2.16. V4L2 videobuf2 functions and data structures¶

enum vb2_memory¶: type of memory model used to make the buffers visible on userspace.

Constants

VB2_MEMORY_UNKNOWN: Buffer status is unknown or it is not used yet on userspace.
VB2_MEMORY_MMAP: The buffers are allocated by the Kernel and it is memory mapped via mmap() ioctl. This model is also used when the user is using the buffers via read() or write() system calls.
VB2_MEMORY_USERPTR: The buffers was allocated in userspace and it is memory mapped via mmap() ioctl.
VB2_MEMORY_DMABUF: The buffers are passed to userspace via DMA buffer.

struct vb2_mem_ops¶: memory handling/memory allocator operations.

Definition

struct vb2_mem_ops {
  void *(*alloc)(struct vb2_buffer *vb,struct device *dev, unsigned long size);
  void (*put)(void *buf_priv);
  struct dma_buf *(*get_dmabuf)(struct vb2_buffer *vb,void *buf_priv, unsigned long flags);
  void *(*get_userptr)(struct vb2_buffer *vb,struct device *dev,unsigned long vaddr, unsigned long size);
  void (*put_userptr)(void *buf_priv);
  void (*prepare)(void *buf_priv);
  void (*finish)(void *buf_priv);
  void *(*attach_dmabuf)(struct vb2_buffer *vb,struct device *dev,struct dma_buf *dbuf, unsigned long size);
  void (*detach_dmabuf)(void *buf_priv);
  int (*map_dmabuf)(void *buf_priv);
  void (*unmap_dmabuf)(void *buf_priv);
  void *(*vaddr)(struct vb2_buffer *vb, void *buf_priv);
  void *(*cookie)(struct vb2_buffer *vb, void *buf_priv);
  unsigned int    (*num_users)(void *buf_priv);
  int (*mmap)(void *buf_priv, struct vm_area_struct *vma);
};

Members

alloc: allocate video memory and, optionally, allocator private data, return ERR_PTR() on failure or a pointer to allocator private, per-buffer data on success; the returned private structure will then be passed as buf_priv argument to other ops in this structure. The size argument to this function shall be page aligned.
put: inform the allocator that the buffer will no longer be used; usually will result in the allocator freeing the buffer (if no other users of this buffer are present); the buf_priv argument is the allocator private per-buffer structure previously returned from the alloc callback.
get_dmabuf: acquire userspace memory for a hardware operation; used for DMABUF memory types.
get_userptr: acquire userspace memory for a hardware operation; used for USERPTR memory types; vaddr is the address passed to the videobuf2 layer when queuing a video buffer of USERPTR type; should return an allocator private per-buffer structure associated with the buffer on success, ERR_PTR() on failure; the returned private structure will then be passed as buf_priv argument to other ops in this structure.
put_userptr: inform the allocator that a USERPTR buffer will no longer be used.
prepare: called every time the buffer is passed from userspace to the driver, useful for cache synchronisation, optional.
finish: called every time the buffer is passed back from the driver to the userspace, also optional.
attach_dmabuf: attach a shared struct dma_buf for a hardware operation; used for DMABUF memory types; dev is the alloc device dbuf is the shared dma_buf; returns ERR_PTR() on failure; allocator private per-buffer structure on success; this needs to be used for further accesses to the buffer.
detach_dmabuf: inform the exporter of the buffer that the current DMABUF buffer is no longer used; the buf_priv argument is the allocator private per-buffer structure previously returned from the attach_dmabuf callback.
map_dmabuf: request for access to the dmabuf from allocator; the allocator of dmabuf is informed that this driver is going to use the dmabuf.
unmap_dmabuf: releases access control to the dmabuf - allocator is notified that this driver is done using the dmabuf for now.
vaddr: return a kernel virtual address to a given memory buffer associated with the passed private structure or NULL if no such mapping exists.
cookie: return allocator specific cookie for a given memory buffer associated with the passed private structure or NULL if not available.
num_users: return the current number of users of a memory buffer; return 1 if the videobuf2 layer (or actually the driver using it) is the only user.
mmap: setup a userspace mapping for a given memory buffer under the provided virtual memory region.

Description

Those operations are used by the videobuf2 core to implement the memory handling/memory allocators for each type of supported streaming I/O method.

Note

Required ops for USERPTR types: get_userptr, put_userptr.
Required ops for MMAP types: alloc, put, num_users, mmap.
Required ops for read/write access types: alloc, put, num_users, vaddr.
Required ops for DMABUF types: attach_dmabuf, detach_dmabuf, map_dmabuf, unmap_dmabuf.

struct vb2_plane¶: plane information.

Definition

struct vb2_plane {
  void *mem_priv;
  struct dma_buf          *dbuf;
  unsigned int            dbuf_mapped;
  unsigned int            bytesused;
  unsigned int            length;
  unsigned int            min_length;
  union {
    unsigned int    offset;
    unsigned long   userptr;
    int fd;
  } m;
  unsigned int            data_offset;
};

Members

mem_priv: private data with this plane.
dbuf: dma_buf - shared buffer object.
dbuf_mapped: flag to show whether dbuf is mapped or not
bytesused: number of bytes occupied by data in the plane (payload).
length: size of this plane (NOT the payload) in bytes. The maximum valid size is MAX_UINT - PAGE_SIZE.
min_length: minimum required size of this plane (NOT the payload) in bytes. length is always greater or equal to min_length, and like length, it is limited to MAX_UINT - PAGE_SIZE.
m: Union with memtype-specific data.
m.offset: when memory in the associated struct vb2_buffer is VB2_MEMORY_MMAP, equals the offset from the start of the device memory for this plane (or is a “cookie” that should be passed to mmap() called on the video node).
m.userptr: when memory is VB2_MEMORY_USERPTR, a userspace pointer pointing to this plane.
m.fd: when memory is VB2_MEMORY_DMABUF, a userspace file descriptor associated with this plane.
data_offset: offset in the plane to the start of data; usually 0, unless there is a header in front of the data.

Description

Should contain enough information to be able to cover all the fields of struct v4l2_plane at videodev2.h.

enum vb2_io_modes¶: queue access methods.

Constants

VB2_MMAP: driver supports MMAP with streaming API.
VB2_USERPTR: driver supports USERPTR with streaming API.
VB2_READ: driver supports read() style access.
VB2_WRITE: driver supports write() style access.
VB2_DMABUF: driver supports DMABUF with streaming API.

enum vb2_buffer_state¶: current video buffer state.

