Hi!
When the FIFO is set to 128, it means that after the initial FIFO fill we will have DMA request coming from McBSP to sDMA with a rate of:
(1000/sampling_rate) * (FIFO-threshold / channels) = DMA_req_distance_in_ms
So in case of 44.1KHz, stereo with 128 FIFO threshold DMA request will come at every 1.45ms. If I'm not mistaken. The whole FIFO (1280) holds 14.51ms of audio in this case.
I don't see this correlate with the 30ms at all.
It seems we easily have a situation where DMA is done buffering to McBSP, and PMIC is playing audio, and we hit idle. At that point there are no immediate timers pending and cpuidle determines we can try to hit a deeper idle mode. As there are no hardware blockers with DMA off and McBSP not blocking, the hardware hits off mode. This cuts power to McBSP.
Ideally we'd configure McBSP activity to block deeper idle states in the hardware but I don't think we have such a configuration available.
I wonder why we have not seen this before? I can not recall anything like this with n900 (Jarkko might know that better) neither with n9/n950. On the n9/n950 I have even put the OMAP3 to OFF during audio playback with the
I was seeing something like that with 4.x kernel on Nokia N900.
Pavel