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On ARM platforms, accessing the PMU registers requires special user access permissions. Since there is no other way to get accurate timers, the current implementation of timers in FFmpeg rely on these registers. Unfortunately, enabling user access to these registers on Linux is not trivial, and generally involve compiling a random and unreliable github kernel module, or patching somehow your kernel. Such module is very unlikely to reach the upstream anytime soon. Quoting Robin Murphin from ARM: > Say you do give userspace direct access to the PMU; now run two or more > programs at once that believe they can use the counters for their own > "minimal-overhead" profiling. Have fun interpreting those results... > > And that's not even getting into the implications of scheduling across > different CPUs, CPUidle, etc. where the PMU state is completely beyond > userspace's control. In general, the plan to provide userspace with > something which might happen to just about work in a few corner cases, > but is meaningless, misleading or downright broken in all others, is to > never do so. As a result, the alternative is to use the Performance Monitoring Linux API which makes use of these registers internally (assuming the PMU of your ARM board is supported in the kernel, which is definitely not a given...). While the Linux API is obviously cross platform, it does have a significant overhead which needs to be taken into account. As a result, that mode is only weakly enabled on ARM platforms exclusively. Note on the non flexibility of the implementation: the timers (native FFmpeg vs Linux API) are selected at compilation time to prevent the need of function calls, which would result in a negative impact on the cycle counters.
FFmpeg README
FFmpeg is a collection of libraries and tools to process multimedia content such as audio, video, subtitles and related metadata.
Libraries
libavcodecprovides implementation of a wider range of codecs.libavformatimplements streaming protocols, container formats and basic I/O access.libavutilincludes hashers, decompressors and miscellaneous utility functions.libavfilterprovides a mean to alter decoded Audio and Video through chain of filters.libavdeviceprovides an abstraction to access capture and playback devices.libswresampleimplements audio mixing and resampling routines.libswscaleimplements color conversion and scaling routines.
Tools
- ffmpeg is a command line toolbox to manipulate, convert and stream multimedia content.
- ffplay is a minimalistic multimedia player.
- ffprobe is a simple analysis tool to inspect multimedia content.
- ffserver is a multimedia streaming server for live broadcasts.
- Additional small tools such as
aviocat,ismindexandqt-faststart.
Documentation
The offline documentation is available in the doc/ directory.
The online documentation is available in the main website and in the wiki.
Examples
Coding examples are available in the doc/examples directory.
License
FFmpeg codebase is mainly LGPL-licensed with optional components licensed under GPL. Please refer to the LICENSE file for detailed information.
Contributing
Patches should be submitted to the ffmpeg-devel mailing list using
git format-patch or git send-email. Github pull requests should be
avoided because they are not part of our review process and will be ignored.