On Wed Jun 9, 2021 at 4:52 PM EDT, Jonathan Cameron wrote:

On Mon, 7 Jun 2021 10:47:18 -0400
Liam Beguin [off-list ref] wrote:

quoted

From: Liam Beguin <redacted>

An ADC is often used to measure other quantities indirectly. This
binding describe such a use case, the measurement of a temperature
through an analog front end connected to a voltage channel.

Signed-off-by: Liam Beguin <redacted>
---
 .../iio/afe/temperature-sense-amplifier.yaml  | 57 +++++++++++++++++++
 MAINTAINERS                                   |  1 +
 2 files changed, 58 insertions(+)
 create mode 100644 Documentation/devicetree/bindings/iio/afe/temperature-sense-amplifier.yaml

diff --git a/Documentation/devicetree/bindings/iio/afe/temperature-sense-amplifier.yaml b/Documentation/devicetree/bindings/iio/afe/temperature-sense-amplifier.yaml
new file mode 100644
index 000000000000..08f97f052a91
--- /dev/null
+++ b/Documentation/devicetree/bindings/iio/afe/temperature-sense-amplifier.yaml

@@ -0,0 +1,57 @@
+# SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/iio/afe/temperature-sense-amplifier.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Temperature Sense Amplifier
+
+maintainers:
+  - Liam Beguin <lvb@xiphos.com>
+
+description: |
+  When an io-channel measures the output voltage of a temperature analog front
+  end such as an RTD (resistance thermometer) or a temperature to current
+  sensor, the interesting measurement is almost always the corresponding
+  temperature, not the voltage output. This binding describes such a circuit.

Perhaps add something about this only covering the linear cases...

Okay, will do.

quoted

+
+properties:
+  compatible:
+    const: temperature-sense-amplifier
+
+  io-channels:
+    maxItems: 1
+    description: |
+      Channel node of a voltage io-channel.
+
+  '#io-channel-cells':
+    const: 1
+
+  sense-gain-mult:
+    $ref: /schemas/types.yaml#/definitions/uint32
+    description: Amplifier gain multiplier. The default is <1>.
+
+  sense-gain-div:
+    $ref: /schemas/types.yaml#/definitions/uint32
+    description: Amplifier gain divider. The default is <1>.
+
+  sense-offset-millicelsius:
+    description: Amplifier offset. The default is <0>.

Whilst it may seem obvious I'd like to see a statement of
how these are used somewhere in here.

temp_celcius = voltage * gain-mult / gain-div + offset

Mainly because those familiar with the IIO usage of offset
would expect
(voltage + offset) * gain-mult/gain-div

which doesn't make sense for this device but might leave
people confused!

You're right, I'll add an explicit statement of how these parameters are
used.

Thanks,
Liam

quoted

+
+additionalProperties: false
+required:
+  - compatible
+  - io-channels
+
+examples:
+  - |
+    pt1000_1: temperature-sensor {
+        compatible = "temperature-sense-amplifier";
+        #io-channel-cells = <1>;
+        io-channels = <&temp_adc 3>;
+
+        sense-gain-mult = <1000000>;
+        sense-gain-div = <3908>;
+        sense-offset-millicelsius = <(-255885)>;
+    };
+...

diff --git a/MAINTAINERS b/MAINTAINERS
index e679d422b472..4f7b4ee9f19b 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS

@@ -8887,6 +8887,7 @@ L:	linux-iio@vger.kernel.org
 S:	Maintained
 F:	Documentation/devicetree/bindings/iio/afe/current-sense-amplifier.yaml
 F:	Documentation/devicetree/bindings/iio/afe/current-sense-shunt.yaml
+F:	Documentation/devicetree/bindings/iio/afe/temperature-sense-amplifier.yaml
 F:	Documentation/devicetree/bindings/iio/afe/voltage-divider.yaml
 F:	drivers/iio/afe/iio-rescale.c