iio: adc: palmas_gpadc: 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 on the device's state structure.

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: Mircea Caprioru <mircea.caprioru@analog.com>
Link: https://lore.kernel.org/r/20200928131333.36646-2-mircea.caprioru@analog.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/adc/palmas_gpadc.c

@@ -90,6 +90,12 @@ static struct palmas_gpadc_info palmas_gpadc_info[] = {
  *			3: 800 uA
  * @extended_delay:	enable the gpadc extended delay mode
  * @auto_conversion_period:	define the auto_conversion_period
+ * @lock:	Lock to protect the device state during a potential concurrent
+ *		read access from userspace. Reading a raw value requires a sequence
+ *		of register writes, then a wait for a completion callback,
+ *		and finally a register read, during which userspace could issue
+ *		another read request. This lock protects a read access from
+ *		ocurring before another one has finished.
  *
  * This is the palmas_gpadc structure to store run-time information
  * and pointers for this driver instance.
@@ -110,6 +116,7 @@ struct palmas_gpadc {
 	bool				wakeup1_enable;
 	bool				wakeup2_enable;
 	int				auto_conversion_period;
+	struct mutex			lock;
 };
 
 /*
@@ -388,7 +395,7 @@ static int palmas_gpadc_read_raw(struct iio_dev *indio_dev,
 	if (adc_chan > PALMAS_ADC_CH_MAX)
 		return -EINVAL;
 
-	mutex_lock(&indio_dev->mlock);
+	mutex_lock(&adc->lock);
 
 	switch (mask) {
 	case IIO_CHAN_INFO_RAW:
@@ -414,12 +421,12 @@ static int palmas_gpadc_read_raw(struct iio_dev *indio_dev,
 		goto out;
 	}
 
-	mutex_unlock(&indio_dev->mlock);
+	mutex_unlock(&adc->lock);
 	return ret;
 
 out:
 	palmas_gpadc_read_done(adc, adc_chan);
-	mutex_unlock(&indio_dev->mlock);
+	mutex_unlock(&adc->lock);
 
 	return ret;
 }
@@ -516,6 +523,9 @@ static int palmas_gpadc_probe(struct platform_device *pdev)
 	adc->dev = &pdev->dev;
 	adc->palmas = dev_get_drvdata(pdev->dev.parent);
 	adc->adc_info = palmas_gpadc_info;
+
+	mutex_init(&adc->lock);
+
 	init_completion(&adc->conv_completion);
 	platform_set_drvdata(pdev, indio_dev);