iio: adc: ad799x: do not use internal iio_dev lock

'mlock' was being grabbed when setting the device frequency. In order to
not introduce any functional change a new lock is added. With that in
mind, the lock also needs to be grabbed in the places where 'mlock' is
since it was also being used to protect st->config against the current
device state.

On the other places the lock was being used, we can just drop
it since we are only doing one i2c bus read/write which is already
safe.

While at it, properly include "mutex.h" for mutex related APIs.

Signed-off-by: Nuno Sá <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jonathan Cameron <[e-mail]>

drivers/iio/adc/ad799x.c

···
 #include <linux/types.h>
 #include <linux/err.h>
 #include <linux/module.h>
+#include <linux/mutex.h>
 #include <linux/bitops.h>
 
 #include <linux/iio/iio.h>
···
 	const struct ad799x_chip_config	*chip_config;
 	struct regulator		*reg;
 	struct regulator		*vref;
+	/* lock to protect against multiple access to the device */
+	struct mutex			lock;
 	unsigned			id;
 	u16				config;
 
···
 		ret = iio_device_claim_direct_mode(indio_dev);
 		if (ret)
 			return ret;
+		mutex_lock(&st->lock);
 		ret = ad799x_scan_direct(st, chan->scan_index);
+		mutex_unlock(&st->lock);
 		iio_device_release_direct_mode(indio_dev);
 
 		if (ret < 0)
···
 	if (ret)
 		return ret;
 
-	mutex_lock(&indio_dev->mlock);
+	mutex_lock(&st->lock);
+
 	ret = i2c_smbus_read_byte_data(st->client, AD7998_CYCLE_TMR_REG);
 	if (ret < 0)
 		goto error_ret_mutex;
···
 	ret = len;
 
 error_ret_mutex:
-	mutex_unlock(&indio_dev->mlock);
+	mutex_unlock(&st->lock);
 
 	return ret;
 }
···
 	if (ret)
 		return ret;
 
+	mutex_lock(&st->lock);
+
 	if (state)
 		st->config |= BIT(chan->scan_index) << AD799X_CHANNEL_SHIFT;
 	else
···
 		st->config &= ~AD7998_ALERT_EN;
 
 	ret = ad799x_write_config(st, st->config);
+	mutex_unlock(&st->lock);
 	iio_device_release_direct_mode(indio_dev);
 	return ret;
 }
···
 	if (val < 0 || val > GENMASK(chan->scan_type.realbits - 1, 0))
 		return -EINVAL;
 
-	mutex_lock(&indio_dev->mlock);
 	ret = i2c_smbus_write_word_swapped(st->client,
 		ad799x_threshold_reg(chan, dir, info),
 		val << chan->scan_type.shift);
-	mutex_unlock(&indio_dev->mlock);
 
 	return ret;
 }
···
 	int ret;
 	struct ad799x_state *st = iio_priv(indio_dev);
 
-	mutex_lock(&indio_dev->mlock);
 	ret = i2c_smbus_read_word_swapped(st->client,
 		ad799x_threshold_reg(chan, dir, info));
-	mutex_unlock(&indio_dev->mlock);
 	if (ret < 0)
 		return ret;
 	*val = (ret >> chan->scan_type.shift) &
···
 		if (ret)
 			goto error_cleanup_ring;
 	}
+
+	mutex_init(&st->lock);
+
 	ret = iio_device_register(indio_dev);
 	if (ret)
 		goto error_cleanup_ring;