A buffer contains data exchanged by application and
driver using one of the Streaming I/O methods. In the multi-planar API, the
data is held in planes, while the buffer structure acts as a container
for the planes. Only pointers to buffers (planes) are exchanged, the data
itself is not copied. These pointers, together with meta-information like
timestamps or field parity, are stored in a struct
v4l2_buffer, argument to
the VIDIOC_QUERYBUF, VIDIOC_QBUF and VIDIOC_DQBUF ioctl.
In the multi-planar API, some plane-specific members of struct
v4l2_buffer, such as pointers and sizes for each
plane, are stored in struct v4l2_plane instead.
In that case, struct v4l2_buffer contains an array of
plane structures.
Dequeued video buffers come with timestamps. The driver
decides at which part of the frame and with which clock the
timestamp is taken. Please see flags in the masks
V4L2_BUF_FLAG_TIMESTAMP_MASK and
V4L2_BUF_FLAG_TSTAMP_SRC_MASK in Table 3.4, “Buffer Flags”. These flags are always valid and constant
across all buffers during the whole video stream. Changes in these
flags may take place as a side effect of VIDIOC_S_INPUT or
VIDIOC_S_OUTPUT however. The
V4L2_BUF_FLAG_TIMESTAMP_COPY timestamp type
which is used by e.g. on mem-to-mem devices is an exception to the
rule: the timestamp source flags are copied from the OUTPUT video
buffer to the CAPTURE video buffer.
Table 3.1. struct v4l2_buffer
| __u32 | index | Number of the buffer, set by the application except
when calling VIDIOC_DQBUF, then it is set by the driver.
This field can range from zero to the number of buffers allocated
with the VIDIOC_REQBUFS ioctl (struct v4l2_requestbuffers count),
plus any buffers allocated with VIDIOC_CREATE_BUFS minus one. | |
| __u32 | type | Type of the buffer, same as struct v4l2_format
type or struct v4l2_requestbuffers
type, set by the application. See Table 3.3, “enum v4l2_buf_type” | |
| __u32 | bytesused | The number of bytes occupied by the data in the
buffer. It depends on the negotiated data format and may change with
each buffer for compressed variable size data like JPEG images.
Drivers must set this field when type
refers to an input stream, applications when it refers to an output stream.
If the application sets this to 0 for an output stream, then
bytesused will be set to the size of the
buffer (see the length field of this struct) by
the driver. For multiplanar formats this field is ignored and the
planes pointer is used instead. | |
| __u32 | flags | Flags set by the application or driver, see Table 3.4, “Buffer Flags”. | |
| __u32 | field | Indicates the field order of the image in the
buffer, see Table 3.9, “enum v4l2_field”. This field is not used when
the buffer contains VBI data. Drivers must set it when
type refers to an input stream,
applications when it refers to an output stream. | |
| struct timeval | timestamp | For input streams this is time when the first data
byte was captured, as returned by the
| |
| struct v4l2_timecode | timecode | When type is
V4L2_BUF_TYPE_VIDEO_CAPTURE and the
V4L2_BUF_FLAG_TIMECODE flag is set in
flags, this structure contains a frame
timecode. In V4L2_FIELD_ALTERNATE
mode the top and bottom field contain the same timecode.
Timecodes are intended to help video editing and are typically recorded on
video tapes, but also embedded in compressed formats like MPEG. This
field is independent of the timestamp and
sequence fields. | |
| __u32 | sequence | Set by the driver, counting the frames (not fields!) in sequence. This field is set for both input and output devices. | |
In V4L2_FIELD_ALTERNATE mode the top and bottom field have the same sequence number. The count starts at zero and includes dropped or repeated frames. A dropped frame was received by an input device but could not be stored due to lack of free buffer space. A repeated frame was displayed again by an output device because the application did not pass new data in time. Note this may count the frames received e.g. over USB, without taking into account the frames dropped by the remote hardware due to limited compression throughput or bus bandwidth. These devices identify by not enumerating any video standards, see the section called “Video Standards”. | |||
| __u32 | memory | This field must be set by applications and/or drivers in accordance with the selected I/O method. See Table 3.5, “enum v4l2_memory” | |
| union | m | ||
| __u32 | offset | For the single-planar API and when
memory is V4L2_MEMORY_MMAP this
is the offset of the buffer from the start of the device memory. The value is
returned by the driver and apart of serving as parameter to the mmap()
function not useful for applications. See the section called “Streaming I/O (Memory Mapping)” for details
| |
| unsigned long | userptr | For the single-planar API and when
memory is V4L2_MEMORY_USERPTR
this is a pointer to the buffer (casted to unsigned long type) in virtual
memory, set by the application. See the section called “Streaming I/O (User Pointers)” for details.
