1 <title>Input/Output</title> 2 3 <para>The V4L2 API defines several different methods to read from or 4write to a device. All drivers exchanging data with applications must 5support at least one of them.</para> 6 7 <para>The classic I/O method using the <function>read()</function> 8and <function>write()</function> function is automatically selected 9after opening a V4L2 device. When the driver does not support this 10method 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 13method with memory mapped or user buffers applications call the 14&VIDIOC-REQBUFS; ioctl. The asynchronous I/O method is not defined 15yet.</para> 16 17 <para>Video overlay can be considered another I/O method, although 18the application does not directly receive the image data. It is 19selected by initiating video overlay with the &VIDIOC-S-FMT; ioctl. 20For more information see <xref linkend="overlay" />.</para> 21 22 <para>Generally exactly one I/O method, including overlay, is 23associated with each file descriptor. The only exceptions are 24applications not exchanging data with a driver ("panel applications", 25see <xref linkend="open" />) and drivers permitting simultaneous video capturing 26and overlay using the same file descriptor, for compatibility with V4L 27and 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, 31but for simplicity drivers need not support switching the I/O method 32(after first switching away from read/write) other than by closing 33and reopening the device.</para> 34 35 <para>The following sections describe the various I/O methods in 36more 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, 43respectively, when the <constant>V4L2_CAP_READWRITE</constant> flag in 44the <structfield>capabilities</structfield> field of &v4l2-capability; 45returned by the &VIDIOC-QUERYCAP; ioctl is set.</para> 46 47 <para>Drivers may need the CPU to copy the data, but they may also 48support DMA to or from user memory, so this I/O method is not 49necessarily less efficient than other methods merely exchanging buffer 50pointers. It is considered inferior though because no meta-information 51like frame counters or timestamps are passed. This information is 52necessary to recognize frame dropping and to synchronize with other 53data streams. However this is also the simplest I/O method, requiring 54little or no setup to exchange data. It permits command line stunts 55like this (the <application>vidctrl</application> tool is 56fictitious):</para> 57 58 <informalexample> 59 <screen> 60> vidctrl /dev/video --input=0 --format=YUYV --size=352x288 61> 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. 67Drivers must implement one I/O method if they 68exchange data with applications, but it need not be this.<footnote> 69 <para>It would be desirable if applications could depend on 70drivers supporting all I/O interfaces, but as much as the complex 71memory mapping I/O can be inadequate for some devices we have no 72reason to require this interface, which is most useful for simple 73applications capturing still images.</para> 74 </footnote> When reading or writing is supported, the driver 75must also support the &func-select; and &func-poll; 76function.<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; 89returned by the &VIDIOC-QUERYCAP; ioctl is set. There are two 90streaming methods, to determine if the memory mapping flavor is 91supported applications must call the &VIDIOC-REQBUFS; ioctl.</para> 92 93 <para>Streaming is an I/O method where only pointers to buffers 94are exchanged between application and driver, the data itself is not 95copied. Memory mapping is primarily intended to map buffers in device 96memory into the application's address space. Device memory can be for 97example the video memory on a graphics card with a video capture 98add-on. However, being the most efficient I/O method available for a 99long time, many other drivers support streaming as well, allocating 100buffers in DMA-able main memory.</para> 101 102 <para>A driver can support many sets of buffers. Each set is 103identified by a unique buffer type value. The sets are independent and 104each set can hold a different type of data. To access different sets 105at the same time different file descriptors must be used.<footnote> 106 <para>One could use one file descriptor and set the buffer 107type field accordingly when calling &VIDIOC-QBUF; etc., but it makes 108the <function>select()</function> function ambiguous. We also like the 109clean approach of one file descriptor per logical stream. Video 110overlay for example is also a logical stream, although the CPU is not 111needed 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 116type, for example <constant>V4L2_BUF_TYPE_VIDEO_CAPTURE</constant>. 117This ioctl can also be used to change the number of buffers or to free 118the allocated memory, provided none of the buffers are still 119mapped.</para> 120 121 <para>Before applications can access the buffers they must map 122them into their address space with the &func-mmap; function. The 123location of the buffers in device memory can be determined with the 124&VIDIOC-QUERYBUF; ioctl. The <structfield>m.offset</structfield> and 125<structfield>length</structfield> returned in a &v4l2-buffer; are 126passed as sixth and second parameter to the 127<function>mmap()</function> function. The offset and length values 128must not be modified. Remember the buffers are allocated in physical 129memory, as opposed to virtual memory which can be swapped out to disk. 130Applications should free the buffers as soon as possible with the 131&func-munmap; function.