1 .. -*- coding: utf-8; mode: rst -*-
9 A buffer contains data exchanged by application and driver using one of
10 the Streaming I/O methods. In the multi-planar API, the data is held in
11 planes, while the buffer structure acts as a container for the planes.
12 Only pointers to buffers (planes) are exchanged, the data itself is not
13 copied. These pointers, together with meta-information like timestamps
14 or field parity, are stored in a struct :ref:`struct v4l2_buffer <v4l2-buffer>`,
15 argument to the :ref:`VIDIOC_QUERYBUF`,
16 :ref:`VIDIOC_QBUF` and
17 :ref:`VIDIOC_DQBUF <VIDIOC_QBUF>` ioctl. In the multi-planar API,
18 some plane-specific members of struct :ref:`struct v4l2_buffer <v4l2-buffer>`,
19 such as pointers and sizes for each plane, are stored in struct
20 :ref:`struct v4l2_plane <v4l2-plane>` instead. In that case, struct
21 :ref:`struct v4l2_buffer <v4l2-buffer>` contains an array of plane structures.
23 Dequeued video buffers come with timestamps. The driver decides at which
24 part of the frame and with which clock the timestamp is taken. Please
25 see flags in the masks ``V4L2_BUF_FLAG_TIMESTAMP_MASK`` and
26 ``V4L2_BUF_FLAG_TSTAMP_SRC_MASK`` in :ref:`buffer-flags`. These flags
27 are always valid and constant across all buffers during the whole video
28 stream. Changes in these flags may take place as a side effect of
29 :ref:`VIDIOC_S_INPUT <VIDIOC_G_INPUT>` or
30 :ref:`VIDIOC_S_OUTPUT <VIDIOC_G_OUTPUT>` however. The
31 ``V4L2_BUF_FLAG_TIMESTAMP_COPY`` timestamp type which is used by e.g. on
32 mem-to-mem devices is an exception to the rule: the timestamp source
33 flags are copied from the OUTPUT video buffer to the CAPTURE video
42 .. tabularcolumns:: |p{3.5cm}|p{3.5cm}|p{3.5cm}|p{7.0cm}|
44 .. flat-table:: struct v4l2_buffer
57 - Number of the buffer, set by the application except when calling
58 :ref:`VIDIOC_DQBUF <VIDIOC_QBUF>`, then it is set by the
59 driver. This field can range from zero to the number of buffers
60 allocated with the :ref:`VIDIOC_REQBUFS` ioctl
61 (struct :ref:`v4l2_requestbuffers <v4l2-requestbuffers>`
62 ``count``), plus any buffers allocated with
63 :ref:`VIDIOC_CREATE_BUFS` minus one.
72 - Type of the buffer, same as struct
73 :ref:`v4l2_format <v4l2-format>` ``type`` or struct
74 :ref:`v4l2_requestbuffers <v4l2-requestbuffers>` ``type``, set
75 by the application. See :ref:`v4l2-buf-type`
84 - The number of bytes occupied by the data in the buffer. It depends
85 on the negotiated data format and may change with each buffer for
86 compressed variable size data like JPEG images. Drivers must set
87 this field when ``type`` refers to a capture stream, applications
88 when it refers to an output stream. If the application sets this
89 to 0 for an output stream, then ``bytesused`` will be set to the
90 size of the buffer (see the ``length`` field of this struct) by
91 the driver. For multiplanar formats this field is ignored and the
92 ``planes`` pointer is used instead.
101 - Flags set by the application or driver, see :ref:`buffer-flags`.
110 - Indicates the field order of the image in the buffer, see
111 :ref:`v4l2-field`. This field is not used when the buffer
112 contains VBI data. Drivers must set it when ``type`` refers to a
113 capture stream, applications when it refers to an output stream.
