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, 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 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. Interactions between formats, controls and buffers

V4L2 exposes parameters that influence the buffer size, or the way data is laid out in the buffer. Those parameters are exposed through both formats and controls. One example of such a control is the V4L2_CID_ROTATE control that modifies the direction in which pixels are stored in the buffer, as well as the buffer size when the selected format includes padding at the end of lines.

The set of information needed to interpret the content of a buffer (e.g. the pixel format, the line stride, the tiling orientation or the rotation) is collectively referred to in the rest of this section as the buffer layout.

Controls that can modify the buffer layout shall set the V4L2_CTRL_FLAG_MODIFY_LAYOUT flag.

Modifying formats or controls that influence the buffer size or layout require the stream to be stopped. Any attempt at such a modification while the stream is active shall cause the ioctl setting the format or the control to return the EBUSY error code. In that case drivers shall also set the V4L2_CTRL_FLAG_GRABBED flag when calling VIDIOC_QUERYCTRL() or VIDIOC_QUERY_EXT_CTRL() for such a control while the stream is active.

注釈

The VIDIOC_S_SELECTION() ioctl can, depending on the hardware (for instance if the device doesn't include a scaler), modify the format in addition to the selection rectangle. Similarly, the VIDIOC_S_INPUT(), VIDIOC_S_OUTPUT(), VIDIOC_S_STD() and VIDIOC_S_DV_TIMINGS() ioctls can also modify the format and selection rectangles. When those ioctls result in a buffer size or layout change, drivers shall handle that condition as they would handle it in the VIDIOC_S_FMT() ioctl in all cases described in this section.

Controls that only influence the buffer layout can be modified at any time when the stream is stopped. As they don't influence the buffer size, no special handling is needed to synchronize those controls with buffer allocation and the V4L2_CTRL_FLAG_GRABBED flag is cleared once the stream is stopped.

Formats and controls that influence the buffer size interact with buffer allocation. The simplest way to handle this is for drivers to always require buffers to be reallocated in order to change those formats or controls. In that case, to perform such changes, userspace applications shall first stop the video stream with the VIDIOC_STREAMOFF() ioctl if it is running and free all buffers with the VIDIOC_REQBUFS() ioctl if they are allocated. After freeing all buffers the V4L2_CTRL_FLAG_GRABBED flag for controls is cleared. The format or controls can then be modified, and buffers shall then be reallocated and the stream restarted. A typical ioctl sequence is

1. VIDIOC_STREAMOFF

2. VIDIOC_REQBUFS(0)

3. VIDIOC_S_EXT_CTRLS

4. VIDIOC_S_FMT

5. VIDIOC_REQBUFS(n)

6. VIDIOC_QBUF

7. VIDIOC_STREAMON

The second VIDIOC_REQBUFS() call will take the new format and control value into account to compute the buffer size to allocate. Applications can also retrieve the size by calling the VIDIOC_G_FMT() ioctl if needed.

注釈

The API doesn't mandate the above order for control (3.) and format (4.) changes. Format and controls can be set in a different order, or even interleaved, depending on the device and use case. For instance some controls might behave differently for different pixel formats, in which case the format might need to be set first.

When reallocation is required, any attempt to modify format or controls that influences the buffer size while buffers are allocated shall cause the format or control set ioctl to return the EBUSY error. Any attempt to queue a buffer too small for the current format or controls shall cause the VIDIOC_QBUF() ioctl to return a EINVAL error.

Buffer reallocation is an expensive operation. To avoid that cost, drivers can (and are encouraged to) allow format or controls that influence the buffer size to be changed with buffers allocated. In that case, a typical ioctl sequence to modify format and controls is

1. VIDIOC_STREAMOFF

2. VIDIOC_S_EXT_CTRLS

3. VIDIOC_S_FMT

4. VIDIOC_QBUF

5. VIDIOC_STREAMON

For this sequence to operate correctly, queued buffers need to be large enough for the new format or controls. Drivers shall return a ENOSPC error in response to format change (VIDIOC_S_FMT()) or control changes (VIDIOC_S_CTRL() or VIDIOC_S_EXT_CTRLS()) if buffers too small for the new format are currently queued. As a simplification, drivers are allowed to return a EBUSY error from these ioctls if any buffer is currently queued, without checking the queued buffers sizes.

