iio: adc: mcp320x: Speed up readout of single-channel ADCs

Single-channel converters such as mcp3001, mcp3201, mcp3301 and the
upcoming mcp3550/1/3 lack a MOSI pin, so there's no need to call
mcp320x_channel_to_tx_data() for them.

Moreover, instead of calling spi_read() for these converters, which
generates an spi_message and spi_transfer on the stack on every readout,
it's more efficient to use the spi_message and spi_transfer[] included
in struct mcp320x (as we do for multi-channel ADCs), but initialize the
spi_message only with the receive transfer.
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/adc/mcp320x.c

@@ -76,10 +76,6 @@ static int mcp320x_channel_to_tx_data(int device_index,
 	int start_bit = 1;
 
 	switch (device_index) {
-	case mcp3001:
-	case mcp3201:
-	case mcp3301:
-		return 0;
 	case mcp3002:
 	case mcp3202:
 		return ((start_bit << 4) | (!differential << 3) |
@@ -100,20 +96,14 @@ static int mcp320x_adc_conversion(struct mcp320x *adc, u8 channel,
 {
 	int ret;
 
-	adc->rx_buf[0] = 0;
-	adc->rx_buf[1] = 0;
-	adc->tx_buf = mcp320x_channel_to_tx_data(device_index,
-						channel, differential);
+	memset(&adc->rx_buf, 0, sizeof(adc->rx_buf));
+	if (adc->chip_info->num_channels > 1)
+		adc->tx_buf = mcp320x_channel_to_tx_data(device_index, channel,
+							 differential);
 
-	if (device_index != mcp3001 && device_index != mcp3201 && device_index != mcp3301) {
 	ret = spi_sync(adc->spi, &adc->msg);
 	if (ret < 0)
 		return ret;
-	} else {
-		ret = spi_read(adc->spi, &adc->rx_buf, sizeof(adc->rx_buf));
-		if (ret < 0)
-			return ret;
-	}
 
 	switch (device_index) {
 	case mcp3001:
@@ -322,7 +312,11 @@ static int mcp320x_probe(struct spi_device *spi)
 	adc->transfer[0].len = sizeof(adc->tx_buf);
 	adc->transfer[1].rx_buf = adc->rx_buf;
 	adc->transfer[1].len = sizeof(adc->rx_buf);
-
+	if (chip_info->num_channels == 1)
+		/* single-channel converters are rx only (no MOSI pin) */
+		spi_message_init_with_transfers(&adc->msg,
+						&adc->transfer[1], 1);
+	else
 		spi_message_init_with_transfers(&adc->msg, adc->transfer,
 						ARRAY_SIZE(adc->transfer));