</para> 132 133 <example> 134 <title>Mapping buffers</title> 135 136 <programlisting> 137&v4l2-requestbuffers; reqbuf; 138struct { 139 void *start; 140 size_t length; 141} *buffers; 142unsigned int i; 143 144memset (&reqbuf, 0, sizeof (reqbuf)); 145reqbuf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE; 146reqbuf.memory = V4L2_MEMORY_MMAP; 147reqbuf.count = 20; 148 149if (-1 == ioctl (fd, &VIDIOC-REQBUFS;, &reqbuf)) { 150 if (errno == EINVAL) 151 printf ("Video capturing or mmap-streaming is not supported\n"); 152 else 153 perror ("VIDIOC_REQBUFS"); 154 155 exit (EXIT_FAILURE); 156} 157 158/* We want at least five buffers. */ 159 160if (reqbuf.count < 5) { 161 /* You may need to free the buffers here. */ 162 printf ("Not enough buffer memory\n"); 163 exit (EXIT_FAILURE); 164} 165 166buffers = calloc (reqbuf.count, sizeof (*buffers)); 167assert (buffers != NULL); 168 169for (i = 0; i < reqbuf.count; i++) { 170 &v4l2-buffer; buffer; 171 172 memset (&buffer, 0, sizeof (buffer)); 173 buffer.type = reqbuf.type; 174 buffer.memory = V4L2_MEMORY_MMAP; 175 buffer.index = i; 176 177 if (-1 == ioctl (fd, &VIDIOC-QUERYBUF;, &buffer)) { 178 perror ("VIDIOC_QUERYBUF"); 179 exit (EXIT_FAILURE); 180 } 181 182 buffers[i].length = buffer.length; /* remember for munmap() */ 183 184 buffers[i].start = mmap (NULL, buffer.length, 185 PROT_READ | PROT_WRITE, /* recommended */ 186 MAP_SHARED, /* recommended */ 187 fd, buffer.m.offset); 188 189 if (MAP_FAILED == buffers[i].start) { 190 /* If you do not exit here you should unmap() and free() 191 the buffers mapped so far. */ 192 perror ("mmap"); 193 exit (EXIT_FAILURE); 194 } 195} 196 197/* Cleanup. */ 198 199for (i = 0; i < reqbuf.count; i++) 200 munmap (buffers[i].start, buffers[i].length); 201 </programlisting> 202 </example> 203 204 <para>Conceptually streaming drivers maintain two buffer queues, an incoming 205and an outgoing queue. They separate the synchronous capture or output 206operation locked to a video clock from the application which is 207subject to random disk or network delays and preemption by 208other processes, thereby reducing the probability of data loss. 209The queues are organized as FIFOs, buffers will be 210output in the order enqueued in the incoming FIFO, and were 211captured in the order dequeued from the outgoing FIFO.</para> 212 213 <para>The driver may require a minimum number of buffers enqueued 214at all times to function, apart of this no limit exists on the number 215of buffers applications can enqueue in advance, or dequeue and 216process. They can also enqueue in a different order than buffers have 217been dequeued, and the driver can <emphasis>fill</emphasis> enqueued 218<emphasis>empty</emphasis> buffers in any order. <footnote> 219 <para>Random enqueue order permits applications processing 220images out of order (such as video codecs) to return buffers earlier, 221reducing the probability of data loss. Random fill order allows 222drivers to reuse buffers on a LIFO-basis, taking advantage of caches 223holding scatter-gather lists and the like.</para> 224 </footnote> The index number of a buffer (&v4l2-buffer; 225<structfield>index</structfield>) plays no role here, it only 226identifies the buffer.</para> 227 228 <para>Initially all mapped buffers are in dequeued state, 229inaccessible by the driver. For capturing applications it is customary 230to first enqueue all mapped buffers, then to start capturing and enter 231the read loop. Here the application waits until a filled buffer can be 232dequeued, and re-enqueues the buffer when the data is no longer 233needed. Output applications fill and enqueue buffers, when enough 234buffers are stacked up the output is started with 235<constant>VIDIOC_STREAMON</constant>. In the write loop, when 236the application runs out of free buffers, it must wait until an empty 237buffer can be dequeued and reused.</para> 238 239 <para>To enqueue and dequeue a buffer applications use the 240&VIDIOC-QBUF; and &VIDIOC-DQBUF; ioctl. The status of a buffer being 241mapped, enqueued, full or empty can be determined at any time using the 242&VIDIOC-QUERYBUF; ioctl. Two methods exist to suspend execution of the 243application until one or more buffers can be dequeued. By default 244<constant>VIDIOC_DQBUF</constant> blocks when no buffer is in the 245outgoing queue. When the <constant>O_NONBLOCK</constant> flag was 246given to the &func-open; function, <constant>VIDIOC_DQBUF</constant> 247returns immediately with an &EAGAIN; when no buffer is available. The 248&func-select; or &func-poll; function are always available.</para> 249 250 <para>To start and stop capturing or output applications call the 251&VIDIOC-STREAMON; and &VIDIOC-STREAMOFF; ioctl. Note 252<constant>VIDIOC_STREAMOFF</constant> removes all buffers from both 253queues as a side effect. Since there is no notion of doing anything 254"now" on a multitasking system, if an application needs to synchronize 255with another event it should examine the &v4l2-buffer; 256<structfield>timestamp</structfield> of captured buffers, or set the 257field before enqueuing buffers for output.</para> 258 259 <para>Drivers implementing memory mapping I/O must 260support the <constant>VIDIOC_REQBUFS</constant>, 261<constant>VIDIOC_QUERYBUF</constant>, 262<constant>VIDIOC_QBUF</constant>, <constant>VIDIOC_DQBUF</constant>, 263<constant>VIDIOC_STREAMON</constant> and 264<constant>VIDIOC_STREAMOFF</constant> ioctl, the 265<function>mmap()</function>, <function>munmap()</function>, 266<function>select()</function> and <function>poll()</function> 267function.<footnote> 268 <para>At the driver level <function>select()</function> and 269<function>poll()</function> are the same, and 270<function>select()</function> is too important to be optional. The 271rest should be evident.