bcm2835-i2s: Reduce the TX DREQ threshold

TX FIFO overrun is thought to be the cause of channel swapping, so
reducing the DREQ threshold seems reasonable and appears to be
effective.

See: #1417

Signed-off-by: Phil Elwell <phil@raspberrypi.org>

sound/soc/bcm/bcm2835-i2s.c

@@ -403,15 +403,22 @@ static int bcm2835_i2s_hw_params(struct snd_pcm_substream *substream,
 
 	/* Setup the DMA parameters */
 	regmap_update_bits(dev->i2s_regmap, BCM2835_I2S_CS_A_REG,
-			BCM2835_I2S_RXTHR(1)
-			| BCM2835_I2S_TXTHR(1)
-			| BCM2835_I2S_DMAEN, 0xffffffff);
+			   BCM2835_I2S_RXTHR(3)
+			   | BCM2835_I2S_TXTHR(3)
+			   | BCM2835_I2S_DMAEN,
+			   BCM2835_I2S_RXTHR(1)
+			   | BCM2835_I2S_TXTHR(1)
+			   | BCM2835_I2S_DMAEN);
 
 	regmap_update_bits(dev->i2s_regmap, BCM2835_I2S_DREQ_A_REG,
-			  BCM2835_I2S_TX_PANIC(0x10)
-			| BCM2835_I2S_RX_PANIC(0x30)
-			| BCM2835_I2S_TX(0x30)
-			| BCM2835_I2S_RX(0x20), 0xffffffff);
+			   BCM2835_I2S_TX_PANIC(0x7f)
+			   | BCM2835_I2S_RX_PANIC(0x7f)
+			   | BCM2835_I2S_TX(0x7f)
+			   | BCM2835_I2S_RX(0x7f),
+			   BCM2835_I2S_TX_PANIC(0x10)
+			   | BCM2835_I2S_RX_PANIC(0x30)
+			   | BCM2835_I2S_TX(0x20)
+			   | BCM2835_I2S_RX(0x20));
 
 	/* Clear FIFOs */
 	bcm2835_i2s_clear_fifos(dev, true, true);