[PATCH] iio: adc: ti-ads7950: inconsistency with spi msg

From: justinpopo6
Date: Thu Jan 24 2019 - 21:05:37 EST

From: Justin Chen <justinpopo6@xxxxxxxxx>

To read a channel we require 3 cycles to send, process, and receive
the data. The transfer buffer for the third transaction is left blank.
This leaves it up to the SPI driver to decide what to do.

In one particular case, if the tx buffer is not set the spi driver
sets it to 0xff. This puts the ADC in a alarm programming state,
therefore the following read to a channel becomes erroneous.

Instead of leaving us to the mercy of the SPI driver, we send the
ADC cmd on the third transaction to prevent inconsistent behavior.

Signed-off-by: Justin Chen <justinpopo6@xxxxxxxxx>
drivers/iio/adc/ti-ads7950.c | 1 +
1 file changed, 1 insertion(+)

diff --git a/drivers/iio/adc/ti-ads7950.c b/drivers/iio/adc/ti-ads7950.c
index 0ad6359..5453e10 100644
--- a/drivers/iio/adc/ti-ads7950.c
+++ b/drivers/iio/adc/ti-ads7950.c
@@ -422,6 +422,7 @@ static int ti_ads7950_probe(struct spi_device *spi)
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->single_tx;
st->scan_single_xfer[2].rx_buf = &st->single_rx;
st->scan_single_xfer[2].len = 2;