</para> 272 </footnote></para> 273 274 <para>[capture example]</para> 275 276 </section> 277 278 <section id="userp"> 279 <title>Streaming I/O (User Pointers)</title> 280 281 <para>Input and output devices support this I/O method when the 282<constant>V4L2_CAP_STREAMING</constant> flag in the 283<structfield>capabilities</structfield> field of &v4l2-capability; 284returned by the &VIDIOC-QUERYCAP; ioctl is set. If the particular user 285pointer method (not only memory mapping) is supported must be 286determined by calling the &VIDIOC-REQBUFS; ioctl.</para> 287 288 <para>This I/O method combines advantages of the read/write and 289memory mapping methods. Buffers are allocated by the application 290itself, and can reside for example in virtual or shared memory. Only 291pointers to data are exchanged, these pointers and meta-information 292are passed in &v4l2-buffer;. The driver must be switched 293into user pointer I/O mode by calling the &VIDIOC-REQBUFS; with the 294desired buffer type. No buffers are allocated beforehands, 295consequently they are not indexed and cannot be queried like mapped 296buffers with the <constant>VIDIOC_QUERYBUF</constant> ioctl.</para> 297 298 <example> 299 <title>Initiating streaming I/O with user pointers</title> 300 301 <programlisting> 302&v4l2-requestbuffers; reqbuf; 303 304memset (&reqbuf, 0, sizeof (reqbuf)); 305reqbuf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE; 306reqbuf.memory = V4L2_MEMORY_USERPTR; 307 308if (ioctl (fd, &VIDIOC-REQBUFS;, &reqbuf) == -1) { 309 if (errno == EINVAL) 310 printf ("Video capturing or user pointer streaming is not supported\n"); 311 else 312 perror ("VIDIOC_REQBUFS"); 313 314 exit (EXIT_FAILURE); 315} 316 </programlisting> 317 </example> 318 319 <para>Buffer addresses and sizes are passed on the fly with the 320&VIDIOC-QBUF; ioctl. Although buffers are commonly cycled, 321applications can pass different addresses and sizes at each 322<constant>VIDIOC_QBUF</constant> call. If required by the hardware the 323driver swaps memory pages within physical memory to create a 324continuous area of memory. This happens transparently to the 325application in the virtual memory subsystem of the kernel. When buffer 326pages have been swapped out to disk they are brought back and finally 327locked in physical memory for DMA.<footnote> 328 <para>We expect that frequently used buffers are typically not 329swapped out. Anyway, the process of swapping, locking or generating 330scatter-gather lists may be time consuming. The delay can be masked by 331the depth of the incoming buffer queue, and perhaps by maintaining 332caches assuming a buffer will be soon enqueued again. On the other 333hand, to optimize memory usage drivers can limit the number of buffers 334locked in advance and recycle the most recently used buffers first. Of 335course, the pages of empty buffers in the incoming queue need not be 336saved to disk. Output buffers must be saved on the incoming and 337outgoing queue because an application may share them with other 338processes.</para> 339 </footnote></para> 340 341 <para>Filled or displayed buffers are dequeued with the 342&VIDIOC-DQBUF; ioctl. The driver can unlock the memory pages at any 343time between the completion of the DMA and this ioctl. The memory is 344also unlocked when &VIDIOC-STREAMOFF; is called, &VIDIOC-REQBUFS;, or 345when the device is closed. Applications must take care not to free 346buffers without dequeuing. For once, the buffers remain locked until 347further, wasting physical memory. Second the driver will not be 348notified when the memory is returned to the application's free list 349and subsequently reused for other purposes, possibly completing the 350requested DMA and overwriting valuable data.</para> 351 352 <para>For capturing applications it is customary to enqueue a 353number of empty buffers, to start capturing and enter the read loop. 354Here the application waits until a filled buffer can be dequeued, and 355re-enqueues the buffer when the data is no longer needed. Output 356applications fill and enqueue buffers, when enough buffers are stacked 357up output is started. In the write loop, when the application 358runs out of free buffers it must wait until an empty buffer can be 359dequeued and reused. Two methods exist to suspend execution of the 360application until one or more buffers can be dequeued. By default 361<constant>VIDIOC_DQBUF</constant> blocks when no buffer is in the 362outgoing queue. When the <constant>O_NONBLOCK</constant> flag was 363given to the &func-open; function, <constant>VIDIOC_DQBUF</constant> 364returns immediately with an &EAGAIN; when no buffer is available. The 365&func-select; or &func-poll; function are always available.</para> 366 367 <para>To start and stop capturing or output applications call the 368&VIDIOC-STREAMON; and &VIDIOC-STREAMOFF; ioctl. Note 369<constant>VIDIOC_STREAMOFF</constant> removes all buffers from both 370queues and unlocks all buffers as a side effect. Since there is no 371notion of doing anything "now" on a multitasking system, if an 372application needs to synchronize with another event it should examine 373the &v4l2-buffer; <structfield>timestamp</structfield> of captured 374buffers, or set the field before enqueuing buffers for output.</para> 375 376 <para>Drivers implementing user pointer I/O must 377support the <constant>VIDIOC_REQBUFS</constant>, 378<constant>VIDIOC_QBUF</constant>, <constant>VIDIOC_DQBUF</constant>, 379<constant>VIDIOC_STREAMON</constant> and 380<constant>VIDIOC_STREAMOFF</constant> ioctl, the 381<function>select()</function> and <function>poll()</function> function.