Constants

VB2_BUF_STATE_DEQUEUED: buffer under userspace control.
VB2_BUF_STATE_IN_REQUEST: buffer is queued in media request.
VB2_BUF_STATE_PREPARING: buffer is being prepared in videobuf2.
VB2_BUF_STATE_QUEUED: buffer queued in videobuf2, but not in driver.
VB2_BUF_STATE_ACTIVE: buffer queued in driver and possibly used in a hardware operation.
VB2_BUF_STATE_DONE: buffer returned from driver to videobuf2, but not yet dequeued to userspace.
VB2_BUF_STATE_ERROR: same as above, but the operation on the buffer has ended with an error, which will be reported to the userspace when it is dequeued.

struct vb2_buffer¶: represents a video buffer.

Definition

struct vb2_buffer {
  struct vb2_queue        *vb2_queue;
  unsigned int            index;
  unsigned int            type;
  unsigned int            memory;
  unsigned int            num_planes;
  u64 timestamp;
  struct media_request    *request;
  struct media_request_object     req_obj;
};

Members

vb2_queue: pointer to struct vb2_queue with the queue to which this driver belongs.
index: id number of the buffer.
type: buffer type.
memory: the method, in which the actual data is passed.
num_planes: number of planes in the buffer on an internal driver queue.
timestamp: frame timestamp in ns.
request: the request this buffer is associated with.
req_obj: used to bind this buffer to a request. This request object has a refcount.

struct vb2_ops¶: driver-specific callbacks.

Definition

struct vb2_ops {
  int (*queue_setup)(struct vb2_queue *q,unsigned int *num_buffers, unsigned int *num_planes, unsigned int sizes[], struct device *alloc_devs[]);
  void (*wait_prepare)(struct vb2_queue *q);
  void (*wait_finish)(struct vb2_queue *q);
  int (*buf_out_validate)(struct vb2_buffer *vb);
  int (*buf_init)(struct vb2_buffer *vb);
  int (*buf_prepare)(struct vb2_buffer *vb);
  void (*buf_finish)(struct vb2_buffer *vb);
  void (*buf_cleanup)(struct vb2_buffer *vb);
  int (*start_streaming)(struct vb2_queue *q, unsigned int count);
  void (*stop_streaming)(struct vb2_queue *q);
  void (*buf_queue)(struct vb2_buffer *vb);
  void (*buf_request_complete)(struct vb2_buffer *vb);
};

Members

queue_setup: called from VIDIOC_REQBUFS() and VIDIOC_CREATE_BUFS() handlers before memory allocation. It can be called twice: if the original number of requested buffers could not be allocated, then it will be called a second time with the actually allocated number of buffers to verify if that is OK. The driver should return the required number of buffers in *num_buffers, the required number of planes per buffer in *num_planes, the size of each plane should be set in the sizes[] array and optional per-plane allocator specific device in the alloc_devs[] array. When called from VIDIOC_REQBUFS(), *num_planes == 0, the driver has to use the currently configured format to determine the plane sizes and *num_buffers is the total number of buffers that are being allocated. When called from VIDIOC_CREATE_BUFS(), *num_planes != 0 and it describes the requested number of planes and sizes[] contains the requested plane sizes. In this case *num_buffers are being allocated additionally to q->num_buffers. If either *num_planes or the requested sizes are invalid callback must return -EINVAL.
wait_prepare: release any locks taken while calling vb2 functions; it is called before an ioctl needs to wait for a new buffer to arrive; required to avoid a deadlock in blocking access type.
wait_finish: reacquire all locks released in the previous callback; required to continue operation after sleeping while waiting for a new buffer to arrive.
buf_out_validate: called when the output buffer is prepared or queued to a request; drivers can use this to validate userspace-provided information; this is required only for OUTPUT queues.
buf_init: called once after allocating a buffer (in MMAP case) or after acquiring a new USERPTR buffer; drivers may perform additional buffer-related initialization; initialization failure (return != 0) will prevent queue setup from completing successfully; optional.
buf_prepare: called every time the buffer is queued from userspace and from the VIDIOC_PREPARE_BUF() ioctl; drivers may perform any initialization required before each hardware operation in this callback; drivers can access/modify the buffer here as it is still synced for the CPU; drivers that support VIDIOC_CREATE_BUFS() must also validate the buffer size; if an error is returned, the buffer will not be queued in driver; optional.
buf_finish: called before every dequeue of the buffer back to userspace; the buffer is synced for the CPU, so drivers can access/modify the buffer contents; drivers may perform any operations required before userspace accesses the buffer; optional. The buffer state can be one of the following: DONE and ERROR occur while streaming is in progress, and the PREPARED state occurs when the queue has been canceled and all pending buffers are being returned to their default DEQUEUED state. Typically you only have to do something if the state is VB2_BUF_STATE_DONE, since in all other cases the buffer contents will be ignored anyway.
buf_cleanup: called once before the buffer is freed; drivers may perform any additional cleanup; optional.
start_streaming: called once to enter ‘streaming’ state; the driver may receive buffers with buf_queue callback before start_streaming is called; the driver gets the number of already queued buffers in count parameter; driver can return an error if hardware fails, in that case all buffers that have been already given by the buf_queue callback are to be returned by the driver by calling vb2_buffer_done() with VB2_BUF_STATE_QUEUED. If you need a minimum number of buffers before you can start streaming, then set vb2_queue->min_buffers_needed. If that is non-zero then start_streaming won’t be called until at least that many buffers have been queued up by userspace.
stop_streaming: called when ‘streaming’ state must be disabled; driver should stop any DMA transactions or wait until they finish and give back all buffers it got from buf_queue callback by calling vb2_buffer_done() with either VB2_BUF_STATE_DONE or VB2_BUF_STATE_ERROR; may use vb2_wait_for_all_buffers() function
buf_queue: passes buffer vb to the driver; driver may start hardware operation on this buffer; driver should give the buffer back by calling vb2_buffer_done() function; it is always called after calling VIDIOC_STREAMON() ioctl; might be called before start_streaming callback if user pre-queued buffers before calling VIDIOC_STREAMON().
buf_request_complete: a buffer that was never queued to the driver but is associated with a queued request was canceled. The driver will have to mark associated objects in the request as completed; required if requests are supported.

Description

These operations are not called from interrupt context except where mentioned specifically.

struct vb2_buf_ops¶: driver-specific callbacks.

