iio: adc: ti-ads7950: allow simultaneous use of buffer and direct mode

This modifies the TI ADS7950 A/DC driver to allow the simultaneous use
of both the triggered buffer and reading channels directly (via in-
kernel API or sysfs).

The use case for this is on LEGO MINDSTORMS EV3. Two of the voltage
channels are used in-kernel by a power supply driver, which reads the
values using iio_read_channel_processed(). These channels are only read
at a slow rate (<= 1Hz).  However, we want to be able to read 12 other
channels at the same time using the triggered buffer at a high rate (>=
100Hz).
Signed-off-by: David Lechner <david@lechnology.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/adc/ti-ads7950.c

@@ -68,6 +68,9 @@ struct ti_ads7950_state {
 	__be16	rx_buf[TI_ADS7950_MAX_CHAN + TI_ADS7950_TIMESTAMP_SIZE]
 							____cacheline_aligned;
 	__be16	tx_buf[TI_ADS7950_MAX_CHAN];
+	__be16			single_tx;
+	__be16			single_rx;
+
 };
 
 struct ti_ads7950_chip_info {
@@ -287,18 +290,26 @@ static irqreturn_t ti_ads7950_trigger_handler(int irq, void *p)
 	return IRQ_HANDLED;
 }
 
-static int ti_ads7950_scan_direct(struct ti_ads7950_state *st, unsigned int ch)
+static int ti_ads7950_scan_direct(struct iio_dev *indio_dev, unsigned int ch)
 {
+	struct ti_ads7950_state *st = iio_priv(indio_dev);
 	int ret, cmd;
 
+	mutex_lock(&indio_dev->mlock);
+
 	cmd = TI_ADS7950_CR_WRITE | TI_ADS7950_CR_CHAN(ch) | st->settings;
-	st->tx_buf[0] = cpu_to_be16(cmd);
+	st->single_tx = cpu_to_be16(cmd);
 
 	ret = spi_sync(st->spi, &st->scan_single_msg);
 	if (ret)
-		return ret;
+		goto out;
+
+	ret = be16_to_cpu(st->single_rx);
 
-	return be16_to_cpu(st->rx_buf[0]);
+out:
+	mutex_unlock(&indio_dev->mlock);
+
+	return ret;
 }
 
 static int ti_ads7950_get_range(struct ti_ads7950_state *st)
@@ -330,13 +341,7 @@ static int ti_ads7950_read_raw(struct iio_dev *indio_dev,
 
 	switch (m) {
 	case IIO_CHAN_INFO_RAW:
-
-		ret = iio_device_claim_direct_mode(indio_dev);
-		if (ret < 0)
-			return ret;
-
-		ret = ti_ads7950_scan_direct(st, chan->address);
-		iio_device_release_direct_mode(indio_dev);
+		ret = ti_ads7950_scan_direct(indio_dev, chan->address);
 		if (ret < 0)
 			return ret;
 
@@ -402,13 +407,13 @@ static int ti_ads7950_probe(struct spi_device *spi)
 	 * was read at the end of the first transfer.
 	 */
 
-	st->scan_single_xfer[0].tx_buf = &st->tx_buf[0];
+	st->scan_single_xfer[0].tx_buf = &st->single_tx;
 	st->scan_single_xfer[0].len = 2;
 	st->scan_single_xfer[0].cs_change = 1;
-	st->scan_single_xfer[1].tx_buf = &st->tx_buf[0];
+	st->scan_single_xfer[1].tx_buf = &st->single_tx;
 	st->scan_single_xfer[1].len = 2;
 	st->scan_single_xfer[1].cs_change = 1;
-	st->scan_single_xfer[2].rx_buf = &st->rx_buf[0];
+	st->scan_single_xfer[2].rx_buf = &st->single_rx;
 	st->scan_single_xfer[2].len = 2;
 
 	spi_message_init_with_transfers(&st->scan_single_msg,