iio: dac: ti-dac7612: 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 from potential concurrent write
accesses from userspace. The write operation requires an SPI write, then
toggling of a GPIO, so the lock aims to protect the sanity of the entire
sequence of operation.

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>
Link: https://lore.kernel.org/r/20200916092535.76527-1-alexandru.ardelean@analog.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/dac/ti-dac7612.c

@@ -22,6 +22,14 @@ struct dac7612 {
 	struct gpio_desc *loaddacs;
 	uint16_t cache[2];
 
+	/*
+	 * Lock to protect the state of the device from potential concurrent
+	 * write accesses from userspace. The write operation requires an
+	 * SPI write, then toggling of a GPIO, so the lock aims to protect
+	 * the sanity of the entire sequence of operation.
+	 */
+	struct mutex lock;
+
 	/*
 	 * DMA (thus cache coherency maintenance) requires the
 	 * transfer buffers to live in their own cache lines.
@@ -101,9 +109,9 @@ static int dac7612_write_raw(struct iio_dev *iio_dev,
 	if (val == priv->cache[chan->channel])
 		return 0;
 
-	mutex_lock(&iio_dev->mlock);
+	mutex_lock(&priv->lock);
 	ret = dac7612_cmd_single(priv, chan->channel, val);
-	mutex_unlock(&iio_dev->mlock);
+	mutex_unlock(&priv->lock);
 
 	return ret;
 }
@@ -145,6 +153,8 @@ static int dac7612_probe(struct spi_device *spi)
 	iio_dev->num_channels = ARRAY_SIZE(priv->cache);
 	iio_dev->name = spi_get_device_id(spi)->name;
 
+	mutex_init(&priv->lock);
+
 	for (i = 0; i < ARRAY_SIZE(priv->cache); i++) {
 		ret = dac7612_cmd_single(priv, i, 0);
 		if (ret)