122 - For capture streams this is time when the first data byte was
123 captured, as returned by the :c:func:`clock_gettime()` function
124 for the relevant clock id; see ``V4L2_BUF_FLAG_TIMESTAMP_*`` in
125 :ref:`buffer-flags`. For output streams the driver stores the
126 time at which the last data byte was actually sent out in the
127 ``timestamp`` field. This permits applications to monitor the
128 drift between the video and system clock. For output streams that
129 use ``V4L2_BUF_FLAG_TIMESTAMP_COPY`` the application has to fill
130 in the timestamp which will be copied by the driver to the capture
135 - struct :ref:`v4l2_timecode <v4l2-timecode>`
140 - When ``type`` is ``V4L2_BUF_TYPE_VIDEO_CAPTURE`` and the
141 ``V4L2_BUF_FLAG_TIMECODE`` flag is set in ``flags``, this
142 structure contains a frame timecode. In
143 :ref:`V4L2_FIELD_ALTERNATE <v4l2-field>` mode the top and
144 bottom field contain the same timecode. Timecodes are intended to
145 help video editing and are typically recorded on video tapes, but
146 also embedded in compressed formats like MPEG. This field is
147 independent of the ``timestamp`` and ``sequence`` fields.
156 - Set by the driver, counting the frames (not fields!) in sequence.
157 This field is set for both input and output devices.
163 In :ref:`V4L2_FIELD_ALTERNATE <v4l2-field>` mode the top and
164 bottom field have the same sequence number. The count starts at
165 zero and includes dropped or repeated frames. A dropped frame was
166 received by an input device but could not be stored due to lack of
167 free buffer space. A repeated frame was displayed again by an
168 output device because the application did not pass new data in
173 This may count the frames received e.g. over USB, without
174 taking into account the frames dropped by the remote hardware due
175 to limited compression throughput or bus bandwidth. These devices
176 identify by not enumerating any video standards, see
187 - This field must be set by applications and/or drivers in
188 accordance with the selected I/O method. See :ref:`v4l2-memory`
203 - For the single-planar API and when ``memory`` is
204 ``V4L2_MEMORY_MMAP`` this is the offset of the buffer from the
205 start of the device memory. The value is returned by the driver
206 and apart of serving as parameter to the
207 :ref:`mmap() <func-mmap>` function not useful for applications.
208 See :ref:`mmap` for details
217 - For the single-planar API and when ``memory`` is
218 ``V4L2_MEMORY_USERPTR`` this is a pointer to the buffer (casted to
219 unsigned long type) in virtual memory, set by the application. See
220 :ref:`userp` for details.
229 - When using the multi-planar API, contains a userspace pointer to
230 an array of struct :ref:`v4l2_plane <v4l2-plane>`. The size of
231 the array should be put in the ``length`` field of this
232 :ref:`struct v4l2_buffer <v4l2-buffer>` structure.
241 - For the single-plane API and when ``memory`` is
242 ``V4L2_MEMORY_DMABUF`` this is the file descriptor associated with
252 - Size of the buffer (not the payload) in bytes for the
253 single-planar API. This is set by the driver based on the calls to
254 :ref:`VIDIOC_REQBUFS` and/or
255 :ref:`VIDIOC_CREATE_BUFS`. For the
256 multi-planar API the application sets this to the number of
257 elements in the ``planes`` array. The driver will fill in the
258 actual number of valid elements in that array.
267 - A place holder for future extensions. Drivers and applications
277 - A place holder for future extensions. Drivers and applications
287 .. tabularcolumns:: |p{3.5cm}|p{3.5cm}|p{3.5cm}|p{7.0cm}|
302 - The number of bytes occupied by data in the plane (its payload).
303 Drivers must set this field when ``type`` refers to a capture
304 stream, applications when it refers to an output stream. If the
305 application sets this to 0 for an output stream, then
306 ``bytesused`` will be set to the size of the plane (see the
307 ``length`` field of this struct) by the driver.
