buffer contents.</para>
678	
679	      <para>Apart of limitations of the video device and natural
680	inaccuracies of all clocks, it should be noted system time itself is
681	not perfectly stable. It can be affected by power saving cycles,
682	warped to insert leap seconds, or even turned back or forth by the
683	system administrator affecting long term measurements. <footnote>
684		  <para>Since no other Linux multimedia
685	API supports unadjusted time it would be foolish to introduce here. We
686	must use a universally supported clock to synchronize different media,
687	hence time of day.</para>
688		</footnote></para>
689	
690	    <table frame="none" pgwide="1" id="v4l2-buffer">
691	      <title>struct <structname>v4l2_buffer</structname></title>
692	      <tgroup cols="4">
693		&cs-ustr;
694		<tbody valign="top">
695		  <row>
696		    <entry>__u32</entry>
697		    <entry><structfield>index</structfield></entry>
698		    <entry></entry>
699		    <entry>Number of the buffer, set by the application. This
700	field is only used for <link linkend="mmap">memory mapping</link> I/O
701	and can range from zero to the number of buffers allocated
702	with the &VIDIOC-REQBUFS; ioctl (&v4l2-requestbuffers; <structfield>count</structfield>) minus one.</entry>
703		  </row>
704		  <row>
705		    <entry>__u32</entry>
706		    <entry><structfield>type</structfield></entry>
707		    <entry></entry>
708		    <entry>Type of the buffer, same as &v4l2-format;
709	<structfield>type</structfield> or &v4l2-requestbuffers;
710	<structfield>type</structfield>, set by the application. See <xref
711	linkend="v4l2-buf-type" /></entry>
712		  </row>
713		  <row>
714		    <entry>__u32</entry>
715		    <entry><structfield>bytesused</structfield></entry>
716		    <entry></entry>
717		    <entry>The number of bytes occupied by the data in the
718	buffer. It depends on the negotiated data format and may change with
719	each buffer for compressed variable size data like JPEG images.
720	Drivers must set this field when <structfield>type</structfield>
721	refers to an input stream, applications when an output stream.</entry>
722		  </row>
723		  <row>
724		    <entry>__u32</entry>
725		    <entry><structfield>flags</structfield></entry>
726		    <entry></entry>
727		    <entry>Flags set by the application or driver, see <xref
728	linkend="buffer-flags" />.</entry>
729		  </row>
730		  <row>
731		    <entry>__u32</entry>
732		    <entry><structfield>field</structfield></entry>
733		    <entry></entry>
734		    <entry>Indicates the field order of the image in the
735	buffer, see <xref linkend="v4l2-field" />. This field is not used when
736	the buffer contains VBI data. Drivers must set it when
737	<structfield>type</structfield> refers to an input stream,
738	applications when an output stream.</entry>
739		  </row>
740		  <row>
741		    <entry>struct timeval</entry>
742		    <entry><structfield>timestamp</structfield></entry>
743		    <entry></entry>
744		    <entry><para>For input streams this is time when the first data
745		    byte was captured, as returned by the
746		    <function>clock_gettime()</function> function for the relevant
747		    clock id; see <constant>V4L2_BUF_FLAG_TIMESTAMP_*</constant> in
748		    <xref linkend="buffer-flags" />. For output streams the data
749		    will not be displayed before this time, secondary to the nominal
750		    frame rate determined by the current video standard in enqueued
751		    order. Applications can for example zero this field to display
752		    frames as soon as possible. The driver stores the time at which
753		    the first data byte was actually sent out in the
754		    <structfield>timestamp</structfield> field. This permits
755		    applications to monitor the drift between the video and system
756		    clock.</para></entry>
757		  </row>
758		  <row>
759		    <entry>&v4l2-timecode;</entry>
760		    <entry><structfield>timecode</structfield></entry>
761		    <entry></entry>
762		    <entry>When <structfield>type</structfield> is
763	<constant>V4L2_BUF_TYPE_VIDEO_CAPTURE</constant> and the
764	<constant>V4L2_BUF_FLAG_TIMECODE</constant> flag is set in
765	<structfield>flags</structfield>, this structure contains a frame
766	timecode. In <link linkend="v4l2-field">V4L2_FIELD_ALTERNATE</link>
767	mode the top and bottom field contain the same timecode.
