linux-kernel - Re: [PATCH] dt-bindings: hwmon: Add nct7802 bindings

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Date:   Thu, 16 Sep 2021 12:34:57 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     Oskar Senft <osk@...gle.com>
Cc:     linux-hwmon@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, Jean Delvare <jdelvare@...e.com>,
        Rob Herring <robh+dt@...nel.org>
Subject: Re: [PATCH] dt-bindings: hwmon: Add nct7802 bindings

On 9/16/21 12:19 PM, Oskar Senft wrote:
>      Hi Guenter
> 
>>> Would it be acceptable to simply number the sensors and document which
>>> sensor has which number?
>>>
>>> Something like this:
>>> 0 = LTD
>>> 1 = RTD1
>>> ...
>>>
>> That might be a possibility, though it would have to be well defined
>> for each chip (nct7802 also has voltage sensors). We'll have to discuss
>> this with Rob.
>>
>> Personally I think I would prefer using a type qualifier - that seems
>> cleaner. But that is really a matter of opinion.
> 
> Another existing way I found is in ltc2978. Following that, we could
> do it as follows:
> 
> i2c {
>      #address-cells = <1>;
>      #size-cells = <0>;
> 
>      nct7802@28 {
>          compatible = "nuvoton,nct7802";
>          reg = <0x28>;
>          #address-cells = <1>;
>          #size-cells = <0>;
> 
>          sensors {
>              ltd {
>                  status = "okay";
>                  label = "my local temperature";
>              };
> 
>              rtd1 {
>                  status = "okay";
>                  mode = <0x2>; /* 3904 transistor */
>                  label = "other temperature";
>              };
> 
>              rtd3 {
>                  status = "okay";
>                  mode = <0x3>; /* thermal diode */
>                  label = "3rd temperature";
>             };
>          };
>      };
> };
> 

Ah, using the node name as indication for both sensor type and
index ? SGTM, though we'd really need input from Rob.
I guess one could also consider something more generic like
"temperature-sensor@0", "voltage-sensor@0", and so on (instead
of [mis-]using reg and a sensor-type field).

Thanks,
Guenter

> 
>>> The NCT7802Y can self-program from an EEPROM, so I assume we should
>>> honor the "power-up configuration" obtained from there? I.e. if no
>>> configuration is provided in the device tree, the driver should use
>>> whatever configuration the chip has when the driver is loaded.
>>>
>> Definitely yes. My question was more what to do if the information
>> in devicetree nodes is incomplete.
> I think there are two cases:
> 1) If the new "sensor" tree is missing, the driver should behave as it
> does today to not break existing users.
> 2) If the new "sensor" tree is present, then each of the sensors that
> should be disabled needs to have "status = 'okay'" and have the mode
> set (unless it's LTD). In the above example, rtd2 is missing and would
> therefore be considered disabled.
> 
> Does that make sense? I still need to find out whether this is
> actually valid DT and how to express that in the YAML, though ...
> 
> Thanks
> Oskar.
>