iio: adc: ad7380: fix oversampling formula

The formula in the datasheet for oversampling time conversion seems to
be valid when device is at full speed using the maximum number of SDO
lines. The driver currently support only 1 SDO line. The correct formula
is: t_convert = T_CONVERT_0_NS + T_CONVERT_X_NS*(x -
1)*num_channel/number_of_sdo_lines.
It will produce larger delays than what is currently set, but some devices
actually require it.

Signed-off-by: Julien Stephan <jstephan@baylibre.com>
Reviewed-by: David Lechner <dlechner@baylibre.com>
Link: https://patch.msgid.link/20241030-ad7380-add-adaq4380-4-support-v4-2-864ff02babae@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Author: justeph

drivers/iio/adc/ad7380.c

@@ -77,6 +77,12 @@
 #define T_CONVERT_X_NS 500		/* xth conversion start time (oversampling) */
 #define T_POWERUP_US 5000		/* Power up */
 
+/*
+ * AD738x support several SDO lines to increase throughput, but driver currently
+ * supports only 1 SDO line (standard SPI transaction)
+ */
+#define AD7380_NUM_SDO_LINES		1
+
 struct ad7380_timing_specs {
 	const unsigned int t_csh_ns;	/* CS minimum high time */
 };
@@ -649,7 +655,8 @@ static int ad7380_set_ch(struct ad7380_state *st, unsigned int ch)
 
 	if (st->oversampling_ratio > 1)
 		xfer.delay.value = T_CONVERT_0_NS +
-			T_CONVERT_X_NS * (st->oversampling_ratio - 1);
+			T_CONVERT_X_NS * (st->oversampling_ratio - 1) *
+			st->chip_info->num_simult_channels / AD7380_NUM_SDO_LINES;
 
 	return spi_sync_transfer(st->spi, &xfer, 1);
 }
@@ -672,7 +679,8 @@ static void ad7380_update_xfers(struct ad7380_state *st,
 	 */
 	if (st->oversampling_ratio > 1)
 		t_convert = T_CONVERT_0_NS + T_CONVERT_X_NS *
-			(st->oversampling_ratio - 1);
+			(st->oversampling_ratio - 1) *
+			st->chip_info->num_simult_channels / AD7380_NUM_SDO_LINES;
 
 	if (st->seq) {
 		xfer[0].delay.value = xfer[1].delay.value = t_convert;
@@ -1021,7 +1029,8 @@ static int ad7380_init(struct ad7380_state *st, bool external_ref_en)
 	/* SPI 1-wire mode */
 	return regmap_update_bits(st->regmap, AD7380_REG_ADDR_CONFIG2,
 				  AD7380_CONFIG2_SDO,
-				  FIELD_PREP(AD7380_CONFIG2_SDO, 1));
+				  FIELD_PREP(AD7380_CONFIG2_SDO,
+					     AD7380_NUM_SDO_LINES));
 }
 
 static int ad7380_probe(struct spi_device *spi)