768	Timecodes are intended to help video editing and are typically recorded on
769	video tapes, but also embedded in compressed formats like MPEG. This
770	field is independent of the <structfield>timestamp</structfield> and
771	<structfield>sequence</structfield> fields.</entry>
772		  </row>
773		  <row>
774		    <entry>__u32</entry>
775		    <entry><structfield>sequence</structfield></entry>
776		    <entry></entry>
777		    <entry>Set by the driver, counting the frames (not fields!) in
778	sequence. This field is set for both input and output devices.</entry>
779		  </row>
780		  <row>
781		    <entry spanname="hspan"><para>In <link
782	linkend="v4l2-field">V4L2_FIELD_ALTERNATE</link> mode the top and
783	bottom field have the same sequence number. The count starts at zero
784	and includes dropped or repeated frames. A dropped frame was received
785	by an input device but could not be stored due to lack of free buffer
786	space. A repeated frame was displayed again by an output device
787	because the application did not pass new data in
788	time.</para><para>Note this may count the frames received
789	e.g. over USB, without taking into account the frames dropped by the
790	remote hardware due to limited compression throughput or bus
791	bandwidth. These devices identify by not enumerating any video
792	standards, see <xref linkend="standard" />.</para></entry>
793		  </row>
794		  <row>
795		    <entry>__u32</entry>
796		    <entry><structfield>memory</structfield></entry>
797		    <entry></entry>
798		    <entry>This field must be set by applications and/or drivers
799	in accordance with the selected I/O method. See <xref linkend="v4l2-memory"
800		    /></entry>
801		  </row>
802		  <row>
803		    <entry>union</entry>
804		    <entry><structfield>m</structfield></entry>
805		  </row>
806		  <row>
807		    <entry></entry>
808		    <entry>__u32</entry>
809		    <entry><structfield>offset</structfield></entry>
810		    <entry>For the single-planar API and when
811	<structfield>memory</structfield> is <constant>V4L2_MEMORY_MMAP</constant> this
812	is the offset of the buffer from the start of the device memory. The value is
813	returned by the driver and apart of serving as parameter to the &func-mmap;
814	function not useful for applications. See <xref linkend="mmap" /> for details
815		  </entry>
816		  </row>
817		  <row>
818		    <entry></entry>
819		    <entry>unsigned long</entry>
820		    <entry><structfield>userptr</structfield></entry>
821		    <entry>For the single-planar API and when
822	<structfield>memory</structfield> is <constant>V4L2_MEMORY_USERPTR</constant>
823	this is a pointer to the buffer (casted to unsigned long type) in virtual
824	memory, set by the application. See <xref linkend="userp" /> for details.
825		    </entry>
826		  </row>
827		  <row>
828		    <entry></entry>
829		    <entry>struct v4l2_plane</entry>
830		    <entry><structfield>*planes</structfield></entry>
831		    <entry>When using the multi-planar API, contains a userspace pointer
832		    to an array of &v4l2-plane;. The size of the array should be put
833		    in the <structfield>length</structfield> field of this
834		    <structname>v4l2_buffer</structname> structure.</entry>
835		  </row>
836		  <row>
837		    <entry></entry>
838		    <entry>int</entry>
839		    <entry><structfield>fd</structfield></entry>
840		    <entry>For the single-plane API and when
841	<structfield>memory</structfield> is <constant>V4L2_MEMORY_DMABUF</constant> this
842	is the file descriptor associated with a DMABUF buffer.</entry>
843		  </row>
844		  <row>
845		    <entry>__u32</entry>
846		    <entry><structfield>length</structfield></entry>
847		    <entry></entry>
848		    <entry>Size of the buffer (not the payload) in bytes for the
849		    single-planar API. For the multi-planar API the application sets
850		    this to the number of elements in the <structfield>planes</structfield>
851		    array. The driver will fill in the actual number of valid elements in
852		    that array.
853		    </entry>
854		  </row>
855		  <row>
856		    <entry>__u32</entry>
857		    <entry><structfield>reserved2</structfield></entry>
858		    <entry></entry>
859		    <entry>A place holder for future extensions. Applications
860	should set this to 0.</entry>
861		  </row>
862		  <row>
863		    <entry>__u32</entry>
864		    <entry><structfield>reserved</structfield></entry>
865		    <entry></entry>
866		    <entry>A place holder for future extensions. Applications
867	should set this to 0.</entry>
868		  </row>
869		</tbody>
870	      </tgroup>
871	    </table>
872	
873	    <table frame="none" pgwide="1" id="v4l2-plane">
874	      <title>struct <structname>v4l2_plane</structname></title>
875	      <tgroup cols="4">
876	        &cs-ustr;
877		<tbody valign="top">
878		  <row>
879		    <entry>__u32</entry>
880		    <entry><structfield>bytesused</structfield></entry>
881		    <entry></entry>
882		    <entry>The number of bytes occupied by data in the plane
883		    (its payload).</entry>
884		  </row>
885		  <row>
886		    <entry>__u32</entry>
887		    <entry><structfield>length</structfield></entry>
888		    <entry></entry>
889		    <entry>Size in bytes of the plane (not its payload).</entry>
890		  </row>
891		  <row>
892		    <entry>union</entry>
893		    <entry><structfield>m</structfield></entry>
894		    <entry></entry>
895		    <entry></entry>
896		  </row>
897		  <row>
898		    <entry></entry>
899		    <entry>__u32</entry>
900		    <entry><structfield>mem_offset</structfield></entry>
901		    <entry>When the memory type in the containing &v4l2-buffer; is
902		      <constant>V4L2_MEMORY_MMAP</constant>, this is the value that
903		      should be passed to &func-mmap;, similar to the
904		      <structfield>offset</structfield> field in &v4l2-buffer;.</entry>
905		  </row>
906		  <row>
907		    <entry></entry>
908		    <entry>unsigned long</entry>
909		    <entry><structfield>userptr</structfield></entry>
910		    <entry>When the memory type in the containing &v4l2-buffer; is
911		      <constant>V4L2_MEMORY_USERPTR</constant>, this is a userspace
912		      pointer to the memory allocated for this plane by an application.