Definition

struct vb2_buf_ops {
  int (*verify_planes_array)(struct vb2_buffer *vb, const void *pb);
  void (*init_buffer)(struct vb2_buffer *vb);
  void (*fill_user_buffer)(struct vb2_buffer *vb, void *pb);
  int (*fill_vb2_buffer)(struct vb2_buffer *vb, struct vb2_plane *planes);
  void (*copy_timestamp)(struct vb2_buffer *vb, const void *pb);
};

Members

verify_planes_array: Verify that a given user space structure contains enough planes for the buffer. This is called for each dequeued buffer.
init_buffer: given a vb2_buffer initialize the extra data after struct vb2_buffer. For V4L2 this is a struct vb2_v4l2_buffer.
fill_user_buffer: given a vb2_buffer fill in the userspace structure. For V4L2 this is a struct v4l2_buffer.
fill_vb2_buffer: given a userspace structure, fill in the vb2_buffer. If the userspace structure is invalid, then this op will return an error.
copy_timestamp: copy the timestamp from a userspace structure to the struct vb2_buffer.

struct vb2_queue¶: a videobuf2 queue.

Definition

struct vb2_queue {
  unsigned int                    type;
  unsigned int                    io_modes;
  struct device                   *dev;
  unsigned long                   dma_attrs;
  unsigned int                    bidirectional:1;
  unsigned int                    fileio_read_once:1;
  unsigned int                    fileio_write_immediately:1;
  unsigned int                    allow_zero_bytesused:1;
  unsigned int               quirk_poll_must_check_waiting_for_buffers:1;
  unsigned int                    supports_requests:1;
  unsigned int                    requires_requests:1;
  unsigned int                    uses_qbuf:1;
  unsigned int                    uses_requests:1;
  unsigned int                    allow_cache_hints:1;
  unsigned int                    non_coherent_mem:1;
  struct mutex                    *lock;
  void *owner;
  const struct vb2_ops            *ops;
  const struct vb2_mem_ops        *mem_ops;
  const struct vb2_buf_ops        *buf_ops;
  void *drv_priv;
  u32 subsystem_flags;
  unsigned int                    buf_struct_size;
  u32 timestamp_flags;
  gfp_t gfp_flags;
  u32 min_buffers_needed;
  struct device                   *alloc_devs[VB2_MAX_PLANES];
};

Members

type: private buffer type whose content is defined by the vb2-core caller. For example, for V4L2, it should match the types defined on enum v4l2_buf_type.
io_modes: supported io methods (see enum vb2_io_modes).
dev: device to use for the default allocation context if the driver doesn’t fill in the alloc_devs array.
dma_attrs: DMA attributes to use for the DMA.
bidirectional: when this flag is set the DMA direction for the buffers of this queue will be overridden with DMA_BIDIRECTIONAL direction. This is useful in cases where the hardware (firmware) writes to a buffer which is mapped as read (DMA_TO_DEVICE), or reads from buffer which is mapped for write (DMA_FROM_DEVICE) in order to satisfy some internal hardware restrictions or adds a padding needed by the processing algorithm. In case the DMA mapping is not bidirectional but the hardware (firmware) trying to access the buffer (in the opposite direction) this could lead to an IOMMU protection faults.
fileio_read_once: report EOF after reading the first buffer
fileio_write_immediately: queue buffer after each write() call
allow_zero_bytesused: allow bytesused == 0 to be passed to the driver
quirk_poll_must_check_waiting_for_buffers: Return EPOLLERR at poll when QBUF has not been called. This is a vb1 idiom that has been adopted also by vb2.
supports_requests: this queue supports the Request API.
requires_requests: this queue requires the Request API. If this is set to 1, then supports_requests must be set to 1 as well.
uses_qbuf: qbuf was used directly for this queue. Set to 1 the first time this is called. Set to 0 when the queue is canceled. If this is 1, then you cannot queue buffers from a request.
uses_requests: requests are used for this queue. Set to 1 the first time a request is queued. Set to 0 when the queue is canceled. If this is 1, then you cannot queue buffers directly.
allow_cache_hints: when set user-space can pass cache management hints in order to skip cache flush/invalidation on ->prepare() or/and ->finish().
non_coherent_mem: when set queue will attempt to allocate buffers using non-coherent memory.
lock: pointer to a mutex that protects the struct vb2_queue. The driver can set this to a mutex to let the v4l2 core serialize the queuing ioctls. If the driver wants to handle locking itself, then this should be set to NULL. This lock is not used by the videobuf2 core API.
owner: The filehandle that ‘owns’ the buffers, i.e. the filehandle that called reqbufs, create_buffers or started fileio. This field is not used by the videobuf2 core API, but it allows drivers to easily associate an owner filehandle with the queue.
ops: driver-specific callbacks
mem_ops: memory allocator specific callbacks
buf_ops: callbacks to deliver buffer information. between user-space and kernel-space.
drv_priv: driver private data.
subsystem_flags: Flags specific to the subsystem (V4L2/DVB/etc.). Not used by the vb2 core.
buf_struct_size: size of the driver-specific buffer structure; “0” indicates the driver doesn’t want to use a custom buffer structure type. In that case a subsystem-specific struct will be used (in the case of V4L2 that is sizeof(struct vb2_v4l2_buffer)). The first field of the driver-specific buffer structure must be the subsystem-specific struct (vb2_v4l2_buffer in the case of V4L2).
timestamp_flags: Timestamp flags; V4L2_BUF_FLAG_TIMESTAMP_* and V4L2_BUF_FLAG_TSTAMP_SRC_*
gfp_flags: additional gfp flags used when allocating the buffers. Typically this is 0, but it may be e.g. GFP_DMA or __GFP_DMA32 to force the buffer allocation to a specific memory zone.
min_buffers_needed: the minimum number of buffers needed before start_streaming can be called. Used when a DMA engine cannot be started unless at least this number of buffers have been queued into the driver.
alloc_devs: struct device memory type/allocator-specific per-plane device

bool vb2_queue_allows_cache_hints(struct vb2_queue *q)¶: Return true if the queue allows cache and memory consistency hints.

Parameters

struct vb2_queue *q: pointer to struct vb2_queue with videobuf2 queue

void *vb2_plane_vaddr(struct vb2_buffer *vb, unsigned int plane_no)¶: Return a kernel virtual address of a given plane.

