start/stop floods with applications using start threshold, e.g. with aplay+S3 #1062

Bug moved from FW to driver (previously thesofproject/sof#1572)

Describe the bug
There seems to be an issue with handling a TRIG_START when following conditions apply:

• stream has been previously running and then restarted (1) S3 suspend+resume, 2) application stopped with SIGSUSPEND, 3) XRUN, or 4) applications which start/stop the stream with snd_pcm_drop() + snd_pcm_prepare() + snd_pcm_start()))
• application has set the "start_threshold" SW parameter to any lower value than the maximum, or explicitly starts the stream with snd_pcm_start()

With these conditions met, FW is not able to start the stream succesfully, but either ends up in a short start/stop loop causing dozens of XRUNs lasting multiple milliseconds (and making a pretty awful nosie), or in infinite loop (application must be terminated to recover).

This problem is at least a contributing factor to Pulseaudio issues tracked in linux#1000

To Reproduce
To simplify testing, I figured out following way to reproduce this with aplay (but this will impact a wide range of other ALSA applications):

1. aplay -R 21000 -Dhw:0 -vv -i testfile.wav
2. suspend aplay with 'ctrl-Z'
3. resume aplay with 'fg'
4. go back to (2) until error occurs

The error happens with 21ms start threshdold in this example, but notably
higher values (-R 200000) also cause the issue. Clearly DSP should have plenty of time to resume the stream with this much buffering in place.

Reproduction Rate
5/10

Expected behavior
Playback should resume succesfully.

Impact
Marked is as P2 as DSP should under no circumstance go into a state machine loop like this.

Environment

1. Branches
   SOF: 0dd6c23 (master)
   Linux: 2712289 (sof-dev head)

Note: attached log file has some additional patches (prints with "KV:"), but the bug is reproduced without these patches with the unmodified version above. The additional patches are to explain the problem better.

2. Name of the topology file: sof-hda-generic-2ch.tplg
3. Platform: WHL laptop with HDA codec
4. Reproducibility Rate: 5/10

Screenshots or console output

Sof-logger file attached.
20190617-soflog.txt