913		      </entry>
914		  </row>
915		  <row>
916		    <entry></entry>
917		    <entry>int</entry>
918		    <entry><structfield>fd</structfield></entry>
919		    <entry>When the memory type in the containing &v4l2-buffer; is
920			<constant>V4L2_MEMORY_DMABUF</constant>, this is a file
921			descriptor associated with a DMABUF buffer, similar to the
922			<structfield>fd</structfield> field in &v4l2-buffer;.</entry>
923		  </row>
924		  <row>
925		    <entry>__u32</entry>
926		    <entry><structfield>data_offset</structfield></entry>
927		    <entry></entry>
928		    <entry>Offset in bytes to video data in the plane, if applicable.
929		    </entry>
930		  </row>
931		  <row>
932		    <entry>__u32</entry>
933		    <entry><structfield>reserved[11]</structfield></entry>
934		    <entry></entry>
935		    <entry>Reserved for future use. Should be zeroed by an
936		    application.</entry>
937		  </row>
938		</tbody>
939	      </tgroup>
940	    </table>
941	
942	    <table frame="none" pgwide="1" id="v4l2-buf-type">
943	      <title>enum v4l2_buf_type</title>
944	      <tgroup cols="3">
945		&cs-def;
946		<tbody valign="top">
947		  <row>
948		    <entry><constant>V4L2_BUF_TYPE_VIDEO_CAPTURE</constant></entry>
949		    <entry>1</entry>
950		    <entry>Buffer of a single-planar video capture stream, see <xref
951			linkend="capture" />.</entry>
952		  </row>
953		  <row>
954		    <entry><constant>V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE</constant>
955		    </entry>
956		    <entry>9</entry>
957		    <entry>Buffer of a multi-planar video capture stream, see <xref
958			linkend="capture" />.</entry>
959		  </row>
960		  <row>
961		    <entry><constant>V4L2_BUF_TYPE_VIDEO_OUTPUT</constant></entry>
962		    <entry>2</entry>
963		    <entry>Buffer of a single-planar video output stream, see <xref
964			linkend="output" />.</entry>
965		  </row>
966		  <row>
967		    <entry><constant>V4L2_BUF_TYPE_VIDEO_OUTPUT_MPLANE</constant>
968		    </entry>
969		    <entry>10</entry>
970		    <entry>Buffer of a multi-planar video output stream, see <xref
971			linkend="output" />.</entry>
972		  </row>
973		  <row>
974		    <entry><constant>V4L2_BUF_TYPE_VIDEO_OVERLAY</constant></entry>
975		    <entry>3</entry>
976		    <entry>Buffer for video overlay, see <xref linkend="overlay" />.</entry>
977		  </row>
978		  <row>
979		    <entry><constant>V4L2_BUF_TYPE_VBI_CAPTURE</constant></entry>
980		    <entry>4</entry>
981		    <entry>Buffer of a raw VBI capture stream, see <xref
982			linkend="raw-vbi" />.</entry>
983		  </row>
984		  <row>
985		    <entry><constant>V4L2_BUF_TYPE_VBI_OUTPUT</constant></entry>
986		    <entry>5</entry>
987		    <entry>Buffer of a raw VBI output stream, see <xref
988			linkend="raw-vbi" />.</entry>
989		  </row>
990		  <row>
991		    <entry><constant>V4L2_BUF_TYPE_SLICED_VBI_CAPTURE</constant></entry>
992		    <entry>6</entry>
993		    <entry>Buffer of a sliced VBI capture stream, see <xref
994			linkend="sliced" />.</entry>
995		  </row>
996		  <row>
997		    <entry><constant>V4L2_BUF_TYPE_SLICED_VBI_OUTPUT</constant></entry>
998		    <entry>7</entry>
999		    <entry>Buffer of a sliced VBI output stream, see <xref
1000			linkend="sliced" />.</entry>
1001		  </row>
1002		  <row>
1003		    <entry><constant>V4L2_BUF_TYPE_VIDEO_OUTPUT_OVERLAY</constant></entry>
1004		    <entry>8</entry>
1005		    <entry>Buffer for video output overlay (OSD), see <xref
1006			linkend="osd" />.</entry>
1007		  </row>
1008		</tbody>
1009	      </tgroup>
1010	    </table>
1011	
1012	    <table frame="none" pgwide="1" id="buffer-flags">
1013	      <title>Buffer Flags</title>
1014	      <tgroup cols="3">
1015		&cs-def;
1016		<tbody valign="top">
1017		  <row>
1018		    <entry><constant>V4L2_BUF_FLAG_MAPPED</constant></entry>
1019		    <entry>0x0001</entry>
1020		    <entry>The buffer resides in device memory and has been mapped
1021	into the application's address space, see <xref linkend="mmap" /> for details.
