ASoC: tlv320adcx140: Add decimation filter support

Add decimation filter selection support.
Per Section 8.3.6.7 the Digital Decimation Filter is selectable between
a Linear Phase, Low Latency, and Ultra Low Latency filer.
Signed-off-by: Dan Murphy <dmurphy@ti.com>
Link: https://lore.kernel.org/r/20200221181358.22526-2-dmurphy@ti.comSigned-off-by: Mark Brown <broonie@kernel.org>

sound/soc/codecs/tlv320adcx140.c

@@ -169,6 +169,17 @@ static DECLARE_TLV_DB_SCALE(agc_thresh_tlv, -3600, 200, 0);
 /* AGC Max Gain. From 3 dB to 42 dB in 3 dB steps */
 static DECLARE_TLV_DB_SCALE(agc_gain_tlv, 300, 300, 0);
 
+static const char * const decimation_filter_text[] = {
+	"Linear Phase", "Low Latency", "Ultra-low Latency"
+};
+
+static SOC_ENUM_SINGLE_DECL(decimation_filter_enum, ADCX140_DSP_CFG0, 4,
+			    decimation_filter_text);
+
+static const struct snd_kcontrol_new decimation_filter_controls[] = {
+	SOC_DAPM_ENUM("Decimation Filter", decimation_filter_enum),
+};
+
 static const char * const resistor_text[] = {
 	"2.5 kOhm", "10 kOhm", "20 kOhm"
 };
@@ -404,6 +415,9 @@ static const struct snd_soc_dapm_widget adcx140_dapm_widgets[] = {
 			in3_resistor_controls),
 	SND_SOC_DAPM_MUX("IN4 Analog Mic Resistor", SND_SOC_NOPM, 0, 0,
 			in4_resistor_controls),
+
+	SND_SOC_DAPM_MUX("Decimation Filter", SND_SOC_NOPM, 0, 0,
+			decimation_filter_controls),
 };
 
 static const struct snd_soc_dapm_route adcx140_audio_map[] = {
@@ -418,6 +432,10 @@ static const struct snd_soc_dapm_route adcx140_audio_map[] = {
 	{"CH3_ASI_EN", "Switch", "CH3_ADC"},
 	{"CH4_ASI_EN", "Switch", "CH4_ADC"},
 
+	{"Decimation Filter", "Linear Phase", "DRE_ENABLE"},
+	{"Decimation Filter", "Low Latency", "DRE_ENABLE"},
+	{"Decimation Filter", "Ultra-low Latency", "DRE_ENABLE"},
+
 	{"DRE_ENABLE", "Switch", "CH1_DRE_EN"},
 	{"DRE_ENABLE", "Switch", "CH2_DRE_EN"},
 	{"DRE_ENABLE", "Switch", "CH3_DRE_EN"},