ASoC: OMAP: Set minimum buffer size constraint for McBSP2 in OMAP3

McBSP2 in OMAP3 has 1 ksample (1k x 32 bit) internal FIFO. During
initial playback startup, this FIFO is keeping the DMA request active
until the FIFO is full.

So now if ALSA buffer size is smaller, DMA is looping around it while
filling up the HW FIFO, generating burst of interrupts as well and SW
doesn't have any change to fill enough data.
Signed-off-by: Jarkko Nikula <jarkko.nikula@nokia.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>

sound/soc/omap/omap-mcbsp.c

@@ -146,6 +146,17 @@ static int omap_mcbsp_dai_startup(struct snd_pcm_substream *substream,
 	struct omap_mcbsp_data *mcbsp_data = to_mcbsp(cpu_dai->private_data);
 	int err = 0;
 
+	if (cpu_is_omap343x() && mcbsp_data->bus_id == 1) {
+		/*
+		 * McBSP2 in OMAP3 has 1024 * 32-bit internal audio buffer.
+		 * Set constraint for minimum buffer size to the same than FIFO
+		 * size in order to avoid underruns in playback startup because
+		 * HW is keeping the DMA request active until FIFO is filled.
+		 */
+		snd_pcm_hw_constraint_minmax(substream->runtime,
+			SNDRV_PCM_HW_PARAM_BUFFER_BYTES, 4096, UINT_MAX);
+	}
+
 	if (!cpu_dai->active)
 		err = omap_mcbsp_request(mcbsp_data->bus_id);