Parameters

struct vb2_buffer *vb: pointer to struct vb2_buffer to which the plane in question belongs to.
unsigned int plane_no: plane number for which the address is to be returned.

Description

This function returns a kernel virtual address of a given plane if such a mapping exist, NULL otherwise.

void *vb2_plane_cookie(struct vb2_buffer *vb, unsigned int plane_no)¶: Return allocator specific cookie for the given plane.

Parameters

struct vb2_buffer *vb: pointer to struct vb2_buffer to which the plane in question belongs to.
unsigned int plane_no: plane number for which the cookie is to be returned.

Description

This function returns an allocator specific cookie for a given plane if available, NULL otherwise. The allocator should provide some simple static inline function, which would convert this cookie to the allocator specific type that can be used directly by the driver to access the buffer. This can be for example physical address, pointer to scatter list or IOMMU mapping.

void vb2_buffer_done(struct vb2_buffer *vb, enum vb2_buffer_state state)¶: inform videobuf2 that an operation on a buffer is finished.

Parameters

struct vb2_buffer *vb: pointer to struct vb2_buffer to be used.
enum vb2_buffer_state state: state of the buffer, as defined by enum vb2_buffer_state. Either VB2_BUF_STATE_DONE if the operation finished successfully, VB2_BUF_STATE_ERROR if the operation finished with an error or VB2_BUF_STATE_QUEUED.

Description

This function should be called by the driver after a hardware operation on a buffer is finished and the buffer may be returned to userspace. The driver cannot use this buffer anymore until it is queued back to it by videobuf by the means of vb2_ops->buf_queue callback. Only buffers previously queued to the driver by vb2_ops->buf_queue can be passed to this function.

While streaming a buffer can only be returned in state DONE or ERROR. The vb2_ops->start_streaming op can also return them in case the DMA engine cannot be started for some reason. In that case the buffers should be returned with state QUEUED to put them back into the queue.

void vb2_discard_done(struct vb2_queue *q)¶: discard all buffers marked as DONE.

Parameters

struct vb2_queue *q: pointer to struct vb2_queue with videobuf2 queue.

Description

This function is intended to be used with suspend/resume operations. It discards all ‘done’ buffers as they would be too old to be requested after resume.

Drivers must stop the hardware and synchronize with interrupt handlers and/or delayed works before calling this function to make sure no buffer will be touched by the driver and/or hardware.

int vb2_wait_for_all_buffers(struct vb2_queue *q)¶: wait until all buffers are given back to vb2.

Parameters

struct vb2_queue *q: pointer to struct vb2_queue with videobuf2 queue.

Description

This function will wait until all buffers that have been given to the driver by vb2_ops->buf_queue are given back to vb2 with vb2_buffer_done(). It doesn’t call vb2_ops->wait_prepare/vb2_ops->wait_finish pair. It is intended to be called with all locks taken, for example from vb2_ops->stop_streaming callback.

void vb2_core_querybuf(struct vb2_queue *q, unsigned int index, void *pb)¶: query video buffer information.

Parameters

struct vb2_queue *q: pointer to struct vb2_queue with videobuf2 queue.
unsigned int index: id number of the buffer.
void *pb: buffer struct passed from userspace.

Description

Videobuf2 core helper to implement VIDIOC_QUERYBUF() operation. It is called internally by VB2 by an API-specific handler, like videobuf2-v4l2.h.

The passed buffer should have been verified.

This function fills the relevant information for the userspace.

Return

returns zero on success; an error code otherwise.

int vb2_core_reqbufs(struct vb2_queue *q, enum vb2_memory memory, unsigned int flags, unsigned int *count)¶: Initiate streaming.

Parameters

struct vb2_queue *q: pointer to struct vb2_queue with videobuf2 queue.
enum vb2_memory memory: memory type, as defined by enum vb2_memory.
unsigned int flags: auxiliary queue/buffer management flags. Currently, the only used flag is V4L2_MEMORY_FLAG_NON_COHERENT.
unsigned int *count: requested buffer count.

Description

Videobuf2 core helper to implement VIDIOC_REQBUF() operation. It is called internally by VB2 by an API-specific handler, like videobuf2-v4l2.h.

This function:

verifies streaming parameters passed from the userspace;
sets up the queue;
negotiates number of buffers and planes per buffer with the driver to be used during streaming;
allocates internal buffer structures (struct vb2_buffer), according to the agreed parameters;
for MMAP memory type, allocates actual video memory, using the memory handling/allocation routines provided during queue initialization.

If req->count is 0, all the memory will be freed instead.

If the queue has been allocated previously by a previous vb2_core_reqbufs() call and the queue is not busy, memory will be reallocated.

Return

returns zero on success; an error code otherwise.

int vb2_core_create_bufs(struct vb2_queue *q, enum vb2_memory memory, unsigned int flags, unsigned int *count, unsigned int requested_planes, const unsigned int requested_sizes[])¶: Allocate buffers and any required auxiliary structs

Parameters

struct vb2_queue *q: pointer to struct vb2_queue with videobuf2 queue.
enum vb2_memory memory: memory type, as defined by enum vb2_memory.
unsigned int flags: auxiliary queue/buffer management flags.
unsigned int *count: requested buffer count.
unsigned int requested_planes: number of planes requested.
const unsigned int requested_sizes[]: array with the size of the planes.

Description

Videobuf2 core helper to implement VIDIOC_CREATE_BUFS() operation. It is called internally by VB2 by an API-specific handler, like videobuf2-v4l2.h.

This function:

verifies parameter sanity;
calls the vb2_ops->queue_setup queue operation;
performs any necessary memory allocations.

Return

returns zero on success; an error code otherwise.

int vb2_core_prepare_buf(struct vb2_queue *q, unsigned int index, void *pb)¶: Pass ownership of a buffer from userspace to the kernel.

Parameters

struct vb2_queue *q: pointer to struct vb2_queue with videobuf2 queue.
unsigned int index: id number of the buffer.
void *pb: buffer structure passed from userspace to v4l2_ioctl_ops->vidioc_prepare_buf handler in driver.

Description

Videobuf2 core helper to implement VIDIOC_PREPARE_BUF() operation. It is called internally by VB2 by an API-specific handler, like videobuf2-v4l2.h.

The passed buffer should have been verified.

This function calls vb2_ops->buf_prepare callback in the driver (if provided), in which driver-specific buffer initialization can be performed.

