sensor: Add support for defining sensor channel in DT

This functionality is needed at least in the case of temperature
sensors. Zephyr specifies three temperature related channels:

- SENSOR_CHAN_DIE_TEMP
- SENSOR_CHAN_AMBIENT_TEMP
- SENSOR_CHAN_GAUGE_TEMP

In addition to that some devices can report temperature from multiple
probes. This normally handled by using a sensor specific channels.

Right now the consumers of such devices need to hardcode the sensor
channel in their logic. Ideally the sensor channel would to use would be
defined in DT. This is necessary to make a "generic" application that
would work with a variety of different sensors. In the future we might
also want to introduce a thermal management framework similar to thermal
zones implemented in Linux. Such logic would need a way to specify a
sensor channel in DT too.

The "sensor-cells" property is marked as optional to retain backward
compatibility with existing sensor DT nodes. For now only temperature
related channels have been redefined in the dt-bindings header.
Note that since dtc can't parse enums, the channel values had to be
redefined as macros.

Signed-off-by: Kornel Dulęba <mindal@semihalf.com>

dts/bindings/sensor/sensor-device.yaml

@@ -17,3 +17,13 @@ properties:
       This property is defined in the Generic Sensor Property Usages of the HID
       Usage Tables specification
       (https://usb.org/sites/default/files/hut1_3_0.pdf, section 22.5).
+
+  "#sensor-cells":
+    type: int
+    required: false
+    description: |
+      Number of items to expect in a sensor specifier.
+      By default it's only one attribute is expected - a sensor channel to use.
+
+sensor-cells:
+  - channel

include/zephyr/drivers/sensor.h

@@ -28,6 +28,8 @@
 #include <zephyr/sys/iterable_sections.h>
 #include <zephyr/types.h>
 
+#include <zephyr/dt-bindings/sensor/sensor.h>
+
 #ifdef __cplusplus
 extern "C" {
 #endif
@@ -202,6 +204,10 @@ enum sensor_channel {
 	SENSOR_CHAN_MAX = INT16_MAX,
 };
 
+BUILD_ASSERT(DT_SENSOR_CHAN_DIE_TEMP == SENSOR_CHAN_DIE_TEMP);
+BUILD_ASSERT(DT_SENSOR_CHAN_AMBIENT_TEMP == SENSOR_CHAN_AMBIENT_TEMP);
+BUILD_ASSERT(DT_SENSOR_CHAN_GAUGE_TEMP == SENSOR_CHAN_GAUGE_TEMP);
+
 /**
  * @brief Sensor trigger types.
  */

include/zephyr/dt-bindings/sensor/sensor.h

@@ -0,0 +1,16 @@
+/*
+ * Copyright 2023 Google LLC
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+#ifndef ZEPHYR_INCLUDE_DT_BINDINGS_SENSOR_H_
+#define ZEPHYR_INCLUDE_DT_BINDINGS_SENSOR_H_
+
+#include <zephyr/dt-bindings/dt-util.h>
+
+/* The values were copied over from drivers/sensor.h */
+#define DT_SENSOR_CHAN_DIE_TEMP 12
+#define DT_SENSOR_CHAN_AMBIENT_TEMP 13
+#define DT_SENSOR_CHAN_GAUGE_TEMP 43
+
+#endif