I'm developing an ASoC driver, and when I play an MP3 with mpg123, my driver's trigger() function is called with the SNDRV_PCM_TRIGGER_STOP command and then is immediately called again with SNDRV_PCM_TRIGGER_START.

Can someone tell me why ALSA would do this?

This only happens when playing an MP3. When I use aplay to play a regular Wave file, I don't see this behavior.

My driver does not have the SNDRV_PCM_INFO_PAUSE bit set, because I haven't gotten around to testing that capability. I suspect that if I did have that bit set, I would be getting SNDRV_PCM_TRIGGER_PAUSE_PUSH and SNDRV_PCM_TRIGGER_PAUSE_RELEASE calls instead.