Return

returns zero on success; an error code otherwise.

int vb2_core_qbuf(struct vb2_queue *q, unsigned int index, void *pb, struct media_request *req)¶: Queue a buffer from userspace

Parameters

struct vb2_queue *q: pointer to struct vb2_queue with videobuf2 queue.
unsigned int index: id number of the buffer
void *pb: buffer structure passed from userspace to v4l2_ioctl_ops->vidioc_qbuf handler in driver
struct media_request *req: pointer to struct media_request, may be NULL.

Description

Videobuf2 core helper to implement VIDIOC_QBUF() operation. It is called internally by VB2 by an API-specific handler, like videobuf2-v4l2.h.

This function:

If req is non-NULL, then the buffer will be bound to this media request and it returns. The buffer will be prepared and queued to the driver (i.e. the next two steps) when the request itself is queued.
if necessary, calls vb2_ops->buf_prepare callback in the driver (if provided), in which driver-specific buffer initialization can be performed;
if streaming is on, queues the buffer in driver by the means of vb2_ops->buf_queue callback for processing.

Return

returns zero on success; an error code otherwise.

int vb2_core_dqbuf(struct vb2_queue *q, unsigned int *pindex, void *pb, bool nonblocking)¶: Dequeue a buffer to the userspace

Parameters

struct vb2_queue *q: pointer to struct vb2_queue with videobuf2 queue
unsigned int *pindex: pointer to the buffer index. May be NULL
void *pb: buffer structure passed from userspace to v4l2_ioctl_ops->vidioc_dqbuf handler in driver.
bool nonblocking: if true, this call will not sleep waiting for a buffer if no buffers ready for dequeuing are present. Normally the driver would be passing (file->f_flags & O_NONBLOCK) here.

Description

Videobuf2 core helper to implement VIDIOC_DQBUF() operation. It is called internally by VB2 by an API-specific handler, like videobuf2-v4l2.h.

This function:

calls buf_finish callback in the driver (if provided), in which driver can perform any additional operations that may be required before returning the buffer to userspace, such as cache sync,
the buffer struct members are filled with relevant information for the userspace.

Return

returns zero on success; an error code otherwise.

int vb2_core_streamon(struct vb2_queue *q, unsigned int type)¶: Implements VB2 stream ON logic

Parameters

struct vb2_queue *q: pointer to struct vb2_queue with videobuf2 queue
unsigned int type: type of the queue to be started. For V4L2, this is defined by enum v4l2_buf_type type.

Description

Videobuf2 core helper to implement VIDIOC_STREAMON() operation. It is called internally by VB2 by an API-specific handler, like videobuf2-v4l2.h.

Return

returns zero on success; an error code otherwise.

int vb2_core_streamoff(struct vb2_queue *q, unsigned int type)¶: Implements VB2 stream OFF logic

Parameters

struct vb2_queue *q: pointer to struct vb2_queue with videobuf2 queue
unsigned int type: type of the queue to be started. For V4L2, this is defined by enum v4l2_buf_type type.

Description

Videobuf2 core helper to implement VIDIOC_STREAMOFF() operation. It is called internally by VB2 by an API-specific handler, like videobuf2-v4l2.h.

Return

returns zero on success; an error code otherwise.

int vb2_core_expbuf(struct vb2_queue *q, int *fd, unsigned int type, unsigned int index, unsigned int plane, unsigned int flags)¶: Export a buffer as a file descriptor.

Parameters

struct vb2_queue *q: pointer to struct vb2_queue with videobuf2 queue.
int *fd: pointer to the file descriptor associated with DMABUF (set by driver).
unsigned int type: buffer type.
unsigned int index: id number of the buffer.
unsigned int plane: index of the plane to be exported, 0 for single plane queues
unsigned int flags: file flags for newly created file, as defined at include/uapi/asm-generic/fcntl.h. Currently, the only used flag is O_CLOEXEC. is supported, refer to manual of open syscall for more details.

Description

Videobuf2 core helper to implement VIDIOC_EXPBUF() operation. It is called internally by VB2 by an API-specific handler, like videobuf2-v4l2.h.

Return

returns zero on success; an error code otherwise.

int vb2_core_queue_init(struct vb2_queue *q)¶: initialize a videobuf2 queue

Parameters

struct vb2_queue *q: pointer to struct vb2_queue with videobuf2 queue. This structure should be allocated in driver

Description

The vb2_queue structure should be allocated by the driver. The driver is responsible of clearing it’s content and setting initial values for some required entries before calling this function.

Note

The following fields at q should be set before calling this function: vb2_queue->ops, vb2_queue->mem_ops, vb2_queue->type.

void vb2_core_queue_release(struct vb2_queue *q)¶: stop streaming, release the queue and free memory

Parameters

struct vb2_queue *q: pointer to struct vb2_queue with videobuf2 queue.

Description

This function stops streaming and performs necessary clean ups, including freeing video buffer memory. The driver is responsible for freeing the struct vb2_queue itself.

void vb2_queue_error(struct vb2_queue *q)¶: signal a fatal error on the queue

Parameters

struct vb2_queue *q: pointer to struct vb2_queue with videobuf2 queue.

Description

Flag that a fatal unrecoverable error has occurred and wake up all processes waiting on the queue. Polling will now set EPOLLERR and queuing and dequeuing buffers will return -EIO.

The error flag will be cleared when canceling the queue, either from vb2_streamoff() or vb2_queue_release(). Drivers should thus not call this function before starting the stream, otherwise the error flag will remain set until the queue is released when closing the device node.

int vb2_mmap(struct vb2_queue *q, struct vm_area_struct *vma)¶: map video buffers into application address space.

Parameters

struct vb2_queue *q: pointer to struct vb2_queue with videobuf2 queue.
struct vm_area_struct *vma: pointer to struct vm_area_struct with the vma passed to the mmap file operation handler in the driver.

Description

Should be called from mmap file operation handler of a driver. This function maps one plane of one of the available video buffers to userspace. To map whole video memory allocated on reqbufs, this function has to be called once per each plane per each buffer previously allocated.

When the userspace application calls mmap, it passes to it an offset returned to it earlier by the means of v4l2_ioctl_ops->vidioc_querybuf handler. That offset acts as a “cookie”, which is then used to identify the plane to be mapped.

This function finds a plane with a matching offset and a mapping is performed by the means of a provided memory operation.