<footnote> 382 <para>At the driver level <function>select()</function> and 383<function>poll()</function> are the same, and 384<function>select()</function> is too important to be optional. The 385rest should be evident.</para> 386 </footnote></para> 387 </section> 388 389 <section id="async"> 390 <title>Asynchronous I/O</title> 391 392 <para>This method is not defined yet.</para> 393 </section> 394 395 <section id="buffer"> 396 <title>Buffers</title> 397 398 <para>A buffer contains data exchanged by application and 399driver using one of the Streaming I/O methods. Only pointers to 400buffers are exchanged, the data itself is not copied. These pointers, 401together with meta-information like timestamps or field parity, are 402stored in a struct <structname>v4l2_buffer</structname>, argument to 403the &VIDIOC-QUERYBUF;, &VIDIOC-QBUF; and &VIDIOC-DQBUF; ioctl.</para> 404 405 <para>Nominally timestamps refer to the first data byte transmitted. 406In practice however the wide range of hardware covered by the V4L2 API 407limits timestamp accuracy. Often an interrupt routine will 408sample the system clock shortly after the field or frame was stored 409completely in memory. So applications must expect a constant 410difference up to one field or frame period plus a small (few scan 411lines) random error. The delay and error can be much 412larger due to compression or transmission over an external bus when 413the frames are not properly stamped by the sender. This is frequently 414the case with USB cameras. Here timestamps refer to the instant the 415field or frame was received by the driver, not the capture time. These 416devices identify by not enumerating any video standards, see <xref 417linkend="standard" />.</para> 418 419 <para>Similar limitations apply to output timestamps. Typically 420the video hardware locks to a clock controlling the video timing, the 421horizontal and vertical synchronization pulses. At some point in the 422line sequence, possibly the vertical blanking, an interrupt routine 423samples the system clock, compares against the timestamp and programs 424the hardware to repeat the previous field or frame, or to display the 425buffer contents.</para> 426 427 <para>Apart of limitations of the video device and natural 428inaccuracies of all clocks, it should be noted system time itself is 429not perfectly stable. It can be affected by power saving cycles, 430warped to insert leap seconds, or even turned back or forth by the 431system administrator affecting long term measurements. <footnote> 432 <para>Since no other Linux multimedia 433API supports unadjusted time it would be foolish to introduce here. We 434must use a universally supported clock to synchronize different media, 435hence time of day.</para> 436 </footnote></para> 437 438 <table frame="none" pgwide="1" id="v4l2-buffer"> 439 <title>struct <structname>v4l2_buffer</structname></title> 440 <tgroup cols="4"> 441 &cs-ustr; 442 <tbody valign="top"> 443 <row> 444 <entry>__u32</entry> 445 <entry><structfield>index</structfield></entry> 446 <entry></entry> 447 <entry>Number of the buffer, set by the application. This 448field is only used for <link linkend="mmap">memory mapping</link> I/O 449and can range from zero to the number of buffers allocated 450with the &VIDIOC-REQBUFS; ioctl (&v4l2-requestbuffers; <structfield>count</structfield>) minus one.</entry> 451 </row> 452 <row> 453 <entry>&v4l2-buf-type;</entry> 454 <entry><structfield>type</structfield></entry> 455 <entry></entry> 456 <entry>Type of the buffer, same as &v4l2-format; 457<structfield>type</structfield> or &v4l2-requestbuffers; 458<structfield>type</structfield>, set by the application.</entry> 459 </row> 460 <row> 461 <entry>__u32</entry> 462 <entry><structfield>bytesused</structfield></entry> 463 <entry></entry> 464 <entry>The number of bytes occupied by the data in the 465buffer. It depends on the negotiated data format and may change with 466each buffer for compressed variable size data like JPEG images. 467Drivers must set this field when <structfield>type</structfield> 468refers to an input stream, applications when an output stream.</entry> 469 </row> 470 <row> 471 <entry>__u32</entry> 472 <entry><structfield>flags</structfield></entry> 473 <entry></entry> 474 <entry>Flags set by the application or driver, see <xref 475linkend="buffer-flags" />.</entry> 476 </row> 477 <row> 478 <entry>&v4l2-field;</entry> 479 <entry><structfield>field</structfield></entry> 480 <entry></entry> 481 <entry>Indicates the field order of the image in the 482buffer, see <xref linkend="v4l2-field" />. This field is not used when 483the buffer contains VBI data. Drivers must set it when 484<structfield>type</structfield> refers to an input stream, 485applications when an output stream.</entry> 486 </row> 487 <row> 488 <entry>struct timeval</entry> 489 <entry><structfield>timestamp</structfield></entry> 490 <entry></entry> 491 <entry><para>For input streams this is the 492system time (as returned by the <function>gettimeofday()</function> 493function) when the first data byte was captured. For output streams 494the data will not be displayed before this time, secondary to the 495nominal frame rate determined by the current video standard in 496enqueued order. Applications can for example zero this field to 497display frames as soon as possible. The driver stores the time at 498which the first data byte was actually sent out in the 499<structfield>timestamp</structfield> field. This permits 500applications to monitor the drift between the video and system 501clock.