[PATCH 2/2] ASoC: fsl_ssi: serialize AC'97 register access operations

[Maciej S. Szmigiero]

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 <m...@maciej.szmigiero.name>
---
 sound/soc/fsl/fsl_ssi.c | 26 ++++++++++++++++++++++----
 1 file changed, 22 insertions(+), 4 deletions(-)

diff --git a/sound/soc/fsl/fsl_ssi.c b/sound/soc/fsl/fsl_ssi.c
index a71bb8391f61..9dea1b16de82 100644
--- a/sound/soc/fsl/fsl_ssi.c
+++ b/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;
 };
 
 /*
@@ -1262,11 +1265,13 @@ 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;
@@ -1280,6 +1285,9 @@ 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,
@@ -1287,16 +1295,18 @@ 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;
@@ -1311,6 +1321,8 @@ 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;
 }
 
@@ -1571,6 +1583,7 @@ 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");
@@ -1665,6 +1678,9 @@ 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);
 
@@ -1683,8 +1699,10 @@ 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;
 }