ASoC: adau17x1: Workaround for noise bug in ADC

The ADC in the ADAU1361 (and possibly other Analog Devices codecs)
exhibits a cyclic variation in the noise floor (in our test setup between
-87 and -93 dB), a new value being attained within this range whenever a
new capture stream is started. The cycle repeats after about 10 or 11
restarts.

The workaround recommended by the manufacturer is to toggle the ADOSR bit
in the Converter Control 0 register each time a new capture stream is
started.

I have verified that the patch fixes this problem on the ADAU1361, and
according to the manufacturer toggling the bit in question in this manner
will at least have no detrimental effect on other chips served by this
driver.

Signed-off-by: Ricard Wanderlof <ricardw@axis.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
Cc: stable@vger.kernel.org

sound/soc/codecs/adau17x1.c

@@ -90,6 +90,27 @@ static int adau17x1_pll_event(struct snd_soc_dapm_widget *w,
 	return 0;
 }
 
+static int adau17x1_adc_fixup(struct snd_soc_dapm_widget *w,
+	struct snd_kcontrol *kcontrol, int event)
+{
+	struct snd_soc_codec *codec = snd_soc_dapm_to_codec(w->dapm);
+	struct adau *adau = snd_soc_codec_get_drvdata(codec);
+
+	/*
+	 * If we are capturing, toggle the ADOSR bit in Converter Control 0 to
+	 * avoid losing SNR (workaround from ADI). This must be done after
+	 * the ADC(s) have been enabled. According to the data sheet, it is
+	 * normally illegal to set this bit when the sampling rate is 96 kHz,
+	 * but according to ADI it is acceptable for this workaround.
+	 */
+	regmap_update_bits(adau->regmap, ADAU17X1_CONVERTER0,
+		ADAU17X1_CONVERTER0_ADOSR, ADAU17X1_CONVERTER0_ADOSR);
+	regmap_update_bits(adau->regmap, ADAU17X1_CONVERTER0,
+		ADAU17X1_CONVERTER0_ADOSR, 0);
+
+	return 0;
+}
+
 static const char * const adau17x1_mono_stereo_text[] = {
 	"Stereo",
 	"Mono Left Channel (L+R)",
@@ -121,7 +142,8 @@ static const struct snd_soc_dapm_widget adau17x1_dapm_widgets[] = {
 	SND_SOC_DAPM_MUX("Right DAC Mode Mux", SND_SOC_NOPM, 0, 0,
 		&adau17x1_dac_mode_mux),
 
-	SND_SOC_DAPM_ADC("Left Decimator", NULL, ADAU17X1_ADC_CONTROL, 0, 0),
+	SND_SOC_DAPM_ADC_E("Left Decimator", NULL, ADAU17X1_ADC_CONTROL, 0, 0,
+			   adau17x1_adc_fixup, SND_SOC_DAPM_POST_PMU),
 	SND_SOC_DAPM_ADC("Right Decimator", NULL, ADAU17X1_ADC_CONTROL, 1, 0),
 	SND_SOC_DAPM_DAC("Left DAC", NULL, ADAU17X1_DAC_CONTROL0, 0, 0),
 	SND_SOC_DAPM_DAC("Right DAC", NULL, ADAU17X1_DAC_CONTROL0, 1, 0),

sound/soc/codecs/adau17x1.h

@@ -129,5 +129,7 @@ bool adau17x1_has_dsp(struct adau *adau);
 
 #define ADAU17X1_CONVERTER0_CONVSR_MASK		0x7
 
+#define ADAU17X1_CONVERTER0_ADOSR		BIT(3)
+
 
 #endif