The return values from this function are intended to be directly returned from the mmap handler in driver.

unsigned long vb2_get_unmapped_area(struct vb2_queue *q, unsigned long addr, unsigned long len, unsigned long pgoff, unsigned long flags)¶: map video buffers into application address space.

Parameters

struct vb2_queue *q: pointer to struct vb2_queue with videobuf2 queue.
unsigned long addr: memory address.
unsigned long len: buffer size.
unsigned long pgoff: page offset.
unsigned long flags: memory flags.

Description

This function is used in noMMU platforms to propose address mapping for a given buffer. It’s intended to be used as a handler for the file_operations->get_unmapped_area operation.

This is called by the mmap() syscall routines will call this to get a proposed address for the mapping, when !CONFIG_MMU.

__poll_t vb2_core_poll(struct vb2_queue *q, struct file *file, poll_table *wait)¶: implements poll syscall() logic.

Parameters

struct vb2_queue *q: pointer to struct vb2_queue with videobuf2 queue.
struct file *file: struct file argument passed to the poll file operation handler.
poll_table *wait: poll_table wait argument passed to the poll file operation handler.

Description

This function implements poll file operation handler for a driver. For CAPTURE queues, if a buffer is ready to be dequeued, the userspace will be informed that the file descriptor of a video device is available for reading. For OUTPUT queues, if a buffer is ready to be dequeued, the file descriptor will be reported as available for writing.

The return values from this function are intended to be directly returned from poll handler in driver.

size_t vb2_read(struct vb2_queue *q, char __user *data, size_t count, loff_t *ppos, int nonblock)¶: implements read() syscall logic.

Parameters

struct vb2_queue *q: pointer to struct vb2_queue with videobuf2 queue.
char __user *data: pointed to target userspace buffer
size_t count: number of bytes to read
loff_t *ppos: file handle position tracking pointer
int nonblock: mode selector (1 means blocking calls, 0 means nonblocking)

size_t vb2_write(struct vb2_queue *q, const char __user *data, size_t count, loff_t *ppos, int nonblock)¶: implements write() syscall logic.

Parameters

struct vb2_queue *q: pointer to struct vb2_queue with videobuf2 queue.
const char __user *data: pointed to target userspace buffer
size_t count: number of bytes to write
loff_t *ppos: file handle position tracking pointer
int nonblock: mode selector (1 means blocking calls, 0 means nonblocking)

vb2_thread_fnc¶: Typedef: callback function for use with vb2_thread.

Syntax

int vb2_thread_fnc (struct vb2_buffer *vb, void *priv)

Parameters

struct vb2_buffer *vb: pointer to struct vb2_buffer.
void *priv: pointer to a private data.

Description

This is called whenever a buffer is dequeued in the thread.

int vb2_thread_start(struct vb2_queue *q, vb2_thread_fnc fnc, void *priv, const char *thread_name)¶: start a thread for the given queue.

Parameters

struct vb2_queue *q: pointer to struct vb2_queue with videobuf2 queue.
vb2_thread_fnc fnc: vb2_thread_fnc callback function.
void *priv: priv pointer passed to the callback function.
const char *thread_name: the name of the thread. This will be prefixed with “vb2-“.

Description

This starts a thread that will queue and dequeue until an error occurs or vb2_thread_stop() is called.

Attention

This function should not be used for anything else but the videobuf2-dvb support. If you think you have another good use-case for this, then please contact the linux-media mailing list first.

int vb2_thread_stop(struct vb2_queue *q)¶: stop the thread for the given queue.

Parameters

struct vb2_queue *q: pointer to struct vb2_queue with videobuf2 queue.

bool vb2_is_streaming(struct vb2_queue *q)¶: return streaming status of the queue.

Parameters

struct vb2_queue *q: pointer to struct vb2_queue with videobuf2 queue.

bool vb2_fileio_is_active(struct vb2_queue *q)¶: return true if fileio is active.

Parameters

struct vb2_queue *q: pointer to struct vb2_queue with videobuf2 queue.

Description

This returns true if read() or write() is used to stream the data as opposed to stream I/O. This is almost never an important distinction, except in rare cases. One such case is that using read() or write() to stream a format using V4L2_FIELD_ALTERNATE is not allowed since there is no way you can pass the field information of each buffer to/from userspace. A driver that supports this field format should check for this in the vb2_ops->queue_setup op and reject it if this function returns true.

bool vb2_is_busy(struct vb2_queue *q)¶: return busy status of the queue.

Parameters

struct vb2_queue *q: pointer to struct vb2_queue with videobuf2 queue.

Description

This function checks if queue has any buffers allocated.

void *vb2_get_drv_priv(struct vb2_queue *q)¶: return driver private data associated with the queue.

Parameters

struct vb2_queue *q: pointer to struct vb2_queue with videobuf2 queue.

void vb2_set_plane_payload(struct vb2_buffer *vb, unsigned int plane_no, unsigned long size)¶: set bytesused for the plane plane_no.

Parameters

struct vb2_buffer *vb: pointer to struct vb2_buffer to which the plane in question belongs to.
unsigned int plane_no: plane number for which payload should be set.
unsigned long size: payload in bytes.

unsigned long vb2_get_plane_payload(struct vb2_buffer *vb, unsigned int plane_no)¶: get bytesused for the plane plane_no

Parameters

struct vb2_buffer *vb: pointer to struct vb2_buffer to which the plane in question belongs to.
unsigned int plane_no: plane number for which payload should be set.

unsigned long vb2_plane_size(struct vb2_buffer *vb, unsigned int plane_no)¶: return plane size in bytes.

Parameters

struct vb2_buffer *vb: pointer to struct vb2_buffer to which the plane in question belongs to.
unsigned int plane_no: plane number for which size should be returned.

bool vb2_start_streaming_called(struct vb2_queue *q)¶: return streaming status of driver.

Parameters

struct vb2_queue *q: pointer to struct vb2_queue with videobuf2 queue.

void vb2_clear_last_buffer_dequeued(struct vb2_queue *q)¶: clear last buffer dequeued flag of queue.

Parameters

struct vb2_queue *q: pointer to struct vb2_queue with videobuf2 queue.

struct vb2_buffer *vb2_get_buffer(struct vb2_queue *q, unsigned int index)¶: get a buffer from a queue

Parameters

struct vb2_queue *q: pointer to struct vb2_queue with videobuf2 queue.
unsigned int index: buffer index

Description

This function obtains a buffer from a queue, by its index. Keep in mind that there is no refcounting involved in this operation, so the buffer lifetime should be taken into consideration.

bool vb2_buffer_in_use(struct vb2_queue *q, struct vb2_buffer *vb)¶: return true if the buffer is in use and the queue cannot be freed (by the means of VIDIOC_REQBUFS(0)) call.

