[alsa-devel] [PATCH v3] ASoC: omap-mcbsp: Add PM QoS support for McBSP to prevent glitches

[Tony Lindgren]

* Peter Ujfalusi <peter.ujfalusi at ti.com> [161205 01:35]:
> On 12/04/2016 03:33 AM, Matt Ranostay wrote:
> >>> + *
> >>> + * Also setting of the QoS latency for the FIFO which varies upon the buffer
> >>> + * size. Approximately 2.3 milliseconds per FIFO location.
> >>>   */
> >>>  void omap_mcbsp_start(struct omap_mcbsp *mcbsp, int tx, int rx)
> >>>  {
> >>>       int enable_srg = 0;
> >>> +     int latency = mcbsp->pdata->buffer_size * 23;
> >>
> >> I think this is not correct.
> >> The McBSP FIFO time depends on the sample rate and on the number of
> >> channels the audio is using. With 8KHz mono you have 12 times more time
> >> per FIFO element compared to 48KHz stereo.
> >>
> >> As it has been discussed with Tony we should calculate the QoS latency
> >> in hw_params:
> >>
> > 
> > Ok yeah missed that part of the thread but it makes sense. Just one
> > question should the latency value be only based on the TX FIFO
> > threshold? I assume capture and transmit have two different settings.
> 
> Yes, I believe we should apply the lowest QoS when both direction is
> active. If the second stream needs lower QoS, we should switch to use
> that, if it would need longer, we should keep the QoS placed for the
> first stream.

Sounds good to me.

Regards,

Tony