311 Note that the actual image data starts at ``data_offset``
321 - Size in bytes of the plane (not its payload). This is set by the
322 driver based on the calls to
323 :ref:`VIDIOC_REQBUFS` and/or
324 :ref:`VIDIOC_CREATE_BUFS`.
342 - When the memory type in the containing struct
343 :ref:`v4l2_buffer <v4l2-buffer>` is ``V4L2_MEMORY_MMAP``, this
344 is the value that should be passed to :ref:`mmap() <func-mmap>`,
345 similar to the ``offset`` field in struct
346 :ref:`v4l2_buffer <v4l2-buffer>`.
355 - When the memory type in the containing struct
356 :ref:`v4l2_buffer <v4l2-buffer>` is ``V4L2_MEMORY_USERPTR``,
357 this is a userspace pointer to the memory allocated for this plane
367 - When the memory type in the containing struct
368 :ref:`v4l2_buffer <v4l2-buffer>` is ``V4L2_MEMORY_DMABUF``,
369 this is a file descriptor associated with a DMABUF buffer, similar
370 to the ``fd`` field in struct :ref:`v4l2_buffer <v4l2-buffer>`.
379 - Offset in bytes to video data in the plane. Drivers must set this
380 field when ``type`` refers to a capture stream, applications when
381 it refers to an output stream.
385 That data_offset is included in ``bytesused``. So the
386 size of the image in the plane is ``bytesused``-``data_offset``
387 at offset ``data_offset`` from the start of the plane.
396 - Reserved for future use. Should be zeroed by drivers and
406 .. tabularcolumns:: |p{6.6cm}|p{2.2cm}|p{8.7cm}|
416 - ``V4L2_BUF_TYPE_VIDEO_CAPTURE``
420 - Buffer of a single-planar video capture stream, see
425 - ``V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE``
429 - Buffer of a multi-planar video capture stream, see
434 - ``V4L2_BUF_TYPE_VIDEO_OUTPUT``
438 - Buffer of a single-planar video output stream, see
443 - ``V4L2_BUF_TYPE_VIDEO_OUTPUT_MPLANE``
447 - Buffer of a multi-planar video output stream, see :ref:`output`.
451 - ``V4L2_BUF_TYPE_VIDEO_OVERLAY``
455 - Buffer for video overlay, see :ref:`overlay`.
459 - ``V4L2_BUF_TYPE_VBI_CAPTURE``
463 - Buffer of a raw VBI capture stream, see :ref:`raw-vbi`.
467 - ``V4L2_BUF_TYPE_VBI_OUTPUT``
471 - Buffer of a raw VBI output stream, see :ref:`raw-vbi`.
475 - ``V4L2_BUF_TYPE_SLICED_VBI_CAPTURE``
479 - Buffer of a sliced VBI capture stream, see :ref:`sliced`.
483 - ``V4L2_BUF_TYPE_SLICED_VBI_OUTPUT``
487 - Buffer of a sliced VBI output stream, see :ref:`sliced`.
491 - ``V4L2_BUF_TYPE_VIDEO_OUTPUT_OVERLAY``
495 - Buffer for video output overlay (OSD), see :ref:`osd`.
499 - ``V4L2_BUF_TYPE_SDR_CAPTURE``
503 - Buffer for Software Defined Radio (SDR) capture stream, see
508 - ``V4L2_BUF_TYPE_SDR_OUTPUT``
512 - Buffer for Software Defined Radio (SDR) output stream, see
522 .. tabularcolumns:: |p{6.6cm}|p{2.2cm}|p{8.7cm}|
530 - .. _`V4L2-BUF-FLAG-MAPPED`:
532 - ``V4L2_BUF_FLAG_MAPPED``
536 - The buffer resides in device memory and has been mapped into the
537 application's address space, see :ref:`mmap` for details.
