7.3. ioctl VIDIOC_CREATE_BUFS

7.3.1. Name

VIDIOC_CREATE_BUFS - Create buffers for Memory Mapped or User Pointer or DMA Buffer I/O

7.3.2. Synopsis

VIDIOC_CREATE_BUFS

int ioctl(int fd, VIDIOC_CREATE_BUFS, struct v4l2_create_buffers *argp)

7.3.3. Arguments

fd

File descriptor returned by open().

argp

Pointer to struct v4l2_create_buffers.

7.3.4. Description

This ioctl is used to create buffers for memory mapped or user pointer or DMA buffer I/O. It can be used as an alternative or in addition to the ioctl VIDIOC_REQBUFS ioctl, when a tighter control over buffers is required. This ioctl can be called multiple times to create buffers of different sizes.

To allocate the device buffers applications must initialize the relevant fields of the struct v4l2_create_buffers structure. The count field must be set to the number of requested buffers, the memory field specifies the requested I/O method and the reserved array must be zeroed.

The format field specifies the image format that the buffers must be able to handle. The application has to fill in this struct v4l2_format. Usually this will be done using the VIDIOC_TRY_FMT or VIDIOC_G_FMT ioctls to ensure that the requested format is supported by the driver. Based on the format’s type field the requested buffer size (for single-planar) or plane sizes (for multi-planar formats) will be used for the allocated buffers. The driver may return an error if the size(s) are not supported by the hardware (usually because they are too small).

The buffers created by this ioctl will have as minimum size the size defined by the format.pix.sizeimage field (or the corresponding fields for other format types). Usually if the format.pix.sizeimage field is less than the minimum required for the given format, then an error will be returned since drivers will typically not allow this. If it is larger, then the value will be used as-is. In other words, the driver may reject the requested size, but if it is accepted the driver will use it unchanged.

When the ioctl is called with a pointer to this structure the driver will attempt to allocate up to the requested number of buffers and store the actual number allocated and the starting index in the count and the index fields respectively. On return count can be smaller than the number requested.

type v4l2_create_buffers

struct v4l2_create_buffers

__u32	index	The starting buffer index, returned by the driver.
__u32	count	The number of buffers requested or granted. If count == 0, then ioctl VIDIOC_CREATE_BUFS will set index to the current number of created buffers, and it will check the validity of memory and format.type. If those are invalid -1 is returned and errno is set to EINVAL error code, otherwise ioctl VIDIOC_CREATE_BUFS returns 0. It will never set errno to EBUSY error code in this particular case.
__u32	memory	Applications set this field to V4L2_MEMORY_MMAP, V4L2_MEMORY_DMABUF or V4L2_MEMORY_USERPTR. See v4l2_memory
struct v4l2_format	format	Filled in by the application, preserved by the driver.
__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. See here for a list of the capabilities. If you want to just query the capabilities without making any other changes, then set count to 0, memory to V4L2_MEMORY_MMAP and format.type to the buffer type.
__u32	flags	Specifies additional buffer management attributes. See Memory Consistency Flags.
__u32	max_num_buffers	If the V4L2_BUF_CAP_SUPPORTS_MAX_NUM_BUFFERS capability flag is set this field indicates the maximum possible number of buffers for this queue.
__u32	reserved[5]	A place holder for future extensions. Drivers and applications must set the array to zero.

7.3.5. 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.

ENOMEM

No memory to allocate buffers for memory mapped I/O.

EINVAL

The buffer type (format.type field), requested I/O method (memory) or format (format field) is not valid.