Based on the sensor datasheet in chapter 7.6 SPI timing, Table 20,
during the SPI transfer there is a minimum time interval requirement
between the CS being asserted and the first clock edge (tHDSS).
This minimum interval of 2.5us is being violated if two consecutive SPI
transfers are queued up.

Fixes: a0858f0cd28e ("iio: pressure: mprls0025pa add SPI driver")
Datasheet: https://prod-edam.honeywell.com/content/dam/honeywell-edam/sps/siot/en-us/products/sensors/pressure-sensors/board-mount-pressure-sensors/micropressure-mpr-series/documents/sps-siot-mpr-series-datasheet-32332628-ciid-172626.pdf?download=false
Signed-off-by: Petre Rodan <petre.rodan@subdimension.ro>
---
v1 -> v2 use xfer.delay.value as per Marcelo Schmitt's review
---
 drivers/iio/pressure/mprls0025pa_spi.c | 19 ++++++++++++++-----
 1 file changed, 14 insertions(+), 5 deletions(-)

diff --git a/drivers/iio/pressure/mprls0025pa_spi.c b/drivers/iio/pressure/mprls0025pa_spi.c
index e6bb75de3411..cf17eb2e7208 100644
--- a/drivers/iio/pressure/mprls0025pa_spi.c
+++ b/drivers/iio/pressure/mprls0025pa_spi.c
@@ -8,6 +8,7 @@
  *  https://prod-edam.honeywell.com/content/dam/honeywell-edam/sps/siot/en-us/products/sensors/pressure-sensors/board-mount-pressure-sensors/micropressure-mpr-series/documents/sps-siot-mpr-series-datasheet-32332628-ciid-172626.pdf
  */
 
+#include <linux/array_size.h>
 #include <linux/device.h>
 #include <linux/errno.h>
 #include <linux/mod_devicetable.h>
@@ -40,17 +41,25 @@ static int mpr_spi_xfer(struct mpr_data *data, const u8 cmd, const u8 pkt_len)
 {
 	struct spi_device *spi = to_spi_device(data->dev);
 	struct mpr_spi_buf *buf = spi_get_drvdata(spi);
-	struct spi_transfer xfer = { };
+	struct spi_transfer xfers[2] = { };
 
 	if (pkt_len > MPR_MEASUREMENT_RD_SIZE)
 		return -EOVERFLOW;
 
 	buf->tx[0] = cmd;
-	xfer.tx_buf = buf->tx;
-	xfer.rx_buf = data->buffer;
-	xfer.len = pkt_len;
 
-	return spi_sync_transfer(spi, &xfer, 1);
+	/*
+	 * Dummy transfer with no data, just cause a 2.5us+ delay between the CS assert
+	 * and the first clock edge as per the datasheet tHDSS timing requirement.
+	 */
+	xfers[0].delay.value = 2500;
+	xfers[0].delay.unit = SPI_DELAY_UNIT_NSECS;
+
+	xfers[1].tx_buf = buf->tx;
+	xfers[1].rx_buf = data->buffer;
+	xfers[1].len = pkt_len;
+
+	return spi_sync_transfer(spi, xfers, ARRAY_SIZE(xfers));
 }
 
 static const struct mpr_ops mpr_spi_ops = {

-- 
2.52.0