ASoC: fsl_ssi: serialize AC'97 register access operations

AC'97 register access operations (both read and write) on SSI use a one,
shared set of SSI registers for AC'97 register address and data.
This means that only one such access is possible at a time and so all these
operations need to be serialized.

Since an AC'97 register access operation in this driver takes 100us+ let's
use a mutex for this.

Use this opportunity to also change a default value returned from AC'97
register read function from -1 to 0, since that's what AC'97 specs require
to be returned when unknown / undefined registers are read.
Signed-off-by: Maciej S. Szmigiero <mail@maciej.szmigiero.name>
Signed-off-by: Mark Brown <broonie@kernel.org>

sound/soc/fsl/fsl_ssi.c

@@ -38,6 +38,7 @@
 #include <linux/ctype.h>
 #include <linux/device.h>
 #include <linux/delay.h>
+#include <linux/mutex.h>
 #include <linux/slab.h>
 #include <linux/spinlock.h>
 #include <linux/of.h>
@@ -265,6 +266,8 @@ struct fsl_ssi_private {
 
 	u32 fifo_watermark;
 	u32 dma_maxburst;
+
+	struct mutex ac97_reg_lock;
 };
 
 /*
@@ -1260,11 +1263,13 @@ static void fsl_ssi_ac97_write(struct snd_ac97 *ac97, unsigned short reg,
 	if (reg > 0x7f)
 		return;
 
+	mutex_lock(&fsl_ac97_data->ac97_reg_lock);
+
 	ret = clk_prepare_enable(fsl_ac97_data->clk);
 	if (ret) {
 		pr_err("ac97 write clk_prepare_enable failed: %d\n",
 			ret);
-		return;
+		goto ret_unlock;
 	}
 
 	lreg = reg <<  12;
@@ -1278,6 +1283,9 @@ static void fsl_ssi_ac97_write(struct snd_ac97 *ac97, unsigned short reg,
 	udelay(100);
 
 	clk_disable_unprepare(fsl_ac97_data->clk);
+
+ret_unlock:
+	mutex_unlock(&fsl_ac97_data->ac97_reg_lock);
 }
 
 static unsigned short fsl_ssi_ac97_read(struct snd_ac97 *ac97,
@@ -1285,16 +1293,18 @@ static unsigned short fsl_ssi_ac97_read(struct snd_ac97 *ac97,
 {
 	struct regmap *regs = fsl_ac97_data->regs;
 
-	unsigned short val = -1;
+	unsigned short val = 0;
 	u32 reg_val;
 	unsigned int lreg;
 	int ret;
 
+	mutex_lock(&fsl_ac97_data->ac97_reg_lock);
+
 	ret = clk_prepare_enable(fsl_ac97_data->clk);
 	if (ret) {
 		pr_err("ac97 read clk_prepare_enable failed: %d\n",
 			ret);
-		return -1;
+		goto ret_unlock;
 	}
 
 	lreg = (reg & 0x7f) <<  12;
@@ -1309,6 +1319,8 @@ static unsigned short fsl_ssi_ac97_read(struct snd_ac97 *ac97,
 
 	clk_disable_unprepare(fsl_ac97_data->clk);
 
+ret_unlock:
+	mutex_unlock(&fsl_ac97_data->ac97_reg_lock);
 	return val;
 }
 
@@ -1569,6 +1581,7 @@ static int fsl_ssi_probe(struct platform_device *pdev)
 	}
 
 	if (fsl_ssi_is_ac97(ssi_private)) {
+		mutex_init(&ssi_private->ac97_reg_lock);
 		ret = snd_soc_set_ac97_ops_of_reset(&fsl_ssi_ac97_ops, pdev);
 		if (ret) {
 			dev_err(&pdev->dev, "could not set AC'97 ops\n");
@@ -1663,6 +1676,9 @@ static int fsl_ssi_probe(struct platform_device *pdev)
 		snd_soc_set_ac97_ops(NULL);
 
 error_ac97_ops:
+	if (fsl_ssi_is_ac97(ssi_private))
+		mutex_destroy(&ssi_private->ac97_reg_lock);
+
 	if (ssi_private->soc->imx)
 		fsl_ssi_imx_clean(pdev, ssi_private);
 
@@ -1681,8 +1697,10 @@ static int fsl_ssi_remove(struct platform_device *pdev)
 	if (ssi_private->soc->imx)
 		fsl_ssi_imx_clean(pdev, ssi_private);
 
-	if (fsl_ssi_is_ac97(ssi_private))
+	if (fsl_ssi_is_ac97(ssi_private)) {
 		snd_soc_set_ac97_ops(NULL);
+		mutex_destroy(&ssi_private->ac97_reg_lock);
+	}
 
 	return 0;
 }