1022	Drivers set or clear this flag when the
1023	<link linkend="vidioc-querybuf">VIDIOC_QUERYBUF</link>, <link
1024		  linkend="vidioc-qbuf">VIDIOC_QBUF</link> or <link
1025		  linkend="vidioc-qbuf">VIDIOC_DQBUF</link> ioctl is called. Set by the driver.</entry>
1026		  </row>
1027		  <row>
1028		    <entry><constant>V4L2_BUF_FLAG_QUEUED</constant></entry>
1029		    <entry>0x0002</entry>
1030		  <entry>Internally drivers maintain two buffer queues, an
1031	incoming and outgoing queue. When this flag is set, the buffer is
1032	currently on the incoming queue. It automatically moves to the
1033	outgoing queue after the buffer has been filled (capture devices) or
1034	displayed (output devices). Drivers set or clear this flag when the
1035	<constant>VIDIOC_QUERYBUF</constant> ioctl is called. After
1036	(successful) calling the <constant>VIDIOC_QBUF </constant>ioctl it is
1037	always set and after <constant>VIDIOC_DQBUF</constant> always
1038	cleared.</entry>
1039		  </row>
1040		  <row>
1041		    <entry><constant>V4L2_BUF_FLAG_DONE</constant></entry>
1042		    <entry>0x0004</entry>
1043		    <entry>When this flag is set, the buffer is currently on
1044	the outgoing queue, ready to be dequeued from the driver. Drivers set
1045	or clear this flag when the <constant>VIDIOC_QUERYBUF</constant> ioctl
1046	is called. After calling the <constant>VIDIOC_QBUF</constant> or
1047	<constant>VIDIOC_DQBUF</constant> it is always cleared. Of course a
1048	buffer cannot be on both queues at the same time, the
1049	<constant>V4L2_BUF_FLAG_QUEUED</constant> and
1050	<constant>V4L2_BUF_FLAG_DONE</constant> flag are mutually exclusive.
1051	They can be both cleared however, then the buffer is in "dequeued"
1052	state, in the application domain to say so.</entry>
1053		  </row>
1054		  <row>
1055		    <entry><constant>V4L2_BUF_FLAG_ERROR</constant></entry>
1056		    <entry>0x0040</entry>
1057		    <entry>When this flag is set, the buffer has been dequeued
1058		    successfully, although the data might have been corrupted.
1059		    This is recoverable, streaming may continue as normal and
1060		    the buffer may be reused normally.
1061		    Drivers set this flag when the <constant>VIDIOC_DQBUF</constant>
1062		    ioctl is called.</entry>
1063		  </row>
1064		  <row>
1065		    <entry><constant>V4L2_BUF_FLAG_KEYFRAME</constant></entry>
1066		    <entry>0x0008</entry>
1067		  <entry>Drivers set or clear this flag when calling the
1068	<constant>VIDIOC_DQBUF</constant> ioctl. It may be set by video
1069	capture devices when the buffer contains a compressed image which is a
1070	key frame (or field), &ie; can be decompressed on its own.</entry>
1071		  </row>
1072		  <row>
1073		    <entry><constant>V4L2_BUF_FLAG_PFRAME</constant></entry>
1074		    <entry>0x0010</entry>
1075		    <entry>Similar to <constant>V4L2_BUF_FLAG_KEYFRAME</constant>
1076	this flags predicted frames or fields which contain only differences to a
1077	previous key frame.</entry>
1078		  </row>
1079		  <row>
1080		    <entry><constant>V4L2_BUF_FLAG_BFRAME</constant></entry>
1081		    <entry>0x0020</entry>
1082		    <entry>Similar to <constant>V4L2_BUF_FLAG_PFRAME</constant>
1083		this is a bidirectional predicted frame or field. [ooc tbd]</entry>
1084		  </row>
1085		  <row>
1086		    <entry><constant>V4L2_BUF_FLAG_TIMECODE</constant></entry>
1087		    <entry>0x0100</entry>
1088		    <entry>The <structfield>timecode</structfield> field is valid.