Parameters

struct vb2_queue *q: pointer to struct vb2_queue with videobuf2 queue.
struct vb2_buffer *vb: buffer for which plane size should be returned.

int vb2_verify_memory_type(struct vb2_queue *q, enum vb2_memory memory, unsigned int type)¶: Check whether the memory type and buffer type passed to a buffer operation are compatible with the queue.

Parameters

struct vb2_queue *q: pointer to struct vb2_queue with videobuf2 queue.
enum vb2_memory memory: memory model, as defined by enum vb2_memory.
unsigned int type: private buffer type whose content is defined by the vb2-core caller. For example, for V4L2, it should match the types defined on enum v4l2_buf_type.

bool vb2_request_object_is_buffer(struct media_request_object *obj)¶: return true if the object is a buffer

Parameters

struct media_request_object *obj: the request object.

unsigned int vb2_request_buffer_cnt(struct media_request *req)¶: return the number of buffers in the request

Parameters

struct media_request *req: the request.

struct vb2_v4l2_buffer¶: video buffer information for v4l2.

Definition

struct vb2_v4l2_buffer {
  struct vb2_buffer       vb2_buf;
  __u32 flags;
  __u32 field;
  struct v4l2_timecode    timecode;
  __u32 sequence;
  __s32 request_fd;
  bool is_held;
  struct vb2_plane        planes[VB2_MAX_PLANES];
};

Members

vb2_buf: embedded struct vb2_buffer.
flags: buffer informational flags.
field: field order of the image in the buffer, as defined by enum v4l2_field.
timecode: frame timecode.
sequence: sequence count of this frame.
request_fd: the request_fd associated with this buffer
is_held: if true, then this capture buffer was held
planes: plane information (userptr/fd, length, bytesused, data_offset).

Description

Should contain enough information to be able to cover all the fields of struct v4l2_buffer at videodev2.h.

struct vb2_buffer *vb2_find_buffer(struct vb2_queue *q, u64 timestamp)¶: Find a buffer with given timestamp

Parameters

struct vb2_queue *q: pointer to struct vb2_queue with videobuf2 queue.
u64 timestamp: the timestamp to find.

Description

Returns the buffer with the given timestamp, or NULL if not found.

int vb2_reqbufs(struct vb2_queue *q, struct v4l2_requestbuffers *req)¶: Wrapper for vb2_core_reqbufs() that also verifies the memory and type values.

Parameters

struct vb2_queue *q: pointer to struct vb2_queue with videobuf2 queue.
struct v4l2_requestbuffers *req: struct v4l2_requestbuffers passed from userspace to v4l2_ioctl_ops->vidioc_reqbufs handler in driver.

int vb2_create_bufs(struct vb2_queue *q, struct v4l2_create_buffers *create)¶: Wrapper for vb2_core_create_bufs() that also verifies the memory and type values.

Parameters

struct vb2_queue *q: pointer to struct vb2_queue with videobuf2 queue.
struct v4l2_create_buffers *create: creation parameters, passed from userspace to v4l2_ioctl_ops->vidioc_create_bufs handler in driver

int vb2_prepare_buf(struct vb2_queue *q, struct media_device *mdev, struct v4l2_buffer *b)¶: Pass ownership of a buffer from userspace to the kernel

Parameters

struct vb2_queue *q: pointer to struct vb2_queue with videobuf2 queue.
struct media_device *mdev: pointer to struct media_device, may be NULL.
struct v4l2_buffer *b: buffer structure passed from userspace to v4l2_ioctl_ops->vidioc_prepare_buf handler in driver

Description

Should be called from v4l2_ioctl_ops->vidioc_prepare_buf ioctl handler of a driver.

This function:

verifies the passed buffer,
calls vb2_ops->buf_prepare callback in the driver (if provided), in which driver-specific buffer initialization can be performed.
if b->request_fd is non-zero and mdev->ops->req_queue is set, then bind the prepared buffer to the request.

The return values from this function are intended to be directly returned from v4l2_ioctl_ops->vidioc_prepare_buf handler in driver.

int vb2_qbuf(struct vb2_queue *q, struct media_device *mdev, struct v4l2_buffer *b)¶: Queue a buffer from userspace

Parameters

struct vb2_queue *q: pointer to struct vb2_queue with videobuf2 queue.
struct media_device *mdev: pointer to struct media_device, may be NULL.
struct v4l2_buffer *b: buffer structure passed from userspace to v4l2_ioctl_ops->vidioc_qbuf handler in driver

Description

Should be called from v4l2_ioctl_ops->vidioc_qbuf handler of a driver.

This function:

verifies the passed buffer;
if b->request_fd is non-zero and mdev->ops->req_queue is set, then bind the buffer to the request.
if necessary, calls vb2_ops->buf_prepare callback in the driver (if provided), in which driver-specific buffer initialization can be performed;
if streaming is on, queues the buffer in driver by the means of vb2_ops->buf_queue callback for processing.

The return values from this function are intended to be directly returned from v4l2_ioctl_ops->vidioc_qbuf handler in driver.

int vb2_expbuf(struct vb2_queue *q, struct v4l2_exportbuffer *eb)¶: Export a buffer as a file descriptor

Parameters

struct vb2_queue *q: pointer to struct vb2_queue with videobuf2 queue.
struct v4l2_exportbuffer *eb: export buffer structure passed from userspace to v4l2_ioctl_ops->vidioc_expbuf handler in driver

Description

The return values from this function are intended to be directly returned from v4l2_ioctl_ops->vidioc_expbuf handler in driver.

int vb2_dqbuf(struct vb2_queue *q, struct v4l2_buffer *b, bool nonblocking)¶: Dequeue a buffer to the userspace

Parameters

struct vb2_queue *q: pointer to struct vb2_queue with videobuf2 queue.
struct v4l2_buffer *b: buffer structure passed from userspace to v4l2_ioctl_ops->vidioc_dqbuf handler in driver
bool nonblocking: if true, this call will not sleep waiting for a buffer if no buffers ready for dequeuing are present. Normally the driver would be passing (file->f_flags & O_NONBLOCK) here

Description

Should be called from v4l2_ioctl_ops->vidioc_dqbuf ioctl handler of a driver.

