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

File descriptor returned by open().
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, however this cannot succeed when any buffers are still mapped. A count value of zero frees all buffers, after aborting or finishing any DMA in progress, an implicit VIDIOC_STREAMOFF.

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 others 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.

__u32 reserved[1] A place holder for future extensions. Drivers and applications must set the array to zero.
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.

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.

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