1089	Drivers set or clear this flag when the <constant>VIDIOC_DQBUF</constant>
1090	ioctl is called.</entry>
1091		  </row>
1092		  <row>
1093		    <entry><constant>V4L2_BUF_FLAG_PREPARED</constant></entry>
1094		    <entry>0x0400</entry>
1095		    <entry>The buffer has been prepared for I/O and can be queued by the
1096	application. Drivers set or clear this flag when the
1097	<link linkend="vidioc-querybuf">VIDIOC_QUERYBUF</link>, <link
1098		  linkend="vidioc-qbuf">VIDIOC_PREPARE_BUF</link>, <link
1099		  linkend="vidioc-qbuf">VIDIOC_QBUF</link> or <link
1100		  linkend="vidioc-qbuf">VIDIOC_DQBUF</link> ioctl is called.</entry>
1101		  </row>
1102		  <row>
1103		    <entry><constant>V4L2_BUF_FLAG_NO_CACHE_INVALIDATE</constant></entry>
1104		    <entry>0x0800</entry>
1105		    <entry>Caches do not have to be invalidated for this buffer.
1106	Typically applications shall use this flag if the data captured in the buffer
1107	is not going to be touched by the CPU, instead the buffer will, probably, be
1108	passed on to a DMA-capable hardware unit for further processing or output.
1109	</entry>
1110		  </row>
1111		  <row>
1112		    <entry><constant>V4L2_BUF_FLAG_NO_CACHE_CLEAN</constant></entry>
1113		    <entry>0x1000</entry>
1114		    <entry>Caches do not have to be cleaned for this buffer.
1115	Typically applications shall use this flag for output buffers if the data
1116	in this buffer has not been created by the CPU but by some DMA-capable unit,
1117	in which case caches have not been used.</entry>
1118		  </row>
1119		  <row>
1120		    <entry><constant>V4L2_BUF_FLAG_TIMESTAMP_MASK</constant></entry>
1121		    <entry>0xe000</entry>
1122		    <entry>Mask for timestamp types below. To test the
1123		    timestamp type, mask out bits not belonging to timestamp
1124		    type by performing a logical and operation with buffer
1125		    flags and timestamp mask.</entry>
1126		  </row>
1127		  <row>
1128		    <entry><constant>V4L2_BUF_FLAG_TIMESTAMP_UNKNOWN</constant></entry>
1129		    <entry>0x0000</entry>
1130		    <entry>Unknown timestamp type. This type is used by
1131		    drivers before Linux 3.9 and may be either monotonic (see
1132		    below) or realtime (wall clock). Monotonic clock has been
1133		    favoured in embedded systems whereas most of the drivers
1134		    use the realtime clock. Either kinds of timestamps are
1135		    available in user space via
1136		    <function>clock_gettime(2)</function> using clock IDs
1137		    <constant>CLOCK_MONOTONIC</constant> and
1138		    <constant>CLOCK_REALTIME</constant>, respectively.</entry>
1139		  </row>
1140		  <row>
1141		    <entry><constant>V4L2_BUF_FLAG_TIMESTAMP_MONOTONIC</constant></entry>
1142		    <entry>0x2000</entry>
1143		    <entry>The buffer timestamp has been taken from the
1144		    <constant>CLOCK_MONOTONIC</constant> clock. To access the
1145		    same clock outside V4L2, use
1146		    <function>clock_gettime(2)</function> .</entry>
1147		  </row>
1148		  <row>
1149		    <entry><constant>V4L2_BUF_FLAG_TIMESTAMP_COPY</constant></entry>
1150		    <entry>0x4000</entry>
1151		    <entry>The CAPTURE buffer timestamp has been taken from the
1152		    corresponding OUTPUT buffer. This flag applies only to mem2mem devices.</entry>
1153		  </row>
1154		</tbody>
1155	      </tgroup>
1156	    </table>
1157	
1158	    <table pgwide="1" frame="none" id="v4l2-memory">
1159	      <title>enum v4l2_memory</title>
1160	      <tgroup cols="3">
1161		&cs-def;
1162		<tbody valign="top">
1163		  <row>
1164		    <entry><constant>V4L2_MEMORY_MMAP</constant></entry>
1165		    <entry>1</entry>
1166		    <entry>The buffer is used for <link linkend="mmap">memory
1167	mapping</link> I/O.</entry>
1168		  </row>
1169		  <row>
1170		    <entry><constant>V4L2_MEMORY_USERPTR</constant></entry>
1171		    <entry>2</entry>
1172		    <entry>The buffer is used for <link linkend="userp">user
1173	pointer</link> I/O.</entry>
1174		  </row>
1175		  <row>
1176		    <entry><constant>V4L2_MEMORY_OVERLAY</constant></entry>
1177		    <entry>3</entry>
1178		    <entry>[to do]</entry>
1179		  </row>
1180		  <row>
1181		    <entry><constant>V4L2_MEMORY_DMABUF</constant></entry>
1182		    <entry>4</entry>
1183		    <entry>The buffer is used for <link linkend="dmabuf">DMA shared
1184	buffer</link> I/O.</entry>
1185		  </row>
1186		</tbody>
1187	      </tgroup>
1188	    </table>
1189	
1190	    <section>
1191	      <title>Timecodes</title>
1192	
1193	      <para>The <structname>v4l2_timecode</structname> structure is
1194	designed to hold a <xref linkend="smpte12m" /> or similar timecode.