</para></entry> 502 </row> 503 <row> 504 <entry>&v4l2-timecode;</entry> 505 <entry><structfield>timecode</structfield></entry> 506 <entry></entry> 507 <entry>When <structfield>type</structfield> is 508<constant>V4L2_BUF_TYPE_VIDEO_CAPTURE</constant> and the 509<constant>V4L2_BUF_FLAG_TIMECODE</constant> flag is set in 510<structfield>flags</structfield>, this structure contains a frame 511timecode. In <link linkend="v4l2-field">V4L2_FIELD_ALTERNATE</link> 512mode the top and bottom field contain the same timecode. 513Timecodes are intended to help video editing and are typically recorded on 514video tapes, but also embedded in compressed formats like MPEG. This 515field is independent of the <structfield>timestamp</structfield> and 516<structfield>sequence</structfield> fields.</entry> 517 </row> 518 <row> 519 <entry>__u32</entry> 520 <entry><structfield>sequence</structfield></entry> 521 <entry></entry> 522 <entry>Set by the driver, counting the frames in the 523sequence.</entry> 524 </row> 525 <row> 526 <entry spanname="hspan"><para>In <link 527linkend="v4l2-field">V4L2_FIELD_ALTERNATE</link> mode the top and 528bottom field have the same sequence number. The count starts at zero 529and includes dropped or repeated frames. A dropped frame was received 530by an input device but could not be stored due to lack of free buffer 531space. A repeated frame was displayed again by an output device 532because the application did not pass new data in 533time.</para><para>Note this may count the frames received 534e.g. over USB, without taking into account the frames dropped by the 535remote hardware due to limited compression throughput or bus 536bandwidth. These devices identify by not enumerating any video 537standards, see <xref linkend="standard" />.</para></entry> 538 </row> 539 <row> 540 <entry>&v4l2-memory;</entry> 541 <entry><structfield>memory</structfield></entry> 542 <entry></entry> 543 <entry>This field must be set by applications and/or drivers 544in accordance with the selected I/O method.</entry> 545 </row> 546 <row> 547 <entry>union</entry> 548 <entry><structfield>m</structfield></entry> 549 </row> 550 <row> 551 <entry></entry> 552 <entry>__u32</entry> 553 <entry><structfield>offset</structfield></entry> 554 <entry>When <structfield>memory</structfield> is 555<constant>V4L2_MEMORY_MMAP</constant> this is the offset of the buffer 556from the start of the device memory. The value is returned by the 557driver and apart of serving as parameter to the &func-mmap; function 558not useful for applications. See <xref linkend="mmap" /> for details.</entry> 559 </row> 560 <row> 561 <entry></entry> 562 <entry>unsigned long</entry> 563 <entry><structfield>userptr</structfield></entry> 564 <entry>When <structfield>memory</structfield> is 565<constant>V4L2_MEMORY_USERPTR</constant> this is a pointer to the 566buffer (casted to unsigned long type) in virtual memory, set by the 567application. See <xref linkend="userp" /> for details.</entry> 568 </row> 569 <row> 570 <entry>__u32</entry> 571 <entry><structfield>length</structfield></entry> 572 <entry></entry> 573 <entry>Size of the buffer (not the payload) in bytes.</entry> 574 </row> 575 <row> 576 <entry>__u32</entry> 577 <entry><structfield>input</structfield></entry> 578 <entry></entry> 579 <entry>Some video capture drivers support rapid and 580synchronous video input changes, a function useful for example in 581video surveillance applications. For this purpose applications set the 582<constant>V4L2_BUF_FLAG_INPUT</constant> flag, and this field to the 583number of a video input as in &v4l2-input; field 584<structfield>index</structfield>.</entry> 585 </row> 586 <row> 587 <entry>__u32</entry> 588 <entry><structfield>reserved</structfield></entry> 589 <entry></entry> 590 <entry>A place holder for future extensions and custom 591(driver defined) buffer types 592<constant>V4L2_BUF_TYPE_PRIVATE</constant> and higher. Applications 593should set this to 0.</entry> 594 </row> 595 </tbody> 596 </tgroup> 597 </table> 598 599 <table frame="none" pgwide="1" id="v4l2-buf-type"> 600 <title>enum v4l2_buf_type</title> 601 <tgroup cols="3"> 602 &cs-def; 603 <tbody valign="top"> 604 <row> 605 <entry><constant>V4L2_BUF_TYPE_VIDEO_CAPTURE</constant></entry> 606 <entry>1</entry> 607 <entry>Buffer of a video capture stream, see <xref 608 linkend="capture" />.</entry> 609 </row> 610 <row> 611 <entry><constant>V4L2_BUF_TYPE_VIDEO_OUTPUT</constant></entry> 612 <entry>2</entry> 613 <entry>Buffer of a video output stream, see <xref 614 linkend="output" />.</entry> 615 </row> 616 <row> 617 <entry><constant>V4L2_BUF_TYPE_VIDEO_OVERLAY</constant></entry> 618 <entry>3</entry> 619 <entry>Buffer for video overlay, see <xref linkend="overlay" />.</entry> 620 </row> 621 <row> 622 <entry><constant>V4L2_BUF_TYPE_VBI_CAPTURE</constant></entry> 623 <entry>4</entry> 624 <entry>Buffer of a raw VBI capture stream, see <xref 625 linkend="raw-vbi" />.</entry> 626 </row> 627 <row> 628 <entry><constant>V4L2_BUF_TYPE_VBI_OUTPUT</constant></entry> 629 <entry>5</entry> 630 <entry>Buffer of a raw VBI output stream, see <xref 631 linkend="raw-vbi" />.</entry> 632 </row> 633 <row> 634 <entry><constant>V4L2_BUF_TYPE_SLICED_VBI_CAPTURE</constant></entry> 635 <entry>6</entry> 636 <entry>Buffer of a sliced VBI capture stream, see <xref 637 linkend="sliced" />.</entry> 638 </row> 639 <row> 640 <entry><constant>V4L2_BUF_TYPE_SLICED_VBI_OUTPUT</constant></entry> 641 <entry>7</entry> 642 <entry>Buffer of a sliced VBI output stream, see <xref 643 linkend="sliced" />.