| |
| struct v4l2_plane | *planes | When using the multi-planar API, contains a userspace pointer
to an array of struct v4l2_plane. The size of the array should be put
in the length field of this
v4l2_buffer structure. | |
| int | fd | For the single-plane API and when
memory is V4L2_MEMORY_DMABUF this
is the file descriptor associated with a DMABUF buffer. | |
| __u32 | length | Size of the buffer (not the payload) in bytes for the
single-planar API. This is set by the driver based on the calls to
VIDIOC_REQBUFS and/or VIDIOC_CREATE_BUFS. For the multi-planar API the application sets
this to the number of elements in the planes
array. The driver will fill in the actual number of valid elements in
that array.
| |
| __u32 | reserved2 | A place holder for future extensions. Applications should set this to 0. | |
| __u32 | reserved | A place holder for future extensions. Applications should set this to 0. | |
Table 3.2. struct v4l2_plane
| __u32 | bytesused | The number of bytes occupied by data in the plane
(its payload). Drivers must set this field when type
refers to an input stream, applications when it refers to an output stream.
If the application sets this to 0 for an output stream, then
bytesused will be set to the size of the
plane (see the length field of this struct)
by the driver. Note that the actual image data starts at
data_offset which may not be 0. | |
| __u32 | length | Size in bytes of the plane (not its payload). This is set by the driver
based on the calls to VIDIOC_REQBUFS and/or VIDIOC_CREATE_BUFS. | |
| union | m | ||
| __u32 | mem_offset | When the memory type in the containing struct v4l2_buffer is
V4L2_MEMORY_MMAP, this is the value that
should be passed to mmap(), similar to the
offset field in struct v4l2_buffer. | |
| unsigned long | userptr | When the memory type in the containing struct v4l2_buffer is
V4L2_MEMORY_USERPTR, this is a userspace
pointer to the memory allocated for this plane by an application.
| |
| int | fd | When the memory type in the containing struct v4l2_buffer is
V4L2_MEMORY_DMABUF, this is a file
descriptor associated with a DMABUF buffer, similar to the
fd field in struct v4l2_buffer. | |
| __u32 | data_offset | Offset in bytes to video data in the plane.
Drivers must set this field when type
refers to an input stream, applications when it refers to an output stream.
Note that data_offset is included in bytesused.
So the size of the image in the plane is
bytesused-data_offset at
offset data_offset from the start of the plane.
| |
| __u32 | reserved[11] | Reserved for future use. Should be zeroed by an application. |
Table 3.3. enum v4l2_buf_type
V4L2_BUF_TYPE_VIDEO_CAPTURE | 1 | Buffer of a single-planar video capture stream, see the section called “Video Capture Interface”. |
V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE
| 9 | Buffer of a multi-planar video capture stream, see the section called “Video Capture Interface”. |
V4L2_BUF_TYPE_VIDEO_OUTPUT | 2 | Buffer of a single-planar video output stream, see the section called “Video Output Interface”. |
V4L2_BUF_TYPE_VIDEO_OUTPUT_MPLANE
| 10 | Buffer of a multi-planar video output stream, see the section called “Video Output Interface”. |
V4L2_BUF_TYPE_VIDEO_OVERLAY | 3 | Buffer for video overlay, see the section called “Video Overlay Interface”. |
V4L2_BUF_TYPE_VBI_CAPTURE | 4 | Buffer of a raw VBI capture stream, see the section called “Raw VBI Data Interface”. |
V4L2_BUF_TYPE_VBI_OUTPUT | 5 | Buffer of a raw VBI output stream, see the section called “Raw VBI Data Interface”. |
V4L2_BUF_TYPE_SLICED_VBI_CAPTURE | 6 | Buffer of a sliced VBI capture stream, see the section called “Sliced VBI Data Interface”. |
V4L2_BUF_TYPE_SLICED_VBI_OUTPUT | 7 | Buffer of a sliced VBI output stream, see the section called “Sliced VBI Data Interface”. |
V4L2_BUF_TYPE_VIDEO_OUTPUT_OVERLAY | 8 | Buffer for video output overlay (OSD), see the section called “Video Output Overlay Interface”. |
