Rework general approach to locking and working with audio worker thread:
- Use flags to signal requested worker action
- Fix submitted buffer calculations to avoid samples starvation
- Protect buffer pointers with locks to fix race condition between callback
and audio worker thread
- Remove unnecessary vchi_service_use
- Do not use lock to serialize VCHI requests since only one thread issues them now
- Fix unloading signaling per hselasky@ suggestion
- Add output to detect inconsistent callback data caused by possible firmware bug
https://github.com/raspberrypi/firmware/issues/696
- Add stats/debug sysctls to troubleshoot possible bugs
PR: 213687, 205979, 215194
MFC after: 1 week
- Move all VCHI activity to worker thread: channel methods are called with
non-sleepable lock held and VCHI uses sleepable lock.
- In worker thread use sx(9) lock instead of mutex(9) for the same reason.
PR: 213801, 205979
- Pre-buffer audio data to VideoCore so there are no audible glitches when
driver is too late to provide samples
- Start actual playback when there is some prebuffered audio,
it fixes audible noisy click in the beginning of playback
- Use #defines instead of hardcoded values where appropriate
- Fix copy-pasted comment
PR: 208678
