[alsa-devel] ASoC: Choppy audio on OMAP using McBSP3 interface

[Pedro Sanchez]

On 05/31/2011 09:21 AM, Pedro Sanchez wrote:
> Hello,
>
> I wrote a ASoC driver for a TI DM3730-based custom board interfacing a
> TI TAS5709 codec over the McBSP2 interface. I'm using the Linux kernel
> 2.6.32. The driver works very well, no problems with the sound quality
> at all.
>
> Unfortunately due to reasons alien to me I needed to move the driver to
> work using the McBSP3 interface instead. All I did was then to change
> the pointer in my machine code to point to the McBSP3 interface:
>
> static struct snd_soc_dai_link bp_audio_dai = {
>       .name = "tas5709",
>       .stream_name = "tas5709",
>       .cpu_dai =&omap_mcbsp_dai[2],<-- from [1] to [2]
>       .codec_dai =&tas5709_dai,
>       .init = bp_audio_init,
>       .ops =&bp_audio_ops,
> };
>
> and in my machine init code:
>
> *(unsigned int *)bp_audio_dai.cpu_dai->private_data = 2;
>
> With this change the audio driver works well when playing local audio
> files via the aplay command. But it fails miserably when playing
> streaming audio from a VoIP connection, the audio is very choppy as it
> happens when there is a large packet loss. However audio is mono 16 bits
> @8 KHz, not really very demanding.
>
> I realize there is a big difference in the internal buffer size for the
> two interfaces, the McBSP2 having 10 times the buffer space of the
> McBSP3. I suspect this is somehow the source of my problems. I'm
> inclined to believe that there is some DMA-related optimization issue
> that has to be addressed somehow, but this is just a guess.
>
> I fail to understand why is it that I can play local audio files but not
> streaming audio. What makes the difference? Any suggestions on how to
> address this issue?
>
> Thank you,
>
>

Fixed!, For future reference here's the change:

--- a/arch/arm/plat-omap/mcbsp.c
+++ b/arch/arm/plat-omap/mcbsp.c
@@ -1254,7 +1254,7 @@ static inline void __devexit 
omap_additional_remove(struct

  static inline void __devinit omap34xx_device_init(struct omap_mcbsp 
*mcbsp)
  {
-       mcbsp->dma_op_mode = MCBSP_DMA_MODE_THRESHOLD;
+    mcbsp->dma_op_mode = MCBSP_DMA_MODE_FRAME;
         if (cpu_is_omap34xx()) {
                 mcbsp->max_tx_thres = max_thres(mcbsp);
                 mcbsp->max_rx_thres = max_thres(mcbsp);

This changes the default DMA operation to be frame-based as opposed to 
be triggered by a threshold (which is probably good just for McBSP2).

-- 
Pedro