3.6. Buffers¶
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 ioctl VIDIOC_QUERYBUF , ioctl VIDIOC_QBUF, VIDIOC_DQBUF 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 struct v4l2_plane instead. In that case, struct 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 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.
3.6.1. struct v4l2_buffer¶
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.
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 Video Standards .
3.6.2. struct v4l2_plane¶
The number of bytes occupied by data in the plane (its payload). Drivers must set this field when type refers to a capture 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.
Offset in bytes to video data in the plane. Drivers must set this field when type refers to a capture stream, applications when it refers to an output stream.
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.
3.6.3. enum v4l2_buf_type¶
| V4L2_BUF_TYPE_VIDEO_CAPTURE | 1 | Buffer of a single-planar video capture stream, see Video Capture Interface . |
| V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE | 9 | Buffer of a multi-planar video capture stream, see Video Capture Interface . |
| V4L2_BUF_TYPE_VIDEO_OUTPUT | 2 | Buffer of a single-planar video output stream, see Video Output Interface . |
| V4L2_BUF_TYPE_VIDEO_OUTPUT_MPLANE | 10 | Buffer of a multi-planar video output stream, see Video Output Interface . |
| V4L2_BUF_TYPE_VIDEO_OVERLAY | 3 | Buffer for video overlay, see Video Overlay Interface . |
| V4L2_BUF_TYPE_VBI_CAPTURE | 4 | Buffer of a raw VBI capture stream, see Raw VBI Data Interface . |
| V4L2_BUF_TYPE_VBI_OUTPUT | 5 | Buffer of a raw VBI output stream, see Raw VBI Data Interface . |
| V4L2_BUF_TYPE_SLICED_VBI_CAPTURE | 6 | Buffer of a sliced VBI capture stream, see Sliced VBI Data Interface . |
| V4L2_BUF_TYPE_SLICED_VBI_OUTPUT | 7 | Buffer of a sliced VBI output stream, see Sliced VBI Data Interface . |
| V4L2_BUF_TYPE_VIDEO_OUTPUT_OVERLAY | 8 | Buffer for video output overlay (OSD), see Video Output Overlay Interface . |
| V4L2_BUF_TYPE_SDR_CAPTURE | 11 | Buffer for Software Defined Radio (SDR) capture stream, see Software Defined Radio Interface (SDR) . |
| V4L2_BUF_TYPE_SDR_OUTPUT | 12 | Buffer for Software Defined Radio (SDR) output stream, see Software Defined Radio Interface (SDR) . |
3.6.4. Buffer Flags¶
The buffer resides in device memory and has been mapped into the application’s address space, see Streaming I/O (Memory Mapping) for details. Drivers set or clear this flag when the ioctl VIDIOC_QUERYBUF , ioctl VIDIOC_QBUF, VIDIOC_DQBUF or VIDIOC_DQBUF ioctl is called. Set by the driver.
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.
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.
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.
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.
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.
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.
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.
The buffer has been prepared for I/O and can be queued by the application. Drivers set or clear this flag when the ioctl VIDIOC_QUERYBUF , VIDIOC_PREPARE_BUF , ioctl VIDIOC_QBUF, VIDIOC_DQBUF or VIDIOC_DQBUF ioctl is called.
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.
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.
Last buffer produced by the hardware. mem2mem codec drivers set this flag on the capture queue for the last buffer when the ioctl VIDIOC_QUERYBUF or VIDIOC_DQBUF ioctl is called. Due to hardware limitations, the last buffer may be empty. In this case the driver will set the bytesused field to 0, regardless of the format. Any Any subsequent call to the VIDIOC_DQBUF ioctl will not block anymore, but return an EPIPE error code.
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.
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() using clock IDs CLOCK_MONOTONIC and CLOCK_REALTIME , respectively.
The buffer timestamp has been taken from the CLOCK_MONOTONIC clock. To access the same clock outside V4L2, use clock_gettime() .
The CAPTURE buffer timestamp has been taken from the corresponding OUTPUT buffer. This flag applies only to mem2mem devices.
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.
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.
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.
3.6.5. 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. |
3.6.6. Timecodes¶
The struct v4l2_timecode structure is designed to hold a SMPTE 12M or similar timecode. (struct struct timeval timestamps are stored in struct v4l2_buffer field timestamp .)
3.6.6.1. struct v4l2_timecode¶
| __u32 | type | Frame rate the timecodes are based on, see Timecode Types . |
| __u32 | flags | Timecode flags, see 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. |
3.6.6.2. 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 |
3.6.6.3. 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. |
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