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
¶
__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 If you want to query the capabilities with a minimum of side-effects,
then this can be called with |
__u32 | reserved [1] |
A place holder for future extensions. Drivers and applications must set the array to zero. |
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. |
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.