V4L2_BUF_TYPE_SDR_CAPTURE | 11 | Buffer for Software Defined Radio (SDR), see the section called “Software Defined Radio Interface (SDR)”. |
Table 3.4. Buffer Flags
V4L2_BUF_FLAG_MAPPED | 0x00000001 | The buffer resides in device memory and has been mapped into the application's address space, see the section called “Streaming I/O (Memory Mapping)” for details. Drivers set or clear this flag when the VIDIOC_QUERYBUF, VIDIOC_QBUF or VIDIOC_DQBUF ioctl is called. Set by the driver. |
V4L2_BUF_FLAG_QUEUED | 0x00000002 | Internally drivers maintain two buffer queues, an
incoming and outgoing queue. When this flag is set, the buffer is
currently on the incoming queue. It automatically moves to the
outgoing queue after the buffer has been filled (capture devices) or
displayed (output devices). Drivers set or clear this flag when the
VIDIOC_QUERYBUF ioctl is called. After
(successful) calling the VIDIOC_QBUF ioctl it is
always set and after VIDIOC_DQBUF always
cleared. |
V4L2_BUF_FLAG_DONE | 0x00000004 | When this flag is set, the buffer is currently on
the outgoing queue, ready to be dequeued from the driver. Drivers set
or clear this flag when the VIDIOC_QUERYBUF ioctl
is called. After calling the VIDIOC_QBUF or
VIDIOC_DQBUF it is always cleared. Of course a
buffer cannot be on both queues at the same time, the
V4L2_BUF_FLAG_QUEUED and
V4L2_BUF_FLAG_DONE flag are mutually exclusive.
They can be both cleared however, then the buffer is in "dequeued"
state, in the application domain so to say. |
V4L2_BUF_FLAG_ERROR | 0x00000040 | When this flag is set, the buffer has been dequeued
successfully, although the data might have been corrupted.
This is recoverable, streaming may continue as normal and
the buffer may be reused normally.
Drivers set this flag when the VIDIOC_DQBUF
ioctl is called. |
V4L2_BUF_FLAG_KEYFRAME | 0x00000008 | Drivers set or clear this flag when calling the
VIDIOC_DQBUF ioctl. It may be set by video
capture devices when the buffer contains a compressed image which is a
key frame (or field), i. e. can be decompressed on its own. Also known as
an I-frame. Applications can set this bit when type
refers to an output stream. |
V4L2_BUF_FLAG_PFRAME | 0x00000010 | Similar to V4L2_BUF_FLAG_KEYFRAME
this flags predicted frames or fields which contain only differences to a
previous key frame. Applications can set this bit when type
refers to an output stream. |
V4L2_BUF_FLAG_BFRAME | 0x00000020 | Similar to V4L2_BUF_FLAG_KEYFRAME
this flags a bi-directional predicted frame or field which contains only
the differences between the current frame and both the preceding and following
key frames to specify its content. Applications can set this bit when
type refers to an output stream. |
V4L2_BUF_FLAG_TIMECODE | 0x00000100 | The timecode field is valid.
Drivers set or clear this flag when the VIDIOC_DQBUF
ioctl is called. Applications can set this bit and the corresponding
timecode structure when type
refers to an output stream. |
V4L2_BUF_FLAG_PREPARED | 0x00000400 | The buffer has been prepared for I/O and can be queued by the application. Drivers set or clear this flag when the VIDIOC_QUERYBUF, VIDIOC_PREPARE_BUF, VIDIOC_QBUF or VIDIOC_DQBUF ioctl is called. |
V4L2_BUF_FLAG_NO_CACHE_INVALIDATE | 0x00000800 | Caches do not have to be invalidated for this buffer. Typically applications shall use this flag if the data captured in the buffer is not going to be touched by the CPU, instead the buffer will, probably, be passed on to a DMA-capable hardware unit for further processing or output. |
V4L2_BUF_FLAG_NO_CACHE_CLEAN | 0x00001000 | Caches do not have to be cleaned for this buffer. Typically applications shall use this flag for output buffers if the data in this buffer has not been created by the CPU but by some DMA-capable unit, in which case caches have not been used. |
