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

[Peter Ujfalusi]

On 12/02/2016 10:23 AM, Peter Ujfalusi wrote:
>> @@ -637,12 +638,21 @@ void omap_mcbsp_free(struct omap_mcbsp *mcbsp)
>>   * Here we start the McBSP, by enabling transmitter, receiver or both.
>>   * If no transmitter or receiver is active prior calling, then sample-rate
>>   * generator and frame sync are started.
>> + *
>> + * 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:
> 
> latency_ms = ((FIFOsize - FIFOthreshold) / channels) * 1000/sampling-rate
> 
> On OMAP3.McBSP2 for example (44.1KHz, stereo):
> FIFO threshold 128
>  - DMA request will be triggered when 128 slots are free in the FIFO
>  - at that point we have still 1152 words in the FIFO.
>  - if the C wakeup latency is longer then what it takes to play out the
> samples from the FIFO (13.06ms), we will drain the FIFO and got underflow.
>  - in this case the QOS should be set as 13.06ms
> 
> FIFO threshold 1024
>  - DMA request will be triggered when 1024 slots are free in the FIFO
>  - at that point we have still 256 words in the FIFO.
>  - if the C wakeup latency is longer then what it takes to play out the
> samples from the FIFO (2.9ms), we will drain the FIFO and got underflow.
>  - in this case the QOS should be 2.9ms
> 
> On other McBSPs with 128 word FIFO the required latency is shorter to ensure
> we don't drain the FIFO.

and we still have the issue of full duplex audio when the FIFO threshold
is different for playback and capture...

-- 
Péter