538 Drivers set or clear this flag when the
539 :ref:`VIDIOC_QUERYBUF`,
540 :ref:`VIDIOC_QBUF` or
541 :ref:`VIDIOC_DQBUF <VIDIOC_QBUF>` ioctl is called. Set by the
544 - .. _`V4L2-BUF-FLAG-QUEUED`:
546 - ``V4L2_BUF_FLAG_QUEUED``
550 - Internally drivers maintain two buffer queues, an incoming and
551 outgoing queue. When this flag is set, the buffer is currently on
552 the incoming queue. It automatically moves to the outgoing queue
553 after the buffer has been filled (capture devices) or displayed
554 (output devices). Drivers set or clear this flag when the
555 ``VIDIOC_QUERYBUF`` ioctl is called. After (successful) calling
556 the ``VIDIOC_QBUF``\ ioctl it is always set and after
557 ``VIDIOC_DQBUF`` always cleared.
559 - .. _`V4L2-BUF-FLAG-DONE`:
561 - ``V4L2_BUF_FLAG_DONE``
565 - When this flag is set, the buffer is currently on the outgoing
566 queue, ready to be dequeued from the driver. Drivers set or clear
567 this flag when the ``VIDIOC_QUERYBUF`` ioctl is called. After
568 calling the ``VIDIOC_QBUF`` or ``VIDIOC_DQBUF`` it is always
569 cleared. Of course a buffer cannot be on both queues at the same
570 time, the ``V4L2_BUF_FLAG_QUEUED`` and ``V4L2_BUF_FLAG_DONE`` flag
571 are mutually exclusive. They can be both cleared however, then the
572 buffer is in "dequeued" state, in the application domain so to
575 - .. _`V4L2-BUF-FLAG-ERROR`:
577 - ``V4L2_BUF_FLAG_ERROR``
581 - When this flag is set, the buffer has been dequeued successfully,
582 although the data might have been corrupted. This is recoverable,
583 streaming may continue as normal and the buffer may be reused
584 normally. Drivers set this flag when the ``VIDIOC_DQBUF`` ioctl is
587 - .. _`V4L2-BUF-FLAG-KEYFRAME`:
589 - ``V4L2_BUF_FLAG_KEYFRAME``
593 - Drivers set or clear this flag when calling the ``VIDIOC_DQBUF``
594 ioctl. It may be set by video capture devices when the buffer
595 contains a compressed image which is a key frame (or field), i. e.
596 can be decompressed on its own. Also known as an I-frame.
597 Applications can set this bit when ``type`` refers to an output
600 - .. _`V4L2-BUF-FLAG-PFRAME`:
602 - ``V4L2_BUF_FLAG_PFRAME``
606 - Similar to ``V4L2_BUF_FLAG_KEYFRAME`` this flags predicted frames
607 or fields which contain only differences to a previous key frame.
608 Applications can set this bit when ``type`` refers to an output
611 - .. _`V4L2-BUF-FLAG-BFRAME`:
613 - ``V4L2_BUF_FLAG_BFRAME``
617 - Similar to ``V4L2_BUF_FLAG_KEYFRAME`` this flags a bi-directional
618 predicted frame or field which contains only the differences
619 between the current frame and both the preceding and following key
620 frames to specify its content. Applications can set this bit when
621 ``type`` refers to an output stream.
623 - .. _`V4L2-BUF-FLAG-TIMECODE`:
625 - ``V4L2_BUF_FLAG_TIMECODE``
629 - The ``timecode`` field is valid. Drivers set or clear this flag
630 when the ``VIDIOC_DQBUF`` ioctl is called. Applications can set
631 this bit and the corresponding ``timecode`` structure when
632 ``type`` refers to an output stream.
634 - .. _`V4L2-BUF-FLAG-PREPARED`:
636 - ``V4L2_BUF_FLAG_PREPARED``
640 - The buffer has been prepared for I/O and can be queued by the
641 application. Drivers set or clear this flag when the
642 :ref:`VIDIOC_QUERYBUF`,
643 :ref:`VIDIOC_PREPARE_BUF <VIDIOC_QBUF>`,
644 :ref:`VIDIOC_QBUF` or
645 :ref:`VIDIOC_DQBUF <VIDIOC_QBUF>` ioctl is called.