1195	(struct <structname>timeval</structname> timestamps are stored in
1196	&v4l2-buffer; field <structfield>timestamp</structfield>.)</para>
1197	
1198	      <table frame="none" pgwide="1" id="v4l2-timecode">
1199		<title>struct <structname>v4l2_timecode</structname></title>
1200		<tgroup cols="3">
1201		  &cs-str;
1202		  <tbody valign="top">
1203		    <row>
1204		      <entry>__u32</entry>
1205		      <entry><structfield>type</structfield></entry>
1206		      <entry>Frame rate the timecodes are based on, see <xref
1207			  linkend="timecode-type" />.</entry>
1208		    </row>
1209		    <row>
1210		      <entry>__u32</entry>
1211		      <entry><structfield>flags</structfield></entry>
1212		      <entry>Timecode flags, see <xref linkend="timecode-flags" />.</entry>
1213		    </row>
1214		    <row>
1215		      <entry>__u8</entry>
1216		      <entry><structfield>frames</structfield></entry>
1217		      <entry>Frame count, 0 ... 23/24/29/49/59, depending on the
1218		    type of timecode.</entry>
1219		    </row>
1220		    <row>
1221		      <entry>__u8</entry>
1222		      <entry><structfield>seconds</structfield></entry>
1223		      <entry>Seconds count, 0 ... 59. This is a binary, not BCD number.</entry>
1224		    </row>
1225		    <row>
1226		      <entry>__u8</entry>
1227		      <entry><structfield>minutes</structfield></entry>
1228		      <entry>Minutes count, 0 ... 59. This is a binary, not BCD number.</entry>
1229		    </row>
1230		    <row>
1231		      <entry>__u8</entry>
1232		      <entry><structfield>hours</structfield></entry>
1233		      <entry>Hours count, 0 ... 29. This is a binary, not BCD number.</entry>
1234		    </row>
1235		    <row>
1236		      <entry>__u8</entry>
1237		      <entry><structfield>userbits</structfield>[4]</entry>
1238		      <entry>The "user group" bits from the timecode.</entry>
1239		    </row>
1240		  </tbody>
1241		</tgroup>
1242	      </table>
1243	
1244	      <table frame="none" pgwide="1" id="timecode-type">
1245		<title>Timecode Types</title>
1246		<tgroup cols="3">
1247		&cs-def;
1248		  <tbody valign="top">
1249		    <row>
1250		      <entry><constant>V4L2_TC_TYPE_24FPS</constant></entry>
1251		      <entry>1</entry>
1252		      <entry>24 frames per second, i.&nbsp;e. film.</entry>
1253		    </row>
1254		    <row>
1255		      <entry><constant>V4L2_TC_TYPE_25FPS</constant></entry>
1256		      <entry>2</entry>
1257		      <entry>25 frames per second, &ie; PAL or SECAM video.</entry>
1258		    </row>
1259		    <row>
1260		      <entry><constant>V4L2_TC_TYPE_30FPS</constant></entry>
1261		      <entry>3</entry>
1262		      <entry>30 frames per second, &ie; NTSC video.</entry>
1263		    </row>
1264		    <row>
1265		      <entry><constant>V4L2_TC_TYPE_50FPS</constant></entry>
1266		      <entry>4</entry>
1267		      <entry></entry>
1268		    </row>
1269		    <row>
1270		      <entry><constant>V4L2_TC_TYPE_60FPS</constant></entry>
1271		      <entry>5</entry>
1272		      <entry></entry>
1273		    </row>
1274		  </tbody>
1275		</tgroup>
1276	      </table>
1277	
1278	      <table frame="none" pgwide="1" id="timecode-flags">
1279		<title>Timecode Flags</title>
1280		<tgroup cols="3">
1281		&cs-def;
1282		  <tbody valign="top">
1283		    <row>
1284		      <entry><constant>V4L2_TC_FLAG_DROPFRAME</constant></entry>
1285		      <entry>0x0001</entry>
1286		      <entry>Indicates "drop frame" semantics for counting frames
1287	in 29.97 fps material. When set, frame numbers 0 and 1 at the start of
1288	each minute, except minutes 0, 10, 20, 30, 40, 50 are omitted from the
1289	count.</entry>
1290		    </row>
1291		    <row>
1292		      <entry><constant>V4L2_TC_FLAG_COLORFRAME</constant></entry>
1293		      <entry>0x0002</entry>
1294		      <entry>The "color frame" flag.</entry>
1295		    </row>
1296		    <row>
1297		      <entry><constant>V4L2_TC_USERBITS_field</constant></entry>
1298		      <entry>0x000C</entry>
1299		      <entry>Field mask for the "binary group flags".</entry>
1300		    </row>
1301		    <row>
1302		      <entry><constant>V4L2_TC_USERBITS_USERDEFINED</constant></entry>
1303		      <entry>0x0000</entry>
1304		      <entry>Unspecified format.</entry>
1305		    </row>
1306		    <row>
