This ioctl is used to initiate memory mapped or user pointer 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.
To allocate device buffers applications initialize three
fields of a v4l2_requestbuffers structure.
They set the type field to the respective
stream or buffer type, the count field to
the desired number of buffers, and memory
must be set to V4L2_MEMORY_MMAP. When the ioctl
is called with a pointer to this structure the driver attempts to
allocate the requested number of buffers and 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 possible when the driver requires
more buffers to function correctly.[1] When memory mapping I/O is not supported the ioctl
returns an EINVAL error code.
Applications can call 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.
To negotiate user pointer I/O, applications initialize only
the type field and set
memory to
V4L2_MEMORY_USERPTR. When the ioctl is called
with a pointer to this structure the driver prepares for user pointer
I/O, when this I/O method is not supported the ioctl returns an
EINVAL error code.
Table 1. struct v4l2_requestbuffers
| __u32 | count | The number of buffers requested or granted. This
field is only used when memory is set to
V4L2_MEMORY_MMAP. |
| enum v4l2_buf_type | type | Type of the stream or buffers, this is the same
as the struct v4l2_format type field. See Table 3-2 for valid values. |
| enum v4l2_memory | memory | Applications set this field to
V4L2_MEMORY_MMAP or
V4L2_MEMORY_USERPTR. |
| __u32 | reserved[2] | A place holder for future extensions and custom
(driver defined) buffer types V4L2_BUF_TYPE_PRIVATE and
higher. |
On success 0 is returned, on error -1 and the errno variable is set appropriately:
The driver supports multiple opens and I/O is already in progress, or reallocation of buffers was attempted although one or more are still mapped.
The buffer type (type field) or the
requested I/O method (memory) is not
supported.
| [1] | For example video output requires at least two buffers, one displayed and one filled by the application. |