647 - .. _`V4L2-BUF-FLAG-NO-CACHE-INVALIDATE`:
649 - ``V4L2_BUF_FLAG_NO_CACHE_INVALIDATE``
653 - Caches do not have to be invalidated for this buffer. Typically
654 applications shall use this flag if the data captured in the
655 buffer is not going to be touched by the CPU, instead the buffer
656 will, probably, be passed on to a DMA-capable hardware unit for
657 further processing or output.
659 - .. _`V4L2-BUF-FLAG-NO-CACHE-CLEAN`:
661 - ``V4L2_BUF_FLAG_NO_CACHE_CLEAN``
665 - Caches do not have to be cleaned for this buffer. Typically
666 applications shall use this flag for output buffers if the data in
667 this buffer has not been created by the CPU but by some
668 DMA-capable unit, in which case caches have not been used.
670 - .. _`V4L2-BUF-FLAG-LAST`:
672 - ``V4L2_BUF_FLAG_LAST``
676 - Last buffer produced by the hardware. mem2mem codec drivers set
677 this flag on the capture queue for the last buffer when the
678 :ref:`VIDIOC_QUERYBUF` or
679 :ref:`VIDIOC_DQBUF <VIDIOC_QBUF>` ioctl is called. Due to
680 hardware limitations, the last buffer may be empty. In this case
681 the driver will set the ``bytesused`` field to 0, regardless of
682 the format. Any Any subsequent call to the
683 :ref:`VIDIOC_DQBUF <VIDIOC_QBUF>` ioctl will not block anymore,
684 but return an ``EPIPE`` error code.
686 - .. _`V4L2-BUF-FLAG-TIMESTAMP-MASK`:
688 - ``V4L2_BUF_FLAG_TIMESTAMP_MASK``
692 - Mask for timestamp types below. To test the timestamp type, mask
693 out bits not belonging to timestamp type by performing a logical
694 and operation with buffer flags and timestamp mask.
696 - .. _`V4L2-BUF-FLAG-TIMESTAMP-UNKNOWN`:
698 - ``V4L2_BUF_FLAG_TIMESTAMP_UNKNOWN``
702 - Unknown timestamp type. This type is used by drivers before Linux
703 3.9 and may be either monotonic (see below) or realtime (wall
704 clock). Monotonic clock has been favoured in embedded systems
705 whereas most of the drivers use the realtime clock. Either kinds
706 of timestamps are available in user space via
707 :c:func:`clock_gettime(2)` using clock IDs ``CLOCK_MONOTONIC``
708 and ``CLOCK_REALTIME``, respectively.
710 - .. _`V4L2-BUF-FLAG-TIMESTAMP-MONOTONIC`:
712 - ``V4L2_BUF_FLAG_TIMESTAMP_MONOTONIC``
716 - The buffer timestamp has been taken from the ``CLOCK_MONOTONIC``
717 clock. To access the same clock outside V4L2, use
718 :c:func:`clock_gettime(2)`.
720 - .. _`V4L2-BUF-FLAG-TIMESTAMP-COPY`:
722 - ``V4L2_BUF_FLAG_TIMESTAMP_COPY``
726 - The CAPTURE buffer timestamp has been taken from the corresponding
727 OUTPUT buffer. This flag applies only to mem2mem devices.
729 - .. _`V4L2-BUF-FLAG-TSTAMP-SRC-MASK`:
731 - ``V4L2_BUF_FLAG_TSTAMP_SRC_MASK``
735 - Mask for timestamp sources below. The timestamp source defines the
736 point of time the timestamp is taken in relation to the frame.
737 Logical 'and' operation between the ``flags`` field and
738 ``V4L2_BUF_FLAG_TSTAMP_SRC_MASK`` produces the value of the
739 timestamp source. Applications must set the timestamp source when
740 ``type`` refers to an output stream and
741 ``V4L2_BUF_FLAG_TIMESTAMP_COPY`` is set.