</entry> 644 </row> 645 <row> 646 <entry><constant>V4L2_BUF_TYPE_VIDEO_OUTPUT_OVERLAY</constant></entry> 647 <entry>8</entry> 648 <entry>Buffer for video output overlay (OSD), see <xref 649 linkend="osd" />. Status: <link 650linkend="experimental">Experimental</link>.</entry> 651 </row> 652 <row> 653 <entry><constant>V4L2_BUF_TYPE_PRIVATE</constant></entry> 654 <entry>0x80</entry> 655 <entry>This and higher values are reserved for custom 656(driver defined) buffer types.</entry> 657 </row> 658 </tbody> 659 </tgroup> 660 </table> 661 662 <table frame="none" pgwide="1" id="buffer-flags"> 663 <title>Buffer Flags</title> 664 <tgroup cols="3"> 665 &cs-def; 666 <tbody valign="top"> 667 <row> 668 <entry><constant>V4L2_BUF_FLAG_MAPPED</constant></entry> 669 <entry>0x0001</entry> 670 <entry>The buffer resides in device memory and has been mapped 671into the application's address space, see <xref linkend="mmap" /> for details. 672Drivers set or clear this flag when the 673<link linkend="vidioc-querybuf">VIDIOC_QUERYBUF</link>, <link 674 linkend="vidioc-qbuf">VIDIOC_QBUF</link> or <link 675 linkend="vidioc-qbuf">VIDIOC_DQBUF</link> ioctl is called. Set by the driver.</entry> 676 </row> 677 <row> 678 <entry><constant>V4L2_BUF_FLAG_QUEUED</constant></entry> 679 <entry>0x0002</entry> 680 <entry>Internally drivers maintain two buffer queues, an 681incoming and outgoing queue. When this flag is set, the buffer is 682currently on the incoming queue. It automatically moves to the 683outgoing queue after the buffer has been filled (capture devices) or 684displayed (output devices). Drivers set or clear this flag when the 685<constant>VIDIOC_QUERYBUF</constant> ioctl is called. After 686(successful) calling the <constant>VIDIOC_QBUF </constant>ioctl it is 687always set and after <constant>VIDIOC_DQBUF</constant> always 688cleared.</entry> 689 </row> 690 <row> 691 <entry><constant>V4L2_BUF_FLAG_DONE</constant></entry> 692 <entry>0x0004</entry> 693 <entry>When this flag is set, the buffer is currently on 694the outgoing queue, ready to be dequeued from the driver. Drivers set 695or clear this flag when the <constant>VIDIOC_QUERYBUF</constant> ioctl 696is called. After calling the <constant>VIDIOC_QBUF</constant> or 697<constant>VIDIOC_DQBUF</constant> it is always cleared. Of course a 698buffer cannot be on both queues at the same time, the 699<constant>V4L2_BUF_FLAG_QUEUED</constant> and 700<constant>V4L2_BUF_FLAG_DONE</constant> flag are mutually exclusive. 701They can be both cleared however, then the buffer is in "dequeued" 702state, in the application domain to say so.</entry> 703 </row> 704 <row> 705 <entry><constant>V4L2_BUF_FLAG_ERROR</constant></entry> 706 <entry>0x0040</entry> 707 <entry>When this flag is set, the buffer has been dequeued 708 successfully, although the data might have been corrupted. 709 This is recoverable, streaming may continue as normal and 710 the buffer may be reused normally. 711 Drivers set this flag when the <constant>VIDIOC_DQBUF</constant> 712 ioctl is called.</entry> 713 </row> 714 <row> 715 <entry><constant>V4L2_BUF_FLAG_KEYFRAME</constant></entry> 716 <entry>0x0008</entry> 717 <entry>Drivers set or clear this flag when calling the 718<constant>VIDIOC_DQBUF</constant> ioctl. It may be set by video 719capture devices when the buffer contains a compressed image which is a 720key frame (or field), &ie; can be decompressed on its own.</entry> 721 </row> 722 <row> 723 <entry><constant>V4L2_BUF_FLAG_PFRAME</constant></entry> 724 <entry>0x0010</entry> 725 <entry>Similar to <constant>V4L2_BUF_FLAG_KEYFRAME</constant> 726this flags predicted frames or fields which contain only differences to a 727previous key frame.</entry> 728 </row> 729 <row> 730 <entry><constant>V4L2_BUF_FLAG_BFRAME</constant></entry> 731 <entry>0x0020</entry> 732 <entry>Similar to <constant>V4L2_BUF_FLAG_PFRAME</constant> 733 this is a bidirectional predicted frame or field. [ooc tbd]</entry> 734 </row> 735 <row> 736 <entry><constant>V4L2_BUF_FLAG_TIMECODE</constant></entry> 737 <entry>0x0100</entry> 738 <entry>The <structfield>timecode</structfield> field is valid. 739Drivers set or clear this flag when the <constant>VIDIOC_DQBUF</constant> 740ioctl is called.</entry> 741 </row> 742 <row> 743 <entry><constant>V4L2_BUF_FLAG_INPUT</constant></entry> 744 <entry>0x0200</entry> 745 <entry>The <structfield>input</structfield> field is valid. 746Applications set or clear this flag before calling the 747<constant>VIDIOC_QBUF</constant> ioctl.</entry> 748 </row> 749 </tbody> 750 </tgroup> 751 </table> 752 753 <table pgwide="1" frame="none" id="v4l2-memory"> 754 <title>enum v4l2_memory</title> 755 <tgroup cols="3"> 756 &cs-def; 757 <tbody valign="top"> 758 <row> 759 <entry><constant>V4L2_MEMORY_MMAP</constant></entry> 760 <entry>1</entry> 761 <entry>The buffer is used for <link linkend="mmap">memory 762mapping</link> I/O.</entry> 763 </row> 764 <row> 765 <entry><constant>V4L2_MEMORY_USERPTR</constant></entry> 766 <entry>2</entry> 767 <entry>The buffer is used for <link linkend="userp">user 768pointer</link> I/O.</entry> 769 </row> 770 <row> 771 <entry><constant>V4L2_MEMORY_OVERLAY</constant></entry> 772 <entry>3</entry> 773 <entry>[to do]</entry> 774 </row> 775 </tbody> 776 </tgroup> 777 </table> 778 779 <section> 780 <title>Timecodes</title> 781 782 <para>The <structname>v4l2_timecode</structname> structure is 783designed to hold a <xref linkend="smpte12m" /> or similar timecode. 784(struct <structname>timeval</structname> timestamps are stored in 785&v4l2-buffer; field <structfield>timestamp</structfield>.)