[RFC PATCH v1] ASoC: wm8904: enable fll with fixed mclk

[Andrejs Cainikovs <andrejs.cainikovs@xxxxxxxxx>]

From: Andrejs Cainikovs <andrejs.cainikovs@xxxxxxxxxxx>

Dear all,

This is an attempt to change wm8904 for a scenario when reference clock
supposed to be fixed to a particular frequency, but is not configured as
a fixed-clock. While this change is working fine, I'm struggling to
finalize it, not being able to find a proper solution of adding a check
whether we want to use fixed MCLK with codec's FLL or not. I could, of
course, introduce a new device tree property, but do not feel this would
be a proper way forward. Hence, I'm sending out this RFC patch to gather
your valuable feedback.

DUT: Dahlia carrier board with Verdin TI AM62 SOM.
Audio card configuration can be found in ti/k3-am62-verdin-dahlia.dtsi.

Thanks,
Andrejs.

On systems with a fixed reference clock output rate it
is impossible to use this clock for all audio frequencies.

Following is an example of playing a 44100Hz audio on a system
with a fixed 25MHz reference clock applied to wm8904 codec,
in combination with simple-audio-card without mclk-fs:

[   27.013564] wm8904 1-001a: Target BCLK is 1411200Hz
[   27.013601] wm8904 1-001a: Using 25000000Hz MCLK
[   27.013611] wm8904 1-001a: CLK_SYS is 12500000Hz
[   27.013654] wm8904 1-001a: Selected CLK_SYS_RATIO of 256
[   27.013663] wm8904 1-001a: Selected SAMPLE_RATE of 44100Hz
[   27.013671] wm8904 1-001a: Selected BCLK_DIV of 80 for 1562500Hz BCLK
[   27.013680] wm8904 1-001a: LRCLK_RATE is 35

This leads to a distorted sound and this configuration is unusable.

On the other hand, configuring simple-audio-card with mclk-fs will
force the system to change MCLK frequency, which supposed to be fixed.

This change forces to use wm8904 FLL while keeping SoC's MCLK
frequency intact:

[  234.108149] wm8904 1-001a: Target BCLK is 1411200Hz
[  234.108304] wm8904 1-001a: Using 0Hz FLL clock
[  234.108722] wm8904 1-001a: FLL configured for 25000000Hz->1411200Hz
[  234.108794] wm8904 1-001a: CLK_SYS is 1411200Hz
[  234.108835] wm8904 1-001a: Selected CLK_SYS_RATIO of 64
[  234.108875] wm8904 1-001a: Selected SAMPLE_RATE of 44100Hz
[  234.108913] wm8904 1-001a: Selected BCLK_DIV of 10 for 1411200Hz BCLK
[  234.108955] wm8904 1-001a: LRCLK_RATE is 32

Signed-off-by: Andrejs Cainikovs <andrejs.cainikovs@xxxxxxxxxxx>
---
 sound/soc/codecs/wm8904.c | 38 +++++++++++++++++++++++---------------
 1 file changed, 23 insertions(+), 15 deletions(-)

diff --git a/sound/soc/codecs/wm8904.c b/sound/soc/codecs/wm8904.c
index 829bf055622a7..033ef1d67ac50 100644
--- a/sound/soc/codecs/wm8904.c
+++ b/sound/soc/codecs/wm8904.c
@@ -311,10 +311,17 @@ static bool wm8904_readable_register(struct device *dev, unsigned int reg)
 	}
 }
 
-static int wm8904_configure_clocking(struct snd_soc_component *component)
+/* Forward declaration */
+static int wm8904_set_fll(struct snd_soc_dai *dai, int fll_id, int source,
+			  unsigned int Fref, unsigned int Fout);
+
+static int wm8904_configure_clocking(struct snd_soc_dai *dai)
 {
+	struct snd_soc_component *component = dai->component;
 	struct wm8904_priv *wm8904 = snd_soc_component_get_drvdata(component);
 	unsigned int clock0, clock2, rate;
+	unsigned long mclk_freq;
+	int ret;
 
 	/* Gate the clock while we're updating to avoid misclocking */
 	clock2 = snd_soc_component_read(component, WM8904_CLOCK_RATES_2);
@@ -338,6 +345,14 @@ static int wm8904_configure_clocking(struct snd_soc_component *component)
 		dev_dbg(component->dev, "Using %dHz FLL clock\n",
 			wm8904->fll_fout);
 
+		mclk_freq = clk_get_rate(wm8904->mclk);
+		ret = wm8904_set_fll(dai, WM8904_FLL_MCLK, WM8904_FLL_MCLK,
+				     mclk_freq, wm8904->bclk);
+		if (ret) {
+			dev_err(component->dev, "Could not configure FLL\n");
+			return ret;
+		}
+
 		clock2 |= WM8904_SYSCLK_SRC;
 		rate = wm8904->fll_fout;
 		break;
@@ -1334,7 +1349,7 @@ static int wm8904_hw_params(struct snd_pcm_substream *substream,
 
 	dev_dbg(component->dev, "Target BCLK is %dHz\n", wm8904->bclk);
 
-	ret = wm8904_configure_clocking(component);
+	ret = wm8904_configure_clocking(dai);
 	if (ret != 0)
 		return ret;
 
@@ -1809,7 +1824,6 @@ static int wm8904_set_sysclk(struct snd_soc_dai *dai, int clk_id,
 	struct snd_soc_component *component = dai->component;
 	struct wm8904_priv *priv = snd_soc_component_get_drvdata(component);
 	unsigned long mclk_freq;
-	int ret;
 
 	switch (clk_id) {
 	case WM8904_CLK_AUTO:
@@ -1820,17 +1834,11 @@ static int wm8904_set_sysclk(struct snd_soc_dai *dai, int clk_id,
 			return 0;
 
 		mclk_freq = clk_get_rate(priv->mclk);
-		/* enable FLL if a different sysclk is desired */
-		if (mclk_freq != freq) {
-			priv->sysclk_src = WM8904_CLK_FLL;
-			ret = wm8904_set_fll(dai, WM8904_FLL_MCLK,
-					     WM8904_FLL_MCLK,
-					     mclk_freq, freq);
-			if (ret)
-				return ret;
-			break;
-		}
-		clk_id = WM8904_CLK_MCLK;
+		/* TODO: add a check to force fll with fixed mclk */
+		if (mclk_freq != freq || 1)
+			clk_id = WM8904_CLK_FLL;
+		else
+			clk_id = WM8904_CLK_MCLK;
 		fallthrough;
 
 	case WM8904_CLK_MCLK:
@@ -1848,7 +1856,7 @@ static int wm8904_set_sysclk(struct snd_soc_dai *dai, int clk_id,
 
 	dev_dbg(dai->dev, "Clock source is %d at %uHz\n", clk_id, freq);
 
-	wm8904_configure_clocking(component);
+	wm8904_configure_clocking(dai);
 
 	return 0;
 }
-- 
2.34.1