743 - .. _`V4L2-BUF-FLAG-TSTAMP-SRC-EOF`:
745 - ``V4L2_BUF_FLAG_TSTAMP_SRC_EOF``
749 - End Of Frame. The buffer timestamp has been taken when the last
750 pixel of the frame has been received or the last pixel of the
751 frame has been transmitted. In practice, software generated
752 timestamps will typically be read from the clock a small amount of
753 time after the last pixel has been received or transmitten,
754 depending on the system and other activity in it.
756 - .. _`V4L2-BUF-FLAG-TSTAMP-SRC-SOE`:
758 - ``V4L2_BUF_FLAG_TSTAMP_SRC_SOE``
762 - Start Of Exposure. The buffer timestamp has been taken when the
763 exposure of the frame has begun. This is only valid for the
764 ``V4L2_BUF_TYPE_VIDEO_CAPTURE`` buffer type.
773 .. tabularcolumns:: |p{6.6cm}|p{2.2cm}|p{8.7cm}|
783 - ``V4L2_MEMORY_MMAP``
787 - The buffer is used for :ref:`memory mapping <mmap>` I/O.
791 - ``V4L2_MEMORY_USERPTR``
795 - The buffer is used for :ref:`user pointer <userp>` I/O.
799 - ``V4L2_MEMORY_OVERLAY``
807 - ``V4L2_MEMORY_DMABUF``
811 - The buffer is used for :ref:`DMA shared buffer <dmabuf>` I/O.
818 The :ref:`struct v4l2_timecode <v4l2-timecode>` structure is designed to hold a
819 :ref:`smpte12m` or similar timecode. (struct
820 :c:type:`struct timeval` timestamps are stored in struct
821 :ref:`v4l2_buffer <v4l2-buffer>` field ``timestamp``.)
829 .. tabularcolumns:: |p{4.4cm}|p{4.4cm}|p{8.7cm}|
843 - Frame rate the timecodes are based on, see :ref:`timecode-type`.
851 - Timecode flags, see :ref:`timecode-flags`.
859 - Frame count, 0 ... 23/24/29/49/59, depending on the type of
868 - Seconds count, 0 ... 59. This is a binary, not BCD number.
876 - Minutes count, 0 ... 59. This is a binary, not BCD number.
884 - Hours count, 0 ... 29. This is a binary, not BCD number.
892 - The "user group" bits from the timecode.
901 .. tabularcolumns:: |p{6.6cm}|p{2.2cm}|p{8.7cm}|
911 - ``V4L2_TC_TYPE_24FPS``
915 - 24 frames per second, i. e. film.
919 - ``V4L2_TC_TYPE_25FPS``
923 - 25 frames per second, i. e. PAL or SECAM video.
927 - ``V4L2_TC_TYPE_30FPS``
931 - 30 frames per second, i. e. NTSC video.
935 - ``V4L2_TC_TYPE_50FPS``
943 - ``V4L2_TC_TYPE_60FPS``
956 .. tabularcolumns:: |p{6.6cm}|p{2.2cm}|p{8.7cm}|
966 - ``V4L2_TC_FLAG_DROPFRAME``
970 - Indicates "drop frame" semantics for counting frames in 29.97 fps
971 material. When set, frame numbers 0 and 1 at the start of each
972 minute, except minutes 0, 10, 20, 30, 40, 50 are omitted from the
977 - ``V4L2_TC_FLAG_COLORFRAME``
981 - The "color frame" flag.
985 - ``V4L2_TC_USERBITS_field``
989 - Field mask for the "binary group flags".
993 - ``V4L2_TC_USERBITS_USERDEFINED``
997 - Unspecified format.
1001 - ``V4L2_TC_USERBITS_8BITCHARS``
1005 - 8-bit ISO characters.