iio: stm32-dac: Replace indio_dev->mlock with own device lock

As part of the general cleanup of indio_dev->mlock, this change replaces
it with a local lock. The lock protects against potential races when
reading the CR reg and then updating, so that the state of pm_runtime
is consistent between the two operations.

This is part of a bigger cleanup.
Link: https://lore.kernel.org/linux-iio/CA+U=Dsoo6YABe5ODLp+eFNPGFDjk5ZeQEceGkqjxXcVEhLWubw@mail.gmail.com/Signed-off-by: Sergiu Cuciurean <sergiu.cuciurean@analog.com>
Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Reviewed-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Link: https://lore.kernel.org/r/20200916092349.75647-1-alexandru.ardelean@analog.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/dac/stm32-dac.c

@@ -26,9 +26,12 @@
 /**
  * struct stm32_dac - private data of DAC driver
  * @common:		reference to DAC common data
+ * @lock:		lock to protect against potential races when reading
+ *			and update CR, to keep it in sync with pm_runtime
  */
 struct stm32_dac {
 	struct stm32_dac_common *common;
+	struct mutex		lock;
 };
 
 static int stm32_dac_is_enabled(struct iio_dev *indio_dev, int channel)
@@ -58,10 +61,10 @@ static int stm32_dac_set_enable_state(struct iio_dev *indio_dev, int ch,
 	int ret;
 
 	/* already enabled / disabled ? */
-	mutex_lock(&indio_dev->mlock);
+	mutex_lock(&dac->lock);
 	ret = stm32_dac_is_enabled(indio_dev, ch);
 	if (ret < 0 || enable == !!ret) {
-		mutex_unlock(&indio_dev->mlock);
+		mutex_unlock(&dac->lock);
 		return ret < 0 ? ret : 0;
 	}
 
@@ -69,13 +72,13 @@ static int stm32_dac_set_enable_state(struct iio_dev *indio_dev, int ch,
 		ret = pm_runtime_get_sync(dev);
 		if (ret < 0) {
 			pm_runtime_put_noidle(dev);
-			mutex_unlock(&indio_dev->mlock);
+			mutex_unlock(&dac->lock);
 			return ret;
 		}
 	}
 
 	ret = regmap_update_bits(dac->common->regmap, STM32_DAC_CR, msk, en);
-	mutex_unlock(&indio_dev->mlock);
+	mutex_unlock(&dac->lock);
 	if (ret < 0) {
 		dev_err(&indio_dev->dev, "%s failed\n", en ?
 			"Enable" : "Disable");
@@ -327,6 +330,8 @@ static int stm32_dac_probe(struct platform_device *pdev)
 	indio_dev->info = &stm32_dac_iio_info;
 	indio_dev->modes = INDIO_DIRECT_MODE;
 
+	mutex_init(&dac->lock);
+
 	ret = stm32_dac_chan_of_init(indio_dev);
 	if (ret < 0)
 		return ret;