</para> 786 787 <table frame="none" pgwide="1" id="v4l2-timecode"> 788 <title>struct <structname>v4l2_timecode</structname></title> 789 <tgroup cols="3"> 790 &cs-str; 791 <tbody valign="top"> 792 <row> 793 <entry>__u32</entry> 794 <entry><structfield>type</structfield></entry> 795 <entry>Frame rate the timecodes are based on, see <xref 796 linkend="timecode-type" />.</entry> 797 </row> 798 <row> 799 <entry>__u32</entry> 800 <entry><structfield>flags</structfield></entry> 801 <entry>Timecode flags, see <xref linkend="timecode-flags" />.</entry> 802 </row> 803 <row> 804 <entry>__u8</entry> 805 <entry><structfield>frames</structfield></entry> 806 <entry>Frame count, 0 ... 23/24/29/49/59, depending on the 807 type of timecode.</entry> 808 </row> 809 <row> 810 <entry>__u8</entry> 811 <entry><structfield>seconds</structfield></entry> 812 <entry>Seconds count, 0 ... 59. This is a binary, not BCD number.</entry> 813 </row> 814 <row> 815 <entry>__u8</entry> 816 <entry><structfield>minutes</structfield></entry> 817 <entry>Minutes count, 0 ... 59. This is a binary, not BCD number.</entry> 818 </row> 819 <row> 820 <entry>__u8</entry> 821 <entry><structfield>hours</structfield></entry> 822 <entry>Hours count, 0 ... 29. This is a binary, not BCD number.</entry> 823 </row> 824 <row> 825 <entry>__u8</entry> 826 <entry><structfield>userbits</structfield>[4]</entry> 827 <entry>The "user group" bits from the timecode.</entry> 828 </row> 829 </tbody> 830 </tgroup> 831 </table> 832 833 <table frame="none" pgwide="1" id="timecode-type"> 834 <title>Timecode Types</title> 835 <tgroup cols="3"> 836 &cs-def; 837 <tbody valign="top"> 838 <row> 839 <entry><constant>V4L2_TC_TYPE_24FPS</constant></entry> 840 <entry>1</entry> 841 <entry>24 frames per second, i. e. film.</entry> 842 </row> 843 <row> 844 <entry><constant>V4L2_TC_TYPE_25FPS</constant></entry> 845 <entry>2</entry> 846 <entry>25 frames per second, &ie; PAL or SECAM video.</entry> 847 </row> 848 <row> 849 <entry><constant>V4L2_TC_TYPE_30FPS</constant></entry> 850 <entry>3</entry> 851 <entry>30 frames per second, &ie; NTSC video.</entry> 852 </row> 853 <row> 854 <entry><constant>V4L2_TC_TYPE_50FPS</constant></entry> 855 <entry>4</entry> 856 <entry></entry> 857 </row> 858 <row> 859 <entry><constant>V4L2_TC_TYPE_60FPS</constant></entry> 860 <entry>5</entry> 861 <entry></entry> 862 </row> 863 </tbody> 864 </tgroup> 865 </table> 866 867 <table frame="none" pgwide="1" id="timecode-flags"> 868 <title>Timecode Flags</title> 869 <tgroup cols="3"> 870 &cs-def; 871 <tbody valign="top"> 872 <row> 873 <entry><constant>V4L2_TC_FLAG_DROPFRAME</constant></entry> 874 <entry>0x0001</entry> 875 <entry>Indicates "drop frame" semantics for counting frames 876in 29.97 fps material. When set, frame numbers 0 and 1 at the start of 877each minute, except minutes 0, 10, 20, 30, 40, 50 are omitted from the 878count.</entry> 879 </row> 880 <row> 881 <entry><constant>V4L2_TC_FLAG_COLORFRAME</constant></entry> 882 <entry>0x0002</entry> 883 <entry>The "color frame" flag.</entry> 884 </row> 885 <row> 886 <entry><constant>V4L2_TC_USERBITS_field</constant></entry> 887 <entry>0x000C</entry> 888 <entry>Field mask for the "binary group flags".</entry> 889 </row> 890 <row> 891 <entry><constant>V4L2_TC_USERBITS_USERDEFINED</constant></entry> 892 <entry>0x0000</entry> 893 <entry>Unspecified format.</entry> 894 </row> 895 <row> 896 <entry><constant>V4L2_TC_USERBITS_8BITCHARS</constant></entry> 897 <entry>0x0008</entry> 898 <entry>8-bit ISO characters.</entry> 899 </row> 900 </tbody> 901 </tgroup> 902 </table> 903 </section> 904 </section> 905 906 <section id="field-order"> 907 <title>Field Order</title> 908 909 <para>We have to distinguish between progressive and interlaced 910video. Progressive video transmits all lines of a video image 911sequentially. Interlaced video divides an image into two fields, 912containing only the odd and even lines of the image, respectively. 913Alternating the so called odd and even field are transmitted, and due 914to a small delay between fields a cathode ray TV displays the lines 915interleaved, yielding the original frame. This curious technique was 916invented because at refresh rates similar to film the image would 917fade out too quickly. Transmitting fields reduces the flicker without 918the necessity of doubling the frame rate and with it the bandwidth 919required for each channel.</para> 920 921 <para>It is important to understand a video camera does not expose 922one frame at a time, merely transmitting the frames separated into 923fields. The fields are in fact captured at two different instances in 924time. An object on screen may well move between one field and the 925next. For applications analysing motion it is of paramount importance 926to recognize which field of a frame is older, the <emphasis>temporal 927order</emphasis>.</para> 928 929 <para>When the driver provides or accepts images field by field 930rather than interleaved, it is also important applications understand 931how the fields combine to frames. We distinguish between top (aka odd) and 932bottom (aka even) fields, the <emphasis>spatial order</emphasis>: The first line 933of the top field is the first line of an interlaced frame, the first 934line of the bottom field is the second line of that frame.</para> 935 936 <para>However because fields were captured one after the other, 937arguing whether a frame commences with the top or bottom field is 938pointless. Any two successive top and bottom, or bottom and top fields 939yield a valid frame. Only when the source was progressive to begin 940with, ⪚ when transferring film to video, two fields may come from 941the same frame, creating a natural order.