V4L2_BUF_FLAG_TIMESTAMP_MASK | 0x0000e000 | Mask for timestamp types below. To test the timestamp type, mask out bits not belonging to timestamp type by performing a logical and operation with buffer flags and timestamp mask. |
V4L2_BUF_FLAG_TIMESTAMP_UNKNOWN | 0x00000000 | Unknown timestamp type. This type is used by
drivers before Linux 3.9 and may be either monotonic (see
below) or realtime (wall clock). Monotonic clock has been
favoured in embedded systems whereas most of the drivers
use the realtime clock. Either kinds of timestamps are
available in user space via
clock_gettime(2) using clock IDs
CLOCK_MONOTONIC and
CLOCK_REALTIME, respectively. |
V4L2_BUF_FLAG_TIMESTAMP_MONOTONIC | 0x00002000 | The buffer timestamp has been taken from the
CLOCK_MONOTONIC clock. To access the
same clock outside V4L2, use
clock_gettime(2) . |
V4L2_BUF_FLAG_TIMESTAMP_COPY | 0x00004000 | The CAPTURE buffer timestamp has been taken from the corresponding OUTPUT buffer. This flag applies only to mem2mem devices. |
V4L2_BUF_FLAG_TSTAMP_SRC_MASK | 0x00070000 | Mask for timestamp sources below. The timestamp source
defines the point of time the timestamp is taken in relation to
the frame. Logical 'and' operation between the
flags field and
V4L2_BUF_FLAG_TSTAMP_SRC_MASK produces the
value of the timestamp source. Applications must set the timestamp
source when type refers to an output stream
and V4L2_BUF_FLAG_TIMESTAMP_COPY is set. |
V4L2_BUF_FLAG_TSTAMP_SRC_EOF | 0x00000000 | End Of Frame. The buffer timestamp has been taken when the last pixel of the frame has been received or the last pixel of the frame has been transmitted. In practice, software generated timestamps will typically be read from the clock a small amount of time after the last pixel has been received or transmitten, depending on the system and other activity in it. |
V4L2_BUF_FLAG_TSTAMP_SRC_SOE | 0x00010000 | Start Of Exposure. The buffer timestamp has been
taken when the exposure of the frame has begun. This is
only valid for the
V4L2_BUF_TYPE_VIDEO_CAPTURE buffer
type. |
Table 3.5. enum v4l2_memory
V4L2_MEMORY_MMAP | 1 | The buffer is used for memory mapping I/O. |
V4L2_MEMORY_USERPTR | 2 | The buffer is used for user pointer I/O. |
V4L2_MEMORY_OVERLAY | 3 | [to do] |
V4L2_MEMORY_DMABUF | 4 | The buffer is used for DMA shared buffer I/O. |
The v4l2_timecode structure is
designed to hold a [SMPTE 12M] or similar timecode.
(struct timeval timestamps are stored in
struct v4l2_buffer field timestamp.)
Table 3.6. struct v4l2_timecode
| __u32 | type | Frame rate the timecodes are based on, see Table 3.7, “Timecode Types”. |
| __u32 | flags | Timecode flags, see Table 3.8, “Timecode Flags”. |
| __u8 | frames | Frame count, 0 ... 23/24/29/49/59, depending on the type of timecode. |
| __u8 | seconds | Seconds count, 0 ... 59. This is a binary, not BCD number. |
| __u8 | minutes | Minutes count, 0 ... 59. This is a binary, not BCD number. |
| __u8 | hours | Hours count, 0 ... 29. This is a binary, not BCD number. |
| __u8 | userbits[4] | The "user group" bits from the timecode. |
Table 3.7. Timecode Types
V4L2_TC_TYPE_24FPS | 1 | 24 frames per second, i. e. film. |
V4L2_TC_TYPE_25FPS | 2 | 25 frames per second, i. e. PAL or SECAM video. |
V4L2_TC_TYPE_30FPS | 3 | 30 frames per second, i. e. NTSC video. |
V4L2_TC_TYPE_50FPS | 4 | |
V4L2_TC_TYPE_60FPS | 5 |
Table 3.8. Timecode Flags
V4L2_TC_FLAG_DROPFRAME | 0x0001 | Indicates "drop frame" semantics for counting frames in 29.97 fps material. When set, frame numbers 0 and 1 at the start of each minute, except minutes 0, 10, 20, 30, 40, 50 are omitted from the count. |
V4L2_TC_FLAG_COLORFRAME | 0x0002 | The "color frame" flag. |
V4L2_TC_USERBITS_field | 0x000C | Field mask for the "binary group flags". |
V4L2_TC_USERBITS_USERDEFINED | 0x0000 | Unspecified format. |
V4L2_TC_USERBITS_8BITCHARS | 0x0008 | 8-bit ISO characters. |