Video 4 Linux 2 (V4L2) Continues With New Features
Aside from GStreamer 1.0, one of the other topics being talked about at this week's GStreamer Conference is V4L2.
Hans Verkuil of Cisco Norway shared a status report on V4L2, the Video 4 Linux 2 implementation within the mainline Linux kernel. Verkuil outlined some of the major improvements recently made within this subsystem that's heavily used by ARM SoCs, web-cameras, etc within the Linux kernel.
- An events API for V4L2 events was introduced in Linux 2.6.35; improved events handling landed in Linux 3.1.
- The videobuf2 framework was merged in Linux 2.6.38 to solve problems with the original videobuf framework, which was very bad code for streaming. Videobuf2 also introduced multi-planar support and memory-to-memory devices.
- Support for Flash controllers was merged in Linux 3.1.
- Codec support for H.264/MPEG4/DiVX and elementary streams was added in Linux 3.1.
- Support for JPEG compression was merged in the Linux 3.4.
- Improved crop and compose support through a new selection API was merged in Linux 3.2.
- Support for multiple frequency bands for radio receivers/transmitters was merged into the forthcoming Linux 3.6 kernel.
- Added libv4l2rds for RDS (Radio Data System) decoding but the TMC decoding support still needs to be done.
- Contiguous Memory Allocator (CMA) from Samsung helps with V4L2 memory handling and was merged in Linux 3.5. CMA allows for pages to be moved elsewhere (or discarded) when needed for DMA buffers by memory being marked as available for DMA buffers or movable pages.
- Buffer sharing is actively being worked on by Linaro for ultimately having zero-copy video piplines. This is being achieved in large part by the much-talked-about-on-Phoronix DMA-BUF support. There is work-in-progress to allow V4L2 through the videobuf2 framework for handling such buffers. Patches for V4L2 are likely not to be merged until the Linux 3.8 kernel.
- Work-in-progress for supporting DVI/HDMI/DIsplayPort/VGA connectors with "odd ends" like EDID handling, hot-plug detect, etc. This work should be merged for the Linux 3.7 kernel.
- CEC (Consumer Electronics Control) support is an open item for V4L2 for allowing one device to discover other devices connected over HDMI, etc. It's tricky and still figuring out how to design this support, but a workshop is going to happen this week.
- A v4l2-compliance tool has been developed since V4L2 is still largely a mess and it's complex. This tool allows for device driver writers for Video 4 Linux 2 to better test a driver's compliance against the API. This compliance tool now covers about 90% of the entire V4L2 API. This tool is stricter than the API itself and new kernel drivers must be in compliance with this tool before being merged. But "there's very few drivers that actually pass." Hans Verkuil is hoping to officially finish this tool in the near future and that the V4L2 driver quality will continue to increase and benefit application developers relying upon V4L2.
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