ASoC: pcm512x: Avoid the PLL for the DAC clock, if possible

The PLL introduces jitter, which in turn introduces noice if used
to clock the DAC. Thus, avoid the PLL output, and use the PLL input
to drive the DAC clock, if possible.

This is described for the PCM5142/PCM5242 chips in the answers to the
forum post "PCM5142/PCM5242 DAC clock source" at the TI E2E community
pages (1).

(1) http://e2e.ti.com/support/data_converters/audio_converters/f/64/t/389994Signed-off-by: Peter Rosin <peda@axentia.se>
Signed-off-by: Mark Brown <broonie@kernel.org>

sound/soc/codecs/pcm512x.c

@@ -105,6 +105,7 @@ static const struct reg_default pcm512x_reg_defaults[] = {
 	{ PCM512x_VCOM_CTRL_2,       0x01 },
 	{ PCM512x_BCLK_LRCLK_CFG,    0x00 },
 	{ PCM512x_MASTER_MODE,       0x7c },
+	{ PCM512x_GPIO_DACIN,        0x00 },
 	{ PCM512x_GPIO_PLLIN,        0x00 },
 	{ PCM512x_SYNCHRONIZE,       0x10 },
 	{ PCM512x_PLL_COEFF_0,       0x00 },
@@ -138,6 +139,7 @@ static bool pcm512x_readable(struct device *dev, unsigned int reg)
 	case PCM512x_MASTER_MODE:
 	case PCM512x_PLL_REF:
 	case PCM512x_DAC_REF:
+	case PCM512x_GPIO_DACIN:
 	case PCM512x_GPIO_PLLIN:
 	case PCM512x_SYNCHRONIZE:
 	case PCM512x_PLL_COEFF_0:
@@ -659,6 +661,37 @@ static int pcm512x_find_pll_coeff(struct snd_soc_dai *dai,
 	return 0;
 }
 
+static unsigned long pcm512x_pllin_dac_rate(struct snd_soc_dai *dai,
+					    unsigned long osr_rate,
+					    unsigned long pllin_rate)
+{
+	struct snd_soc_codec *codec = dai->codec;
+	struct pcm512x_priv *pcm512x = snd_soc_codec_get_drvdata(codec);
+	unsigned long dac_rate;
+
+	if (!pcm512x->pll_out)
+		return 0; /* no PLL to bypass, force SCK as DAC input */
+
+	if (pllin_rate % osr_rate)
+		return 0; /* futile, quit early */
+
+	/* run DAC no faster than 6144000 Hz */
+	for (dac_rate = rounddown(6144000, osr_rate);
+	     dac_rate;
+	     dac_rate -= osr_rate) {
+
+		if (pllin_rate / dac_rate > 128)
+			return 0; /* DAC divider would be too big */
+
+		if (!(pllin_rate % dac_rate))
+			return dac_rate;
+
+		dac_rate -= osr_rate;
+	}
+
+	return 0;
+}
+
 static int pcm512x_set_dividers(struct snd_soc_dai *dai,
 				struct snd_pcm_hw_params *params)
 {
@@ -672,6 +705,7 @@ static int pcm512x_set_dividers(struct snd_soc_dai *dai,
 	unsigned long bclk_rate;
 	unsigned long sample_rate;
 	unsigned long osr_rate;
+	unsigned long dacsrc_rate;
 	int bclk_div;
 	int lrclk_div;
 	int dsp_div;
@@ -679,11 +713,10 @@ static int pcm512x_set_dividers(struct snd_soc_dai *dai,
 	unsigned long dac_rate;
 	int ncp_div;
 	int osr_div;
-	unsigned long dac_mul;
-	unsigned long sck_mul;
 	int ret;
 	int idac;
 	int fssp;
+	int gpio;
 
 	lrclk_div = snd_soc_params_to_frame_size(params);
 	if (lrclk_div == 0) {
@@ -772,9 +805,38 @@ static int pcm512x_set_dividers(struct snd_soc_dai *dai,
 	/* run DSP no faster than 50 MHz */
 	dsp_div = mck_rate > 50000000 ? 2 : 1;
 
