[PATCH] ASoC: bcm2835: Add enable/disable clock functions

[Allen Martin]

Add functions to control enable/disable of BCLK output of bcm2835 I2S
controller so that BCLK output only starts when dma starts.  This
resolves issues of audio pop with DACs such as max98357 on rpi.  The
LRCLK output of bcm2835 only starts when the frame size has been
configured and there is data in the FIFO.  The max98357 dac makes a
loud popping sound when BCLK is toggling but LRCLK is not.

Signed-off-by: Allen Martin <armartin at gmail.com>
---
 sound/soc/bcm/bcm2835-i2s.c | 35 +++++++++++++++++++++++++++++++++++
 1 file changed, 35 insertions(+)

diff --git a/sound/soc/bcm/bcm2835-i2s.c b/sound/soc/bcm/bcm2835-i2s.c
index e6a12e271b07..5c8649864c0d 100644
--- a/sound/soc/bcm/bcm2835-i2s.c
+++ b/sound/soc/bcm/bcm2835-i2s.c
@@ -122,9 +122,27 @@ struct bcm2835_i2s_dev {
 	struct regmap				*i2s_regmap;
 	struct clk				*clk;
 	bool					clk_prepared;
+	bool					clk_enabled;
 	int					clk_rate;
 };
 
+static void bcm2835_i2s_enable_clock(struct bcm2835_i2s_dev *dev)
+{
+	if (dev->clk_enabled)
+		return;
+
+	regmap_update_bits(dev->i2s_regmap, BCM2835_I2S_MODE_A_REG, BCM2835_I2S_CLKDIS, 0);
+	dev->clk_enabled = true;
+}
+
+static void bcm2835_i2s_disable_clock(struct bcm2835_i2s_dev *dev)
+{
+	if (dev->clk_enabled)
+		regmap_update_bits(dev->i2s_regmap, BCM2835_I2S_MODE_A_REG, BCM2835_I2S_CLKDIS, BCM2835_I2S_CLKDIS);
+
+	dev->clk_enabled = false;
+}
+
 static void bcm2835_i2s_start_clock(struct bcm2835_i2s_dev *dev)
 {
 	unsigned int master = dev->fmt & SND_SOC_DAIFMT_MASTER_MASK;
@@ -145,6 +163,7 @@ static void bcm2835_i2s_start_clock(struct bcm2835_i2s_dev *dev)
 
 static void bcm2835_i2s_stop_clock(struct bcm2835_i2s_dev *dev)
 {
+	bcm2835_i2s_disable_clock(dev);
 	if (dev->clk_prepared)
 		clk_disable_unprepare(dev->clk);
 	dev->clk_prepared = false;
@@ -158,6 +177,7 @@ static void bcm2835_i2s_clear_fifos(struct bcm2835_i2s_dev *dev,
 	uint32_t csreg;
 	uint32_t i2s_active_state;
 	bool clk_was_prepared;
+	bool clk_was_enabled;
 	uint32_t off;
 	uint32_t clr;
 
@@ -176,6 +196,11 @@ static void bcm2835_i2s_clear_fifos(struct bcm2835_i2s_dev *dev,
 	if (!clk_was_prepared)
 		bcm2835_i2s_start_clock(dev);
 
+	/* Enable clock if not enabled */
+	clk_was_enabled = dev->clk_enabled;
+	if (!clk_was_enabled)
+		bcm2835_i2s_enable_clock(dev);
+
 	/* Stop I2S module */
 	regmap_update_bits(dev->i2s_regmap, BCM2835_I2S_CS_A_REG, off, 0);
 
@@ -207,6 +232,10 @@ static void bcm2835_i2s_clear_fifos(struct bcm2835_i2s_dev *dev,
 	if (!timeout)
 		dev_err(dev->dev, "I2S SYNC error!\n");
 
+	/* Disable clock if it was not enabled before */
+	if (!clk_was_enabled)
+		bcm2835_i2s_disable_clock(dev);
+
 	/* Stop clock if it was not running before */
 	if (!clk_was_prepared)
 		bcm2835_i2s_stop_clock(dev);
@@ -414,6 +443,8 @@ static int bcm2835_i2s_hw_params(struct snd_pcm_substream *substream,
 	/* Clock should only be set up here if CPU is clock master */
 	if (bit_clock_master &&
 	    (!dev->clk_prepared || dev->clk_rate != bclk_rate)) {
+		if (dev->clk_enabled)
+			bcm2835_i2s_disable_clock(dev);
 		if (dev->clk_prepared)
 			bcm2835_i2s_stop_clock(dev);
 
@@ -534,6 +565,8 @@ static int bcm2835_i2s_hw_params(struct snd_pcm_substream *substream,
 		mode |= BCM2835_I2S_FTXP | BCM2835_I2S_FRXP;
 	}
 
+	if (!dev->clk_enabled)
+		mode |= BCM2835_I2S_CLKDIS;
 	mode |= BCM2835_I2S_FLEN(frame_length - 1);
 	mode |= BCM2835_I2S_FSLEN(framesync_length);
 
@@ -668,6 +701,7 @@ static int bcm2835_i2s_trigger(struct snd_pcm_substream *substream, int cmd,
 	case SNDRV_PCM_TRIGGER_RESUME:
 	case SNDRV_PCM_TRIGGER_PAUSE_RELEASE:
 		bcm2835_i2s_start_clock(dev);
+		bcm2835_i2s_enable_clock(dev);
 
 		if (substream->stream == SNDRV_PCM_STREAM_CAPTURE)
 			mask = BCM2835_I2S_RXON;
@@ -839,6 +873,7 @@ static int bcm2835_i2s_probe(struct platform_device *pdev)
 
 	/* get the clock */
 	dev->clk_prepared = false;
+	dev->clk_enabled = false;
 	dev->clk = devm_clk_get(&pdev->dev, NULL);
 	if (IS_ERR(dev->clk)) {
 		dev_err(&pdev->dev, "could not get clk: %ld\n",
-- 
2.20.1