ASoC: davinci-mcasp: Logic low for inactive output slots

The default state when serializers are in inactive slots is Hi-Z.
In some cases, there are no additional components driving the data
lines to a safe state so they might have noise.

While in inactive slots, the McASP AXR pins configured as outputs
can be driven low through the serializer pin drive mode setting
(DISMOD) to prevent such noise.
Signed-off-by: Misael Lopez Cruz <misael.lopez@ti.com>
Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

sound/soc/davinci/davinci-mcasp.c

@@ -686,6 +686,8 @@ static int mcasp_common_hw_param(struct davinci_mcasp *mcasp, int stream,
 		if (mcasp->serial_dir[i] == TX_MODE &&
 					tx_ser < max_active_serializers) {
 			mcasp_set_bits(mcasp, DAVINCI_MCASP_PDIR_REG, AXR(i));
+			mcasp_mod_bits(mcasp, DAVINCI_MCASP_XRSRCTL_REG(i),
+				       DISMOD_LOW, DISMOD_MASK);
 			tx_ser++;
 		} else if (mcasp->serial_dir[i] == RX_MODE &&
 					rx_ser < max_active_serializers) {

sound/soc/davinci/davinci-mcasp.h

@@ -215,7 +215,10 @@
  * DAVINCI_MCASP_XRSRCTL_BASE_REG -  Serializer Control Register Bits
  */
 #define MODE(val)	(val)
-#define DISMOD		(val)(val<<2)
+#define DISMOD_3STATE	(0x0)
+#define DISMOD_LOW	(0x2 << 2)
+#define DISMOD_HIGH	(0x3 << 2)
+#define DISMOD_MASK	DISMOD_HIGH
 #define TXSTATE		BIT(4)
 #define RXSTATE		BIT(5)
 #define SRMOD_MASK	3