iio: adc: ad7793: claim direct mode when writing frequency

Driver was checking for direct mode and trying to lock it, but
left a gap where mode could change before the desired operation.
Use iio_device_claim_direct_mode() to guarantee device stays in
direct mode.

Refactor function to clarify look-up followed by lock sequence.
Signed-off-by: Alison Schofield <amsfield22@gmail.com>
Cc: Daniel Baluta <daniel.baluta@gmail.com>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

drivers/iio/adc/ad7793.c

@@ -369,13 +369,6 @@ static ssize_t ad7793_write_frequency(struct device *dev,
 	long lval;
 	int i, ret;
 
-	mutex_lock(&indio_dev->mlock);
-	if (iio_buffer_enabled(indio_dev)) {
-		mutex_unlock(&indio_dev->mlock);
-		return -EBUSY;
-	}
-	mutex_unlock(&indio_dev->mlock);
-
 	ret = kstrtol(buf, 10, &lval);
 	if (ret)
 		return ret;
@@ -383,20 +376,21 @@ static ssize_t ad7793_write_frequency(struct device *dev,
 	if (lval == 0)
 		return -EINVAL;
 
-	ret = -EINVAL;
-
 	for (i = 0; i < 16; i++)
-		if (lval == st->chip_info->sample_freq_avail[i]) {
-			mutex_lock(&indio_dev->mlock);
-			st->mode &= ~AD7793_MODE_RATE(-1);
-			st->mode |= AD7793_MODE_RATE(i);
-			ad_sd_write_reg(&st->sd, AD7793_REG_MODE,
-					 sizeof(st->mode), st->mode);
-			mutex_unlock(&indio_dev->mlock);
-			ret = 0;
-		}
+		if (lval == st->chip_info->sample_freq_avail[i])
+			break;
+	if (i == 16)
+		return -EINVAL;
 
-	return ret ? ret : len;
+	ret = iio_device_claim_direct_mode(indio_dev);
+	if (ret)
+		return ret;
+	st->mode &= ~AD7793_MODE_RATE(-1);
+	st->mode |= AD7793_MODE_RATE(i);
+	ad_sd_write_reg(&st->sd, AD7793_REG_MODE, sizeof(st->mode), st->mode);
+	iio_device_release_direct_mode(indio_dev);
+
+	return len;
 }
 
 static IIO_DEV_ATTR_SAMP_FREQ(S_IWUSR | S_IRUGO,