[RFC PATCH v2 1/2] dt-bindings: hwmon: emc2305: Add fan-shutdown-percent property

[Guenter Roeck]

On 3/13/26 10:38, Conor Dooley wrote:
> On Fri, Mar 13, 2026 at 07:14:33PM +0200, Florin Leotescu wrote:
>> On Thu, Mar 12, 2026 at 05:37:51PM +0000, Conor Dooley wrote:
>>> On Thu, Mar 12, 2026 at 02:22:47PM +0200, florin.leotescu at oss.nxp.com wrote:
>>>> From: Florin Leotescu <florin.leotescu at nxp.com>
>>>>
>>>> The EMC2305 fan controller supports multiple independent PWM fan
>>>> outputs. Some