This function:

verifies the passed buffer;
calls vb2_ops->buf_finish callback in the driver (if provided), in which driver can perform any additional operations that may be required before returning the buffer to userspace, such as cache sync;
the buffer struct members are filled with relevant information for the userspace.

The return values from this function are intended to be directly returned from v4l2_ioctl_ops->vidioc_dqbuf handler in driver.

int vb2_streamon(struct vb2_queue *q, enum v4l2_buf_type type)¶: start streaming

Parameters

struct vb2_queue *q: pointer to struct vb2_queue with videobuf2 queue.
enum v4l2_buf_type type: type argument passed from userspace to vidioc_streamon handler, as defined by enum v4l2_buf_type.

Description

Should be called from v4l2_ioctl_ops->vidioc_streamon handler of a driver.

This function:

verifies current state
passes any previously queued buffers to the driver and starts streaming

The return values from this function are intended to be directly returned from v4l2_ioctl_ops->vidioc_streamon handler in the driver.

int vb2_streamoff(struct vb2_queue *q, enum v4l2_buf_type type)¶: stop streaming

Parameters

struct vb2_queue *q: pointer to struct vb2_queue with videobuf2 queue.
enum v4l2_buf_type type: type argument passed from userspace to vidioc_streamoff handler

Description

Should be called from vidioc_streamoff handler of a driver.

This function:

verifies current state,
stop streaming and dequeues any queued buffers, including those previously passed to the driver (after waiting for the driver to finish).

This call can be used for pausing playback. The return values from this function are intended to be directly returned from vidioc_streamoff handler in the driver

int vb2_queue_init(struct vb2_queue *q)¶: initialize a videobuf2 queue

Parameters

struct vb2_queue *q: pointer to struct vb2_queue with videobuf2 queue.

Description

The vb2_queue structure should be allocated by the driver. The driver is responsible of clearing it’s content and setting initial values for some required entries before calling this function. q->ops, q->mem_ops, q->type and q->io_modes are mandatory. Please refer to the struct vb2_queue description in include/media/videobuf2-core.h for more information.

int vb2_queue_init_name(struct vb2_queue *q, const char *name)¶: initialize a videobuf2 queue with a name

Parameters

struct vb2_queue *q: pointer to struct vb2_queue with videobuf2 queue.
const char *name: the queue name

Description

This function initializes the vb2_queue exactly like vb2_queue_init(), and additionally sets the queue name. The queue name is used for logging purpose, and should uniquely identify the queue within the context of the device it belongs to. This is useful to attribute kernel log messages to the right queue for m2m devices or other devices that handle multiple queues.

void vb2_queue_release(struct vb2_queue *q)¶: stop streaming, release the queue and free memory

Parameters

struct vb2_queue *q: pointer to struct vb2_queue with videobuf2 queue.

Description

This function stops streaming and performs necessary clean ups, including freeing video buffer memory. The driver is responsible for freeing the vb2_queue structure itself.

int vb2_queue_change_type(struct vb2_queue *q, unsigned int type)¶: change the type of an inactive vb2_queue

Parameters

struct vb2_queue *q: pointer to struct vb2_queue with videobuf2 queue.
unsigned int type: the type to change to (V4L2_BUF_TYPE_VIDEO_*)

Description

This function changes the type of the vb2_queue. This is only possible if the queue is not busy (i.e. no buffers have been allocated).

vb2_queue_change_type() can be used to support multiple buffer types using the same queue. The driver can implement v4l2_ioctl_ops.vidioc_reqbufs and v4l2_ioctl_ops.vidioc_create_bufs functions and call vb2_queue_change_type() before calling vb2_ioctl_reqbufs() or vb2_ioctl_create_bufs(), and thus “lock” the buffer type until the buffers have been released.

__poll_t vb2_poll(struct vb2_queue *q, struct file *file, poll_table *wait)¶: implements poll userspace operation

Parameters

struct vb2_queue *q: pointer to struct vb2_queue with videobuf2 queue.
struct file *file: file argument passed to the poll file operation handler
poll_table *wait: wait argument passed to the poll file operation handler

Description

This function implements poll file operation handler for a driver. For CAPTURE queues, if a buffer is ready to be dequeued, the userspace will be informed that the file descriptor of a video device is available for reading. For OUTPUT queues, if a buffer is ready to be dequeued, the file descriptor will be reported as available for writing.

If the driver uses struct v4l2_fh, then vb2_poll() will also check for any pending events.

The return values from this function are intended to be directly returned from poll handler in driver.

bool vb2_queue_is_busy(struct vb2_queue *q, struct file *file)¶: check if the queue is busy

Parameters

struct vb2_queue *q: pointer to struct vb2_queue with videobuf2 queue.
struct file *file: file through which the vb2 queue access is performed

Description

The queue is considered busy if it has an owner and the owner is not the file.

Queue ownership is acquired and checked by some of the v4l2_ioctl_ops helpers below. Drivers can also use this function directly when they need to open-code ioctl handlers, for instance to add additional checks between the queue ownership test and the call to the corresponding vb2 operation.

void vb2_video_unregister_device(struct video_device *vdev)¶: unregister the video device and release queue

Parameters

struct video_device *vdev: pointer to struct video_device

Description

If the driver uses vb2_fop_release()/_vb2_fop_release(), then it should use vb2_video_unregister_device() instead of video_unregister_device().

This function will call video_unregister_device() and then release the vb2_queue if streaming is in progress. This will stop streaming and this will simplify the unbind sequence since after this call all subdevs will have stopped streaming as well.

void vb2_ops_wait_prepare(struct vb2_queue *vq)¶: helper function to lock a struct vb2_queue

Parameters

struct vb2_queue *vq: pointer to struct vb2_queue

Description

..note:: only use if vq->lock is non-NULL.

void vb2_ops_wait_finish(struct vb2_queue *vq)¶: helper function to unlock a struct vb2_queue

Parameters

struct vb2_queue *vq: pointer to struct vb2_queue

Description

..note:: only use if vq->lock is non-NULL.

struct vb2_vmarea_handler¶: common vma refcount tracking handler.

Definition

struct vb2_vmarea_handler {
  refcount_t *refcount;
  void (*put)(void *arg);
  void *arg;
};

Members

refcount: pointer to refcount_t entry in the buffer.
put: callback to function that decreases buffer refcount.
arg: argument for put callback.