iio: adc: ti-ads7950: Allow to use on ACPI platforms

ACPI enabled platforms do not have a mean of regulators. Instead we use
hard coded voltage value for reference pin. When value is 0 (default) we
fall back to request a regulator.
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/adc/ti-ads7950.c

@@ -21,6 +21,7 @@
  * GNU General Public License for more details.
  */
 
+#include <linux/acpi.h>
 #include <linux/bitops.h>
 #include <linux/device.h>
 #include <linux/err.h>
@@ -37,6 +38,12 @@
 #include <linux/iio/trigger_consumer.h>
 #include <linux/iio/triggered_buffer.h>
 
+/*
+ * In case of ACPI, we use the 5000 mV as default for the reference pin.
+ * Device tree users encode that via the vref-supply regulator.
+ */
+#define TI_ADS7950_VA_MV_ACPI_DEFAULT	5000
+
 #define TI_ADS7950_CR_MANUAL	BIT(12)
 #define TI_ADS7950_CR_WRITE	BIT(11)
 #define TI_ADS7950_CR_CHAN(ch)	((ch) << 7)
@@ -58,6 +65,7 @@ struct ti_ads7950_state {
 	struct spi_message	scan_single_msg;
 
 	struct regulator	*reg;
+	unsigned int		vref_mv;
 
 	unsigned int		settings;
 
@@ -305,11 +313,15 @@ static int ti_ads7950_get_range(struct ti_ads7950_state *st)
 {
 	int vref;
 
+	if (st->vref_mv) {
+		vref = st->vref_mv;
+	} else {
 		vref = regulator_get_voltage(st->reg);
 		if (vref < 0)
 			return vref;
 
 		vref /= 1000;
+	}
 
 	if (st->settings & TI_ADS7950_CR_RANGE_5V)
 		vref *= 2;
@@ -411,6 +423,10 @@ static int ti_ads7950_probe(struct spi_device *spi)
 	spi_message_init_with_transfers(&st->scan_single_msg,
 					st->scan_single_xfer, 3);
 
+	/* Use hard coded value for reference voltage in ACPI case */
+	if (ACPI_COMPANION(&spi->dev))
+		st->vref_mv = TI_ADS7950_VA_MV_ACPI_DEFAULT;
+
 	st->reg = devm_regulator_get(&spi->dev, "vref");
 	if (IS_ERR(st->reg)) {
 		dev_err(&spi->dev, "Failed get get regulator \"vref\"\n");