7.52. ioctl VIDIOC_REQBUFS

7.52.1. Name

VIDIOC_REQBUFS - Initiate Memory Mapping, User Pointer I/O or DMA buffer I/O

7.52.2. Synopsis

int ioctl(int fd, VIDIOC_REQBUFS, struct v4l2_requestbuffers *argp)

7.52.3. Arguments

fd

File descriptor returned by open().

argp

Pointer to struct v4l2_requestbuffers.

7.52.4. Description

This ioctl is used to initiate memory mapped, user pointer or DMABUF based I/O. Memory mapped buffers are located in device memory and must be allocated with this ioctl before they can be mapped into the application’s address space. User buffers are allocated by applications themselves, and this ioctl is merely used to switch the driver into user pointer I/O mode and to setup some internal structures. Similarly, DMABUF buffers are allocated by applications through a device driver, and this ioctl only configures the driver into DMABUF I/O mode without performing any direct allocation.

To allocate device buffers applications initialize all fields of the struct v4l2_requestbuffers structure. They set the type field to the respective stream or buffer type, the count field to the desired number of buffers, memory must be set to the requested I/O method and the reserved array must be zeroed. When the ioctl is called with a pointer to this structure the driver will attempt to allocate the requested number of buffers and it stores the actual number allocated in the count field. It can be smaller than the number requested, even zero, when the driver runs out of free memory. A larger number is also possible when the driver requires more buffers to function correctly. For example video output requires at least two buffers, one displayed and one filled by the application.

When the I/O method is not supported the ioctl returns an EINVAL error code.

Applications can call ioctl VIDIOC_REQBUFS again to change the number of buffers. Note that if any buffers are still mapped or exported via DMABUF, then ioctl VIDIOC_REQBUFS can only succeed if the V4L2_BUF_CAP_SUPPORTS_ORPHANED_BUFS capability is set. Otherwise ioctl VIDIOC_REQBUFS will return the EBUSY error code. If V4L2_BUF_CAP_SUPPORTS_ORPHANED_BUFS is set, then these buffers are orphaned and will be freed when they are unmapped or when the exported DMABUF fds are closed. A count value of zero frees or orphans all buffers, after aborting or finishing any DMA in progress, an implicit VIDIOC_STREAMOFF.

v4l2_requestbuffers
struct v4l2_requestbuffers

__u32

count

The number of buffers requested or granted.

__u32

type

Type of the stream or buffers, this is the same as the struct v4l2_format type field. See v4l2_buf_type for valid values.

__u32

memory

Applications set this field to V4L2_MEMORY_MMAP, V4L2_MEMORY_DMABUF or V4L2_MEMORY_USERPTR. See v4l2_memory.

__u32

capabilities

Set by the driver. If 0, then the driver doesn’t support capabilities. In that case all you know is that the driver is guaranteed to support V4L2_MEMORY_MMAP and might support other v4l2_memory types. It will not support any other capabilities.

If you want to query the capabilities with a minimum of side-effects, then this can be called with count set to 0, memory set to V4L2_MEMORY_MMAP and type set to the buffer type. This will free any previously allocated buffers, so this is typically something that will be done at the start of the application.

union {

(anonymous)

__u32

flags

Specifies additional buffer management attributes. See Memory Consistency Flags.

__u32

reserved[1]

Kept for backwards compatibility. Use flags instead.

}

V4L2 Buffer Capabilities Flags

V4L2_BUF_CAP_SUPPORTS_MMAP

0x00000001

This buffer type supports the V4L2_MEMORY_MMAP streaming mode.

V4L2_BUF_CAP_SUPPORTS_USERPTR

0x00000002

This buffer type supports the V4L2_MEMORY_USERPTR streaming mode.

V4L2_BUF_CAP_SUPPORTS_DMABUF

0x00000004

This buffer type supports the V4L2_MEMORY_DMABUF streaming mode.

V4L2_BUF_CAP_SUPPORTS_REQUESTS

0x00000008

This buffer type supports requests.

V4L2_BUF_CAP_SUPPORTS_ORPHANED_BUFS

0x00000010

The kernel allows calling ioctl VIDIOC_REQBUFS while buffers are still mapped or exported via DMABUF. These orphaned buffers will be freed when they are unmapped or when the exported DMABUF fds are closed.

V4L2_BUF_CAP_SUPPORTS_M2M_HOLD_CAPTURE_BUF

0x00000020

Only valid for stateless decoders. If set, then userspace can set the V4L2_BUF_FLAG_M2M_HOLD_CAPTURE_BUF flag to hold off on returning the capture buffer until the OUTPUT timestamp changes.

V4L2_BUF_CAP_SUPPORTS_MMAP_CACHE_HINTS

0x00000040

This capability is set by the driver to indicate that the queue supports cache and memory management hints. However, it’s only valid when the queue is used for memory mapping streaming I/O. See V4L2_FLAG_MEMORY_NON_CONSISTENT, V4L2_BUF_FLAG_NO_CACHE_INVALIDATE and V4L2_BUF_FLAG_NO_CACHE_CLEAN.

7.52.5. Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately. The generic error codes are described at the Generic Error Codes chapter.

EINVAL

The buffer type (type field) or the requested I/O method (memory) is not supported.