iio: adc: xilinx-xadc: Fix sequencer mode in postdisable for dual mux

commit 852534744c2d35626a604f128ff0b8ec12805591 upstream.

xadc_postdisable() unconditionally sets the sequencer to continuous
mode. For dual external multiplexer configurations this is incorrect:
simultaneous sampling mode is required so that ADC-A samples through
the mux on VAUX[0-7] while ADC-B simultaneously samples through the
mux on VAUX[8-15]. In continuous mode only ADC-A is active, so
VAUX[8-15] channels return incorrect data.

Since postdisable is also called from xadc_probe() to set the initial
idle state, the wrong sequencer mode is active from the moment the
driver loads.

The preenable path already uses xadc_get_seq_mode() which returns
SIMULTANEOUS for dual mux. Fix postdisable to do the same.

Fixes: bdc8cda1d0 ("iio:adc: Add Xilinx XADC driver")
Cc: stable@vger.kernel.org
Signed-off-by: Christofer Jonason <christofer.jonason@guidelinegeo.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Reviewed-by: Nuno Sá <nuno.sa@analog.com>
Reviewed-by: Salih Erim <salih.erim@amd.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Ulrich Hecht <uli@kernel.org>

drivers/iio/adc/xilinx-xadc-core.c

@@ -766,6 +766,7 @@ static int xadc_postdisable(struct iio_dev *indio_dev)
 {
 	struct xadc *xadc = iio_priv(indio_dev);
 	unsigned long scan_mask;
+	int seq_mode;
 	int ret;
 	int i;
 
@@ -773,6 +774,12 @@ static int xadc_postdisable(struct iio_dev *indio_dev)
 	for (i = 0; i < indio_dev->num_channels; i++)
 		scan_mask |= BIT(indio_dev->channels[i].scan_index);
 
+	/*
+	 * Use the correct sequencer mode for the idle state: simultaneous
+	 * mode for dual external mux configurations, continuous otherwise.
+	 */
+	seq_mode = xadc_get_seq_mode(xadc, scan_mask);
+
 	/* Enable all channels and calibration */
 	ret = xadc_write_adc_reg(xadc, XADC_REG_SEQ(0), scan_mask & 0xffff);
 	if (ret)
@@ -783,11 +790,11 @@ static int xadc_postdisable(struct iio_dev *indio_dev)
 		return ret;
 
 	ret = xadc_update_adc_reg(xadc, XADC_REG_CONF1, XADC_CONF1_SEQ_MASK,
-		XADC_CONF1_SEQ_CONTINUOUS);
+				  seq_mode);
 	if (ret)
 		return ret;
 
-	return xadc_power_adc_b(xadc, XADC_CONF1_SEQ_CONTINUOUS);
+	return xadc_power_adc_b(xadc, seq_mode);
 }
 
 static int xadc_preenable(struct iio_dev *indio_dev)