1307		      <entry><constant>V4L2_TC_USERBITS_8BITCHARS</constant></entry>
1308		      <entry>0x0008</entry>
1309		      <entry>8-bit ISO characters.</entry>
1310		    </row>
1311		  </tbody>
1312		</tgroup>
1313	      </table>
1314	    </section>
1315	  </section>
1316	
1317	  <section id="field-order">
1318	    <title>Field Order</title>
1319	
1320	    <para>We have to distinguish between progressive and interlaced
1321	video. Progressive video transmits all lines of a video image
1322	sequentially. Interlaced video divides an image into two fields,
1323	containing only the odd and even lines of the image, respectively.
1324	Alternating the so called odd and even field are transmitted, and due
1325	to a small delay between fields a cathode ray TV displays the lines
1326	interleaved, yielding the original frame. This curious technique was
1327	invented because at refresh rates similar to film the image would
1328	fade out too quickly. Transmitting fields reduces the flicker without
1329	the necessity of doubling the frame rate and with it the bandwidth
1330	required for each channel.</para>
1331	
1332	    <para>It is important to understand a video camera does not expose
1333	one frame at a time, merely transmitting the frames separated into
1334	fields. The fields are in fact captured at two different instances in
1335	time. An object on screen may well move between one field and the
1336	next. For applications analysing motion it is of paramount importance
1337	to recognize which field of a frame is older, the <emphasis>temporal
1338	order</emphasis>.</para>
1339	
1340	    <para>When the driver provides or accepts images field by field
1341	rather than interleaved, it is also important applications understand
1342	how the fields combine to frames. We distinguish between top (aka odd) and
1343	bottom (aka even) fields, the <emphasis>spatial order</emphasis>: The first line
1344	of the top field is the first line of an interlaced frame, the first
1345	line of the bottom field is the second line of that frame.</para>
1346	
1347	    <para>However because fields were captured one after the other,
1348	arguing whether a frame commences with the top or bottom field is
1349	pointless. Any two successive top and bottom, or bottom and top fields
1350	yield a valid frame. Only when the source was progressive to begin
1351	with, &eg; when transferring film to video, two fields may come from
1352	the same frame, creating a natural order.</para>
1353	
1354	    <para>Counter to intuition the top field is not necessarily the
1355	older field. Whether the older field contains the top or bottom lines
1356	is a convention determined by the video standard. Hence the
1357	distinction between temporal and spatial order of fields. The diagrams
1358	below should make this clearer.</para>
1359	
1360	    <para>All video capture and output devices must report the current
1361	field order. Some drivers may permit the selection of a different
1362	order, to this end applications initialize the
1363	<structfield>field</structfield> field of &v4l2-pix-format; before
1364	calling the &VIDIOC-S-FMT; ioctl. If this is not desired it should
1365	have the value <constant>V4L2_FIELD_ANY</constant> (0).</para>
1366	
1367	    <table frame="none" pgwide="1" id="v4l2-field">
1368	      <title>enum v4l2_field</title>
1369	      <tgroup cols="3">
1370		&cs-def;
1371		<tbody valign="top">
1372		  <row>
1373		    <entry><constant>V4L2_FIELD_ANY</constant></entry>
1374		    <entry>0</entry>
1375		    <entry>Applications request this field order when any
1376	one of the <constant>V4L2_FIELD_NONE</constant>,
1377	<constant>V4L2_FIELD_TOP</constant>,
1378	<constant>V4L2_FIELD_BOTTOM</constant>, or
1379	<constant>V4L2_FIELD_INTERLACED</constant> formats is acceptable.
1380	Drivers choose depending on hardware capabilities or e.&nbsp;g. the
1381	requested image size, and return the actual field order. &v4l2-buffer;
1382	<structfield>field</structfield> can never be
1383	<constant>V4L2_FIELD_ANY</constant>.</entry>
1384		  </row>
1385		  <row>
1386		    <entry><constant>V4L2_FIELD_NONE</constant></entry>
1387		    <entry>1</entry>
1388		    <entry>Images are in progressive format, not interlaced.
