ALSA: ASoC: cs4271: add optional soft reset workaround

The CS4271 requires its LRCLK and MCLK to be stable before its RESET
line is de-asserted. That also means that clocks cannot be changed
without putting the chip back into hardware reset, which also requires
a complete re-initialization of all registers.

One (undocumented) workaround is to assert and de-assert the PDN bit
in the MODE2 register.

This patch adds a new flag to both the DT bindings as well as to the
platform data to enable that workaround.
Signed-off-by: Daniel Mack <zonque@gmail.com>
Acked-by: Alexander Sverdlin <subaparts@yandex.ru>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>

Documentation/devicetree/bindings/sound/cs4271.txt

@@ -20,6 +20,18 @@ Optional properties:
 		!RESET pin
  - cirrus,amuteb-eq-bmutec:	When given, the Codec's AMUTEB=BMUTEC flag
 				is enabled.
+ - cirrus,enable-soft-reset:
+	The CS4271 requires its LRCLK and MCLK to be stable before its RESET
+	line is de-asserted. That also means that clocks cannot be changed
+	without putting the chip back into hardware reset, which also requires
+	a complete re-initialization of all registers.
+
+	One (undocumented) workaround is to assert and de-assert the PDN bit
+	in the MODE2 register. This workaround can be enabled with this DT
+	property.
+
+	Note that this is not needed in case the clocks are stable
+	throughout the entire runtime of the codec.
 
 Examples:
 

include/sound/cs4271.h

@@ -20,6 +20,21 @@
 struct cs4271_platform_data {
 	int gpio_nreset;	/* GPIO driving Reset pin, if any */
 	bool amutec_eq_bmutec;	/* flag to enable AMUTEC=BMUTEC */
+
+	/*
+	 * The CS4271 requires its LRCLK and MCLK to be stable before its RESET
+	 * line is de-asserted. That also means that clocks cannot be changed
+	 * without putting the chip back into hardware reset, which also requires
+	 * a complete re-initialization of all registers.
+	 *
+	 * One (undocumented) workaround is to assert and de-assert the PDN bit
+	 * in the MODE2 register. This workaround can be enabled with the
+	 * following flag.
+	 *
+	 * Note that this is not needed in case the clocks are stable
+	 * throughout the entire runtime of the codec.
+	 */
+	bool enable_soft_reset;
 };
 
 #endif /* __CS4271_H */

sound/soc/codecs/cs4271.c

@@ -167,6 +167,8 @@ struct cs4271_private {
 	int				gpio_nreset;
 	/* GPIO that disable serial bus, if any */
 	int				gpio_disable;
+	/* enable soft reset workaround */
+	bool				enable_soft_reset;
 };
 
 /*
@@ -325,6 +327,33 @@ static int cs4271_hw_params(struct snd_pcm_substream *substream,
 	int i, ret;
 	unsigned int ratio, val;
 
+	if (cs4271->enable_soft_reset) {
+		/*
+		 * Put the codec in soft reset and back again in case it's not
+		 * currently streaming data. This way of bringing the codec in
+		 * sync to the current clocks is not explicitly documented in
+		 * the data sheet, but it seems to work fine, and in contrast
+		 * to a read hardware reset, we don't have to sync back all
+		 * registers every time.
+		 */
+
+		if ((substream->stream == SNDRV_PCM_STREAM_PLAYBACK &&
+		     !dai->capture_active) ||
+		    (substream->stream == SNDRV_PCM_STREAM_CAPTURE &&
+		     !dai->playback_active)) {
+			ret = snd_soc_update_bits(codec, CS4271_MODE2,
+						  CS4271_MODE2_PDN,
+						  CS4271_MODE2_PDN);
+			if (ret < 0)
+				return ret;
+
+			ret = snd_soc_update_bits(codec, CS4271_MODE2,
+						  CS4271_MODE2_PDN, 0);
+			if (ret < 0)
+				return ret;
+		}
+	}
+
 	cs4271->rate = params_rate(params);
 
 	/* Configure DAC */
@@ -484,6 +513,10 @@ static int cs4271_probe(struct snd_soc_codec *codec)
 		if (of_get_property(codec->dev->of_node,
 				     "cirrus,amutec-eq-bmutec", NULL))
 			amutec_eq_bmutec = true;
+
+		if (of_get_property(codec->dev->of_node,
+				     "cirrus,enable-soft-reset", NULL))
+			cs4271->enable_soft_reset = true;
 	}
 #endif
 
@@ -492,6 +525,7 @@ static int cs4271_probe(struct snd_soc_codec *codec)
 			gpio_nreset = cs4271plat->gpio_nreset;
 
 		amutec_eq_bmutec = cs4271plat->amutec_eq_bmutec;
+		cs4271->enable_soft_reset = cs4271plat->enable_soft_reset;
 	}
 
 	if (gpio_nreset >= 0)