Correctly set the water level based on min_avail to avoid infinite
polling loops.
Applications tested: chrome, musescore, aplay and arecord .
PR: 261767
Approved by: pi (implicit)
When computing the oss_pointer() return value, use the application
pointer value as reference, so that ALSA audio applications at any
time don't read/write more audio data than is possible. Following the
cyclic DMA pointer is not a good idea, because it doesn't account for
under- or overruns and this quickly leads to a situation where some
applications may want to write more data, ending in infinite loops.
The SNDCTL_DSP_GET space ioctls operate on buffer fragments. Any
size less than bi.fragsize is reported as zero bytes. Don't set the
water level less than bi.fragsize, because it leads to infinite
polling the way ALSA PCM is working.
Approved by: pi (implicit)
files/alsa-plugins.patch contains an assortment of FreeBSD-specific
improvements that may not work on alternative OSS implementations.
So, restore the ability to exclude the patch for debugging.
1) When read() or write() fails due to EAGAIN because no buffer is available or
buffer is full, return 0 instead of failure. Else clients may simply close
the device and re-open it instantly chopping the audio.
2) Implement OSS delay function to fix an issue with negative delays.
3) Use at least 16ms of HW buffer by default to avoid jitter.
Approved by: pi, jbeich