1389	The driver may also indicate this order when it cannot distinguish
1390	between <constant>V4L2_FIELD_TOP</constant> and
1391	<constant>V4L2_FIELD_BOTTOM</constant>.</entry>
1392		  </row>
1393		  <row>
1394		    <entry><constant>V4L2_FIELD_TOP</constant></entry>
1395		    <entry>2</entry>
1396		    <entry>Images consist of the top (aka odd) field only.</entry>
1397		  </row>
1398		  <row>
1399		    <entry><constant>V4L2_FIELD_BOTTOM</constant></entry>
1400		    <entry>3</entry>
1401		    <entry>Images consist of the bottom (aka even) field only.
1402	Applications may wish to prevent a device from capturing interlaced
1403	images because they will have "comb" or "feathering" artefacts around
1404	moving objects.</entry>
1405		  </row>
1406		  <row>
1407		    <entry><constant>V4L2_FIELD_INTERLACED</constant></entry>
1408		    <entry>4</entry>
1409		    <entry>Images contain both fields, interleaved line by
1410	line. The temporal order of the fields (whether the top or bottom
1411	field is first transmitted) depends on the current video standard.
1412	M/NTSC transmits the bottom field first, all other standards the top
1413	field first.</entry>
1414		  </row>
1415		  <row>
1416		    <entry><constant>V4L2_FIELD_SEQ_TB</constant></entry>
1417		    <entry>5</entry>
1418		    <entry>Images contain both fields, the top field lines
1419	are stored first in memory, immediately followed by the bottom field
1420	lines. Fields are always stored in temporal order, the older one first
1421	in memory. Image sizes refer to the frame, not fields.</entry>
1422		  </row>
1423		  <row>
1424		    <entry><constant>V4L2_FIELD_SEQ_BT</constant></entry>
1425		    <entry>6</entry>
1426		    <entry>Images contain both fields, the bottom field
1427	lines are stored first in memory, immediately followed by the top
1428	field lines. Fields are always stored in temporal order, the older one
1429	first in memory. Image sizes refer to the frame, not fields.</entry>
1430		  </row>
1431		  <row>
1432		    <entry><constant>V4L2_FIELD_ALTERNATE</constant></entry>
1433		    <entry>7</entry>
1434		    <entry>The two fields of a frame are passed in separate
1435	buffers, in temporal order, &ie; the older one first. To indicate the field
1436	parity (whether the current field is a top or bottom field) the driver
1437	or application, depending on data direction, must set &v4l2-buffer;
1438	<structfield>field</structfield> to
1439	<constant>V4L2_FIELD_TOP</constant> or
1440	<constant>V4L2_FIELD_BOTTOM</constant>. Any two successive fields pair
1441	to build a frame. If fields are successive, without any dropped fields
1442	between them (fields can drop individually), can be determined from
1443	the &v4l2-buffer; <structfield>sequence</structfield> field. Image
1444	sizes refer to the frame, not fields. This format cannot be selected
1445	when using the read/write I/O method.<!-- Where it's indistinguishable
1446	from V4L2_FIELD_SEQ_*. --></entry>
1447		  </row>
1448		  <row>
1449		    <entry><constant>V4L2_FIELD_INTERLACED_TB</constant></entry>
1450		    <entry>8</entry>
1451		    <entry>Images contain both fields, interleaved line by
1452	line, top field first. The top field is transmitted first.</entry>
1453		  </row>
1454		  <row>
1455		    <entry><constant>V4L2_FIELD_INTERLACED_BT</constant></entry>
1456		    <entry>9</entry>
1457		    <entry>Images contain both fields, interleaved line by
1458	line, top field first. The bottom field is transmitted first.</entry>
1459		  </row>
1460		</tbody>
1461	      </tgroup>
1462	    </table>
1463	
1464	    <figure id="fieldseq-tb">
1465		<title>Field Order, Top Field First Transmitted</title>
1466		<mediaobject>
1467		  <imageobject>
1468		    <imagedata fileref="fieldseq_tb.pdf" format="PS" />
1469		  </imageobject>
1470		  <imageobject>
1471		    <imagedata fileref="fieldseq_tb.gif" format="GIF" />
1472		  </imageobject>
1473		</mediaobject>
1474	    </figure>
1475	
1476	    <figure id="fieldseq-bt">
1477		<title>Field Order, Bottom Field First Transmitted</title>
1478		<mediaobject>
1479		  <imageobject>
1480		    <imagedata fileref="fieldseq_bt.pdf" format="PS" />
1481		  </imageobject>
1482		  <imageobject>
1483		    <imagedata fileref="fieldseq_bt.gif" format="GIF" />
1484		  </imageobject>
1485		</mediaobject>
1486	    </figure>
1487	  </section>
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