+	dac_rate = pcm512x_pllin_dac_rate(dai, osr_rate, pllin_rate);
+	if (dac_rate) {
+		/* the desired clock rate is "compatible" with the pll input
+		 * clock, so use that clock as dac input instead of the pll
+		 * output clock since the pll will introduce jitter and thus
+		 * noise.
+		 */
+		dev_dbg(dev, "using pll input as dac input\n");
+		ret = regmap_update_bits(pcm512x->regmap, PCM512x_DAC_REF,
+					 PCM512x_SDAC, PCM512x_SDAC_GPIO);
+		if (ret != 0) {
+			dev_err(codec->dev,
+				"Failed to set gpio as dacref: %d\n", ret);
+			return ret;
+		}
+
+		gpio = PCM512x_GREF_GPIO1 + pcm512x->pll_in - 1;
+		ret = regmap_update_bits(pcm512x->regmap, PCM512x_GPIO_DACIN,
+					 PCM512x_GREF, gpio);
+		if (ret != 0) {
+			dev_err(codec->dev,
+				"Failed to set gpio %d as dacin: %d\n",
+				pcm512x->pll_in, ret);
+			return ret;
+		}
+
+		dacsrc_rate = pllin_rate;
+	} else {
 		/* run DAC no faster than 6144000 Hz */
-	dac_mul = 6144000 / osr_rate;
-	sck_mul = sck_rate / osr_rate;
+		unsigned long dac_mul = 6144000 / osr_rate;
+		unsigned long sck_mul = sck_rate / osr_rate;
+
 		for (; dac_mul; dac_mul--) {
 			if (!(sck_mul % dac_mul))
 				break;
@@ -785,18 +847,30 @@ static int pcm512x_set_dividers(struct snd_soc_dai *dai,
 		}
 
 		dac_rate = dac_mul * osr_rate;
-	dev_dbg(dev, "dac_rate %lu sample_rate %lu\n", dac_rate, sample_rate);
+		dev_dbg(dev, "dac_rate %lu sample_rate %lu\n",
+			dac_rate, sample_rate);
+
+		ret = regmap_update_bits(pcm512x->regmap, PCM512x_DAC_REF,
+					 PCM512x_SDAC, PCM512x_SDAC_SCK);
+		if (ret != 0) {
+			dev_err(codec->dev,
+				"Failed to set sck as dacref: %d\n", ret);
+			return ret;
+		}
+
+		dacsrc_rate = sck_rate;
+	}
 
-	dac_div = DIV_ROUND_CLOSEST(sck_rate, dac_rate);
+	dac_div = DIV_ROUND_CLOSEST(dacsrc_rate, dac_rate);
 	if (dac_div > 128) {
 		dev_err(dev, "Failed to find DAC divider\n");
 		return -EINVAL;
 	}
 
-	ncp_div = DIV_ROUND_CLOSEST(sck_rate / dac_div, 1536000);
-	if (ncp_div > 128 || sck_rate / dac_div / ncp_div > 2048000) {
+	ncp_div = DIV_ROUND_CLOSEST(dacsrc_rate / dac_div, 1536000);
+	if (ncp_div > 128 || dacsrc_rate / dac_div / ncp_div > 2048000) {
 		/* run NCP no faster than 2048000 Hz, but why? */
-		ncp_div = DIV_ROUND_UP(sck_rate / dac_div, 2048000);
+		ncp_div = DIV_ROUND_UP(dacsrc_rate / dac_div, 2048000);
 		if (ncp_div > 128) {
 			dev_err(dev, "Failed to find NCP divider\n");
 			return -EINVAL;
@@ -1011,13 +1085,6 @@ static int pcm512x_hw_params(struct snd_pcm_substream *substream,
 	if (ret != 0)
 		return ret;
 
-	ret = regmap_update_bits(pcm512x->regmap, PCM512x_DAC_REF,
-				 PCM512x_SDAC, PCM512x_SDAC_SCK);
-	if (ret != 0) {
-		dev_err(codec->dev, "Failed to set sck as dacref: %d\n", ret);
-		return ret;
-	}
-
 	if (pcm512x->pll_out) {
 		ret = regmap_update_bits(pcm512x->regmap, PCM512x_PLL_REF,
 					 PCM512x_SREF, PCM512x_SREF_GPIO);

sound/soc/codecs/pcm512x.h

@@ -38,6 +38,7 @@
 #define PCM512x_MASTER_MODE       (PCM512x_PAGE_BASE(0) +  12)
 #define PCM512x_PLL_REF           (PCM512x_PAGE_BASE(0) +  13)
 #define PCM512x_DAC_REF           (PCM512x_PAGE_BASE(0) +  14)
+#define PCM512x_GPIO_DACIN        (PCM512x_PAGE_BASE(0) +  16)
 #define PCM512x_GPIO_PLLIN        (PCM512x_PAGE_BASE(0) +  18)
 #define PCM512x_SYNCHRONIZE       (PCM512x_PAGE_BASE(0) +  19)
 #define PCM512x_PLL_COEFF_0       (PCM512x_PAGE_BASE(0) +  20)
@@ -162,8 +163,9 @@
 #define PCM512x_SDAC_PLL    (1 << 4)
 #define PCM512x_SDAC_SCK    (3 << 4)
 #define PCM512x_SDAC_BCK    (4 << 4)
+#define PCM512x_SDAC_GPIO   (5 << 4)
 
-/* Page 0, Register 18 - GPIO source for PLL */
+/* Page 0, Register 16, 18 - GPIO source for DAC, PLL */
 #define PCM512x_GREF        (7 << 0)
 #define PCM512x_GREF_SHIFT  0
 #define PCM512x_GREF_GPIO1  (0 << 0)