On Sun Jul 4, 2021 at 1:02 PM EDT, Jonathan Cameron wrote:

On Wed, 30 Jun 2021 21:00:33 -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 one case, the measurement of a temperature through the
voltage across an RTD resistor such as a PT1000.

Signed-off-by: Liam Beguin <redacted>
---
 .../iio/afe/temperature-sense-rtd.yaml        | 101 ++++++++++++++++++
 MAINTAINERS                                   |   7 ++
 2 files changed, 108 insertions(+)
 create mode 100644 Documentation/devicetree/bindings/iio/afe/temperature-sense-rtd.yaml

diff --git a/Documentation/devicetree/bindings/iio/afe/temperature-sense-rtd.yaml b/Documentation/devicetree/bindings/iio/afe/temperature-sense-rtd.yaml
new file mode 100644
index 000000000000..e23e74e63ec5
--- /dev/null
+++ b/Documentation/devicetree/bindings/iio/afe/temperature-sense-rtd.yaml

@@ -0,0 +1,101 @@
+# SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/iio/afe/temperature-sense-rtd.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Temperature Sense RTD
+
+maintainers:
+  - Liam Beguin <lvb@xiphos.com>
+
+description: |
+  RTDs (Resistance Temperature Detectors) are a kind of temperature sensors
+  used to get a linear voltage to temperature reading within a give range
+  (usually 0 to 100 degrees Celsius).
+
+  When an io-channel measures the output voltage across an RTD such as a
+  PT1000, the interesting measurement is almost always the corresponding
+  temperature, not the voltage output. This binding describes such a circuit.
+
+  The general transfer function here is (using SI units)
+
+    V = R(T) * iexc
+    R(T) = r0 * (1 + alpha * T)
+    T = 1 / (alpha * r0 * iexc) * (V - r0 * iexc)
+
+  The following circuit matches what's in the examples section.
+
+           5V0
+          -----
+            |
+        +---+----+
+        |  R 5k  |
+        +---+----+
+            |
+            V 1mA
+            |
+            +---- Vout
+            |
+        +---+----+
+        | PT1000 |
+        +---+----+
+            |
+          -----
+           GND
+
+properties:
+  compatible:
+    const: temperature-sense-rtd
+
+  io-channels:
+    maxItems: 1
+    description: |
+      Channel node of a voltage io-channel.
+
+  '#io-channel-cells':
+    const: 1

Only 1 channel, so this should be 0. For consumers of this driver
they only need to identify the device, not the device + channel.

https://github.com/devicetree-org/dt-schema/blob/master/schemas/iio/iio.yaml

Thanks for pointing that out. will fix.

quoted

+
+  excitation-current-microamp:
+    description: The current fed through the RTD sensor.
+
+  alpha-ppm-per-celsius:
+    description: |
+      alpha can also be expressed in micro-ohms per ohm Celsius. It's a linear
+      approximation of the resistance versus temperature relationship
+      between 0 and 100 degrees Celsius.
+
+      alpha = (R_100 - R_0) / (100 * R_0)
+
+      Where, R_100 is the resistance of the sensor at 100 degrees Celsius, and
+      R_0 (or r-naught-ohms) is the resistance of the sensor at 0 degrees
+      Celsius.
+
+      Pure platinum has an alpha of 3925. Industry standards such as IEC60751
+      and ASTM E-1137 specify an alpha of 3850.
+
+  r-naught-ohms:
+    description: |
+      Resistance of the sensor at 0 degrees Celsius.
+      Common values are 100 for PT100, 500 for PT500, and 1000 for PT1000
+
+additionalProperties: false
+required:
+  - compatible
+  - io-channels
+  - excitation-current-microamp
+  - alpha-ppm-per-celsius
+  - r-naught-ohms
+
+examples:
+  - |
+    pt1000_1: temperature-sensor0 {
+        compatible = "temperature-sense-rtd";
+        #io-channel-cells = <1>;
+        io-channels = <&temp_adc1 0>;
+
+        excitation-current-microamp = <1000>; /* i = U/R = 5 / 5000 */
+        alpha-ppm-per-celsius = <3908>;
+        r-naught-ohms = <1000>;
+    };
+...

diff --git a/MAINTAINERS b/MAINTAINERS
index 9bf553e53f0f..ed80e671a16a 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS

@@ -8897,6 +8897,13 @@ F:	Documentation/devicetree/bindings/iio/afe/current-sense-shunt.yaml
 F:	Documentation/devicetree/bindings/iio/afe/voltage-divider.yaml
 F:	drivers/iio/afe/iio-rescale.c
 
+IIO UNIT CONVERTER (TEMPERATURE)
+M:	Liam Beguin <liambeguin@gmail.com>
+R:	Peter Rosin <peda@axentia.se>
+L:	linux-iio@vger.kernel.org
+S:	Maintained
+F:	Documentation/devicetree/bindings/iio/afe/temperature-sense-rtd.yaml

I'm not sure we'd normally bother with a MAINTAINERS entry when it is
just the binding doc
(as rest is in the driver). The binding doc itself has it's own local
maintainers entry
which is the more useful one.

You're right, it makes sense to drop the changes to MAINTAINERS as the
information is already contained it the bindings.
I added this after briefly talking about it with Peter.

Thanks again for reviewing these changes,
Liam

quoted

+
 IKANOS/ADI EAGLE ADSL USB DRIVER
 M:	Matthieu Castet [off-list ref]
 M:	Stanislaw Gruszka [off-list ref]