dmabuf-v4l2-demo

Simple example application to show how to allocate dmabufs from user space (from a dmabuf heap) and use them for v4l2 capture.

dmabuf heaps

If the entries in /dev/dma_heap are missing, maybe the dmabuf heaps are not enabled. Have a look a these config symbols:

CONFIG_DMABUF_HEAPS
CONFIG_DMABUF_SYSFS_STATS
CONFIG_DMABUF_HEAPS_SYSTEM
CONFIG_DMABUF_HEAPS_CMA

Configure CMA area

There are basically three ways to configure the CMA area:

• Kernel configuration (Priority 3)
• Kernel command line (Priority 2)
• Device tree entry (Priority 1)

By using a device tree node it is also possible to specify the location of the CMA area. Such a device tree node could look like this example:

linux,cma {
  compatible = "shared-dma-pool";
  reusable;
  size = <0 0x30000000>;
  alloc-ranges = <0 0x80000000 0 0x30000000>;
  linux,cma-default;
};

Note: Depending on the configuration method, the name of the dmabuf-heap device node changes. If the mentioned device tree node is used, the node will be /dev/dma_heap/linux,cma, otherwise the node will be /dev/dma_heap/reserved.

v4l2 test device

For quick testing, the vivid v4l2 test driver can be used. If this module is not yet available, have a look a these config symbols:

CONFIG_MEDIA_TEST_SUPPORT
CONFIG_V4L_TEST_DRIVERS
CONFIG_VIDEO_VIVID

To create a v4l2 capture device with vivid run:

modprobe vivid num_inputs=1

Running the example

To capture 5 frames from /dev/video0 and dump them into /tmp run:

dmabuf-v4l2 -d /dev/video0 -o /tmp -l 5

One might also be interested in the dma-buf documentation.