ASoC: add start treshold to prevent underflows in some cases
Grazvydas Ignotas [Thu, 10 Jun 2010 20:34:51 +0000 (23:34 +0300)]
This is useful for apps that don't rely on audio blocking and
feed fixed amount of samples over time period (like emulators).

sound/soc/omap/omap-pcm.c

index acd68ef..b411eb4 100644 (file)
@@ -168,6 +168,31 @@ static int omap_pcm_prepare(struct snd_pcm_substream *substream)
 
        omap_enable_dma_irq(prtd->dma_ch, OMAP_DMA_FRAME_IRQ);
 
+       /*
+        * To handle realtime streaming sources properly, we need to be sure
+        * we have at least 2 periods of size larger than FIFO in the buffer
+        * to start playing without underflowing. This is because DMA is always
+        * ahead of playback by amount close to FIFO size and needs to have
+        * next period ready when previous one finishes transfering to FIFO.
+        */
+       if (substream->stream == SNDRV_PCM_STREAM_PLAYBACK &&
+           runtime->periods > 1) {
+               struct snd_soc_pcm_runtime *rtd = substream->private_data;
+               int *mcbsp_bus_id = rtd->dai->cpu_dai->private_data;
+               int fifo_samples = (*mcbsp_bus_id == 1) ? 1024 : 256;
+               int period_samples, period_frames;
+               
+               period_samples = bytes_to_samples(runtime,
+                               snd_pcm_lib_period_bytes(substream));
+               if (period_samples < fifo_samples + runtime->channels)
+                       period_samples = fifo_samples + runtime->channels;
+               period_frames = bytes_to_frames(runtime,
+                               samples_to_bytes(runtime, period_samples)) * 2;
+
+               if (runtime->start_threshold < period_frames)
+                       runtime->start_threshold = period_frames;
+       }
+
        return 0;
 }