drivers: adc: voltage divider

adds DT macro initializer and scaling function for voltage divider.

Signed-off-by: Jason Yuan <jasonyuan@google.com>

include/zephyr/drivers/adc/voltage_divider.h

@@ -0,0 +1,59 @@
+/*
+ * Copyright (c) 2023 The ChromiumOS Authors
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#ifndef ZEPHYR_INCLUDE_DRIVERS_ADC_VOLTAGE_DIVIDER_H_
+#define ZEPHYR_INCLUDE_DRIVERS_ADC_VOLTAGE_DIVIDER_H_
+
+#include <zephyr/drivers/adc.h>
+
+struct voltage_divider_dt_spec {
+	const struct adc_dt_spec port;
+	uint32_t full_ohms;
+	uint32_t output_ohms;
+};
+
+/**
+ * @brief Get voltage divider information from devicetree.
+ *
+ * This returns a static initializer for a @p voltage_divider_dt_spec structure
+ * given a devicetree node.
+ *
+ * @param node_id Devicetree node identifier.
+ *
+ * @return Static initializer for an voltage_divider_dt_spec structure.
+ */
+#define VOLTAGE_DIVIDER_DT_SPEC_GET(node_id)                                                       \
+	{                                                                                          \
+		.port = ADC_DT_SPEC_GET(node_id),                                                  \
+		.full_ohms = DT_PROP_OR(node_id, full_ohms, 0),                                    \
+		.output_ohms = DT_PROP(node_id, output_ohms),                                      \
+	}
+
+/**
+ * @brief Calculates the actual voltage from the measured voltage
+ *
+ * @param[in] spec voltage divider specification from Devicetree.
+ * @param[in,out] v_to_v Pointer to the measured voltage on input, and the
+ * corresponding scaled voltage value on output.
+ *
+ * @retval 0 on success
+ * @retval -ENOTSUP if "full_ohms" is not specified
+ */
+static inline int voltage_divider_scale_dt(const struct voltage_divider_dt_spec *spec,
+					   int32_t *v_to_v)
+{
+	/* cannot be scaled if "full_ohms" is not specified */
+	if (spec->full_ohms == 0) {
+		return -ENOTSUP;
+	}
+
+	/* voltage scaled by voltage divider values using DT binding */
+	*v_to_v = *v_to_v * spec->full_ohms / spec->output_ohms;
+
+	return 0;
+}
+
+#endif /* ZEPHYR_INCLUDE_DRIVERS_ADC_VOLTAGE_DIVIDER_H_ */