Additionally, drivers shall return a EINVAL error from the VIDIOC_QBUF() ioctl if the buffer being queued is too small for the current format or controls. Together, these requirements ensure that queued buffers will always be large enough for the configured format and controls.

Userspace applications can query the buffer size required for a given format and controls by first setting the desired control values and then trying the desired format. The VIDIOC_TRY_FMT() ioctl will return the required buffer size.

1. VIDIOC_S_EXT_CTRLS(x)

2. VIDIOC_TRY_FMT()

3. VIDIOC_S_EXT_CTRLS(y)

4. VIDIOC_TRY_FMT()

The VIDIOC_CREATE_BUFS() ioctl can then be used to allocate buffers based on the queried sizes (for instance by allocating a set of buffers large enough for all the desired formats and controls, or by allocating separate set of appropriately sized buffers for each use case).

v4l2_buffer

3.6.2. struct v4l2_buffer

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 ioctl VIDIOC_REQBUFS ioctl (struct v4l2_requestbuffers count), plus any buffers allocated with ioctl 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 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 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 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 Buffer Flags.
__u32	field		Indicates the field order of the image in the buffer, see v4l2_field. This field is not used when the buffer contains VBI data. Drivers must set it when type refers to a capture stream, applications when it refers to an output stream.
struct timeval	timestamp		For capture streams this is time when the first data byte was captured, as returned by the clock_gettime() function for the relevant clock id; see V4L2_BUF_FLAG_TIMESTAMP_* in Buffer Flags. For output streams the driver stores the time at which the last data byte was actually sent out in the timestamp field. This permits applications to monitor the drift between the video and system clock. For output streams that use V4L2_BUF_FLAG_TIMESTAMP_COPY the application has to fill in the timestamp which will be copied by the driver to the capture stream.
struct v4l2_timecode	timecode		When 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. 注釈 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.
__u32	memory		This field must be set by applications and/or drivers in accordance with the selected I/O method. See 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 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 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 struct 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 ioctl VIDIOC_REQBUFS and/or ioctl 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. Drivers and applications must set this to 0.
__u32	request_fd		The file descriptor of the request to queue the buffer to. If the flag V4L2_BUF_FLAG_REQUEST_FD is set, then the buffer will be queued to this request. If the flag is not set, then this field will be ignored. The V4L2_BUF_FLAG_REQUEST_FD flag and this field are only used by ioctl VIDIOC_QBUF and ignored by other ioctls that take a v4l2_buffer as argument. Applications should not set V4L2_BUF_FLAG_REQUEST_FD for any ioctls other than VIDIOC_QBUF. If the device does not support requests, then EBADR will be returned. If requests are supported but an invalid request file descriptor is given, then EINVAL will be returned.

v4l2_plane

3.6.3. 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 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. 注釈 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 ioctl VIDIOC_REQBUFS and/or ioctl 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 a capture stream, applications when it refers to an output stream. 注釈 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 drivers and applications.

v4l2_buf_type

3.6.4. 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).
V4L2_BUF_TYPE_META_CAPTURE	13	Buffer for metadata capture, see Metadata Interface.
V4L2_BUF_TYPE_META_OUTPUT	14	Buffer for metadata output, see Metadata Interface.

3.6.5. Buffer Flags

V4L2_BUF_FLAG_MAPPED	0x00000001	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.
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_QBUFioctl 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_IN_REQUEST	0x00000080	This buffer is part of a request that hasn't been queued yet.
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 ioctl VIDIOC_QUERYBUF, VIDIOC_PREPARE_BUF, ioctl VIDIOC_QBUF, VIDIOC_DQBUF 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_LAST	0x00100000	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.
V4L2_BUF_FLAG_REQUEST_FD	0x00800000	The request_fd field contains a valid file descriptor.
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() 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().
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.

v4l2_memory

3.6.6. 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.

3.6.7. Timecodes

The v4l2_buffer_timecode structure is designed to hold a SMPTE 12M or similar timecode. (struct timeval timestamps are stored in the struct v4l2_buffer timestamp field.)

v4l2_timecode

3.6.7.1. 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.

3.6.7.2. 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

3.6.7.3. 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.