</para> 942 943 <para>Counter to intuition the top field is not necessarily the 944older field. Whether the older field contains the top or bottom lines 945is a convention determined by the video standard. Hence the 946distinction between temporal and spatial order of fields. The diagrams 947below should make this clearer.</para> 948 949 <para>All video capture and output devices must report the current 950field order. Some drivers may permit the selection of a different 951order, to this end applications initialize the 952<structfield>field</structfield> field of &v4l2-pix-format; before 953calling the &VIDIOC-S-FMT; ioctl. If this is not desired it should 954have the value <constant>V4L2_FIELD_ANY</constant> (0).</para> 955 956 <table frame="none" pgwide="1" id="v4l2-field"> 957 <title>enum v4l2_field</title> 958 <tgroup cols="3"> 959 &cs-def; 960 <tbody valign="top"> 961 <row> 962 <entry><constant>V4L2_FIELD_ANY</constant></entry> 963 <entry>0</entry> 964 <entry>Applications request this field order when any 965one of the <constant>V4L2_FIELD_NONE</constant>, 966<constant>V4L2_FIELD_TOP</constant>, 967<constant>V4L2_FIELD_BOTTOM</constant>, or 968<constant>V4L2_FIELD_INTERLACED</constant> formats is acceptable. 969Drivers choose depending on hardware capabilities or e. g. the 970requested image size, and return the actual field order. &v4l2-buffer; 971<structfield>field</structfield> can never be 972<constant>V4L2_FIELD_ANY</constant>.</entry> 973 </row> 974 <row> 975 <entry><constant>V4L2_FIELD_NONE</constant></entry> 976 <entry>1</entry> 977 <entry>Images are in progressive format, not interlaced. 978The driver may also indicate this order when it cannot distinguish 979between <constant>V4L2_FIELD_TOP</constant> and 980<constant>V4L2_FIELD_BOTTOM</constant>.</entry> 981 </row> 982 <row> 983 <entry><constant>V4L2_FIELD_TOP</constant></entry> 984 <entry>2</entry> 985 <entry>Images consist of the top (aka odd) field only.</entry> 986 </row> 987 <row> 988 <entry><constant>V4L2_FIELD_BOTTOM</constant></entry> 989 <entry>3</entry> 990 <entry>Images consist of the bottom (aka even) field only. 991Applications may wish to prevent a device from capturing interlaced 992images because they will have "comb" or "feathering" artefacts around 993moving objects.</entry> 994 </row> 995 <row> 996 <entry><constant>V4L2_FIELD_INTERLACED</constant></entry> 997 <entry>4</entry> 998 <entry>Images contain both fields, interleaved line by 999line. The temporal order of the fields (whether the top or bottom 1000field is first transmitted) depends on the current video standard. 1001M/NTSC transmits the bottom field first, all other standards the top 1002field first.</entry> 1003 </row> 1004 <row> 1005 <entry><constant>V4L2_FIELD_SEQ_TB</constant></entry> 1006 <entry>5</entry> 1007 <entry>Images contain both fields, the top field lines 1008are stored first in memory, immediately followed by the bottom field 1009lines. Fields are always stored in temporal order, the older one first 1010in memory. Image sizes refer to the frame, not fields.</entry> 1011 </row> 1012 <row> 1013 <entry><constant>V4L2_FIELD_SEQ_BT</constant></entry> 1014 <entry>6</entry> 1015 <entry>Images contain both fields, the bottom field 1016lines are stored first in memory, immediately followed by the top 1017field lines. Fields are always stored in temporal order, the older one 1018first in memory. Image sizes refer to the frame, not fields.</entry> 1019 </row> 1020 <row> 1021 <entry><constant>V4L2_FIELD_ALTERNATE</constant></entry> 1022 <entry>7</entry> 1023 <entry>The two fields of a frame are passed in separate 1024buffers, in temporal order, &ie; the older one first. To indicate the field 1025parity (whether the current field is a top or bottom field) the driver 1026or application, depending on data direction, must set &v4l2-buffer; 1027<structfield>field</structfield> to 1028<constant>V4L2_FIELD_TOP</constant> or 1029<constant>V4L2_FIELD_BOTTOM</constant>. Any two successive fields pair 1030to build a frame. If fields are successive, without any dropped fields 1031between them (fields can drop individually), can be determined from 1032the &v4l2-buffer; <structfield>sequence</structfield> field. Image 1033sizes refer to the frame, not fields. This format cannot be selected 1034when using the read/write I/O method.<!-- Where it's indistinguishable 1035from V4L2_FIELD_SEQ_*. --></entry> 1036 </row> 1037 <row> 1038 <entry><constant>V4L2_FIELD_INTERLACED_TB</constant></entry> 1039 <entry>8</entry> 1040 <entry>Images contain both fields, interleaved line by 1041line, top field first. The top field is transmitted first.</entry> 1042 </row> 1043 <row> 1044 <entry><constant>V4L2_FIELD_INTERLACED_BT</constant></entry> 1045 <entry>9</entry> 1046 <entry>Images contain both fields, interleaved line by 1047line, top field first. The bottom field is transmitted first.</entry> 1048 </row> 1049 </tbody> 1050 </tgroup> 1051 </table> 1052 1053 <figure id="fieldseq-tb"> 1054 <title>Field Order, Top Field First Transmitted</title> 1055 <mediaobject> 1056 <imageobject> 1057 <imagedata fileref="fieldseq_tb.pdf" format="PS" /> 1058 </imageobject> 1059 <imageobject> 1060 <imagedata fileref="fieldseq_tb.gif" format="GIF" /> 1061 </imageobject> 1062 </mediaobject> 1063 </figure> 1064 1065 <figure id="fieldseq-bt"> 1066 <title>Field Order, Bottom Field First Transmitted</title> 1067 <mediaobject> 1068 <imageobject> 1069 <imagedata fileref="fieldseq_bt.pdf" format="PS" /> 1070 </imageobject> 1071 <imageobject> 1072 <imagedata fileref="fieldseq_bt.gif" format="GIF" /> 1073 </imageobject> 1074 </mediaobject> 1075 </figure> 1076 </section> 1077 1078 <!-- 1079Local Variables: 1080mode: sgml 1081sgml-parent-document: "v4l2.sgml" 1082indent-tabs-mode: nil 1083End: 1084 --> 1085
