ASoC: sgtl5000: Allow SCLK pad drive strength to be changed

This patch introduces "sclk-strength" property to allow SCLK pad drive
strength to be changed via device tree.

When running playback test on LS1028ARDB, Tx Frame sync error interrupt
will occur sometimes. Some noises also exist. After changing SCLK pad
drive strength to the maximum value, the issues are gone.

Signed-off-by: Alison Wang <alison.wang@nxp.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/sound/sgtl5000.txt

@@ -37,6 +37,15 @@ VDDIO		1.8V		2.5V		3.3V
 2 =		3.33 mA		5.74 mA		8.03  mA
 3 =		4.99 mA		8.61 mA		12.05 mA
 
+- sclk-strength: the SCLK pad strength. Possible values are:
+0, 1, 2 and 3 as per the table below:
+
+VDDIO		1.8V		2.5V		3.3V
+0 = 		Disable
+1 =		1.66 mA		2.87 mA		4.02  mA
+2 =		3.33 mA		5.74 mA		8.03  mA
+3 =		4.99 mA		8.61 mA		12.05 mA
+
 Example:
 
 sgtl5000: codec@a {

sound/soc/codecs/sgtl5000.c

@@ -116,6 +116,13 @@ enum  {
 	I2S_LRCLK_STRENGTH_HIGH,
 };
 
+enum  {
+	I2S_SCLK_STRENGTH_DISABLE,
+	I2S_SCLK_STRENGTH_LOW,
+	I2S_SCLK_STRENGTH_MEDIUM,
+	I2S_SCLK_STRENGTH_HIGH,
+};
+
 /* sgtl5000 private structure in codec */
 struct sgtl5000_priv {
 	int sysclk;	/* sysclk rate */
@@ -129,6 +136,7 @@ struct sgtl5000_priv {
 	u8 micbias_resistor;
 	u8 micbias_voltage;
 	u8 lrclk_strength;
+	u8 sclk_strength;
 };
 
 /*
@@ -1302,7 +1310,9 @@ static int sgtl5000_probe(struct snd_soc_component *component)
 			SGTL5000_DAC_MUTE_RIGHT |
 			SGTL5000_DAC_MUTE_LEFT);
 
-	reg = ((sgtl5000->lrclk_strength) << SGTL5000_PAD_I2S_LRCLK_SHIFT | 0x5f);
+	reg = ((sgtl5000->lrclk_strength) << SGTL5000_PAD_I2S_LRCLK_SHIFT |
+	       (sgtl5000->sclk_strength) << SGTL5000_PAD_I2S_SCLK_SHIFT |
+	       0x1f);
 	snd_soc_component_write(component, SGTL5000_CHIP_PAD_STRENGTH, reg);
 
 	snd_soc_component_write(component, SGTL5000_CHIP_ANA_CTRL,
@@ -1542,6 +1552,13 @@ static int sgtl5000_i2c_probe(struct i2c_client *client,
 		sgtl5000->lrclk_strength = value;
 	}
 
+	sgtl5000->sclk_strength = I2S_SCLK_STRENGTH_LOW;
+	if (!of_property_read_u32(np, "sclk-strength", &value)) {
+		if (value > I2S_SCLK_STRENGTH_HIGH)
+			value = I2S_SCLK_STRENGTH_LOW;
+		sgtl5000->sclk_strength = value;
+	}
+
 	/* Ensure sgtl5000 will start with sane register values */
 	sgtl5000_fill_defaults(client);