3.2.16. ioctl DMX_REQBUFS¶
DMX_REQBUFS - Initiate Memory Mapping and/or DMA buffer I/O
this API is still experimental
File descriptor returned by
Pointer to struct
This ioctl is used to initiate a memory mapped or DMABUF based demux 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
dmx_requestbuffers structure. They set the
to the desired number of buffers, and
size to the size of each
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. The
count 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. The actual allocated buffer size can is returned
size, and can be smaller than what’s requested.
When this I/O method is not supported, the ioctl returns an
Applications can call ioctl DMX_REQBUFS again to change the number of
buffers, however this cannot succeed when any buffers are still mapped.
count value of zero frees all buffers, after aborting or finishing
any DMA in progress.