linux-kernel - Re: [PATCH 2/4] dt-bindings: hwmon: Added new properties to the devicetree

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Date:   Wed, 13 Sep 2023 23:07:36 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>,
        Daniel Matyas <daniel.matyas@...log.com>
Cc:     Jean Delvare <jdelvare@...e.com>, Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Conor Dooley <conor+dt@...nel.org>,
        Jonathan Corbet <corbet@....net>, linux-hwmon@...r.kernel.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org
Subject: Re: [PATCH 2/4] dt-bindings: hwmon: Added new properties to the
 devicetree

On 9/13/23 22:41, Krzysztof Kozlowski wrote:
> On 13/09/2023 18:43, Guenter Roeck wrote:
>> On 9/13/23 08:40, Krzysztof Kozlowski wrote:
>>> On 13/09/2023 17:21, Daniel Matyas wrote:
>>>
>>> Subject: not much improved. I am sorry, but you are not adding new
>>> properties to entire devicetree of entire world. You are actually not
>>> adding anything to any devicetree, because these are bindings (which is
>>> obvious, as said by prefix).
>>>
>>> You got comments on this.
>>>
>>>> These attributes are:
>>>> 	- adi,comp-int - boolean property
>>>> 	- adi,alrm-pol - can be 0, 1 (if not present, default value)
>>>> 	- adi,flt-q - can be 1, 2, 4, 8 (if not present, default value)
>>>> 	- adi,timeout-enable - boolean property
>>>
>>> Don't repeat what the code does. Explain why you are adding it, what is
>>> the purpose.
>>>
>>>>
>>>> These modify the corresponding bits in the configuration register.
>>>>
>>>> Signed-off-by: Daniel Matyas <daniel.matyas@...log.com>
>>>> ---
>>>>    .../bindings/hwmon/adi,max31827.yaml          | 35 +++++++++++++++++++
>>>>    1 file changed, 35 insertions(+)
>>>>
>>>> diff --git a/Documentation/devicetree/bindings/hwmon/adi,max31827.yaml b/Documentation/devicetree/bindings/hwmon/adi,max31827.yaml
>>>> index 2dc8b07b4d3b..6bde71bdb8dd 100644
>>>> --- a/Documentation/devicetree/bindings/hwmon/adi,max31827.yaml
>>>> +++ b/Documentation/devicetree/bindings/hwmon/adi,max31827.yaml
>>>> @@ -32,6 +32,37 @@ properties:
>>>>          Must have values in the interval (1.6V; 3.6V) in order for the device to
>>>>          function correctly.
>>>>    
>>>> +  adi,comp-int:
>>>> +    description:
>>>> +      If present interrupt mode is used. If not present comparator mode is used
>>>> +      (default).
>>>
>>> Why this is a property of hardware?
>>>
>>
>> Because it affects the behavior of the interrupt signal and whatever
>> it is connected to. For example, it could be connected to an interrupt
>> controller (interupt mode), or it could be connected to a fan which is
>> enabled while the signal is active (comparator mode).
> 
> That makes sense. Pardon my naive questions, I just could not figure out
> use case out of the field description. Based on this very short
> description itself, I could imagine sysfs entry.
> 
>>
>>>> +    type: boolean
>>>> +
>>>> +  adi,alrm-pol:
>>>> +    description:
>>>> +      Sets the alarms active state.
>>>> +            - 0 = active low
>>>> +            - 1 = active high
>>>> +      For max31827 and max31828 the default alarm polarity is low. For max31829
>>>> +      it is high.
>>>> +    $ref: /schemas/types.yaml#/definitions/uint32
>>>> +    enum: [0, 1]
>>>> +
>>>> +  adi,flt-q:
>>>> +    description:
>>>> +      Select how many consecutive temperature faults must occur before
>>>> +      overtemperature or undertemperature faults are indicated in the
>>>> +      corresponding status bits.
>>>> +      For max31827 default fault queue is 1. For max31828 and max31829 it is 4.
>>>> +    $ref: /schemas/types.yaml#/definitions/uint32
>>>> +    enum: [1, 2, 4, 8]
>>>> +
>>>> +  adi,timeout-enable:
>>>> +    description:
>>>> +      Enables timeout. Bus timeout resets the I2C-compatible interface when SCL
>>>> +      is low for more than 30ms (nominal).
>>>
>>> Why this is a property of hardware?
>>>
>>
>> Because it affects i2c bus operation.
>>
>> I am not sure I understand what you are trying to say here.
>> This is a much a "hardware" property as the i2c bus speed
>> and many similar properties, and the need for it is system
>> dependent (for example, a system with many devices on the
>> i2c bus or with a less than perfect i2c controller may need
>> it because the bus tends to get stuck).
>>
>> Those are not properties where one would, at runtime,
>> decide to enable bus timeout or the interrupt mode or
>> the bus speed. Typically that kind of functionality
>> has to be configured early when the system is started.
>> If devicetree must not or no longer be used to describe the
>> system to a point where it can be configured to get it
>> to a working state, what is the suggested alternative ?
> 
> I could imagine enabling it always, unconditionally. I wanted to
> understand why different boards with this chip will have it enabled or
> disabled.
> 

Some boards may need a larger timeout, for example if there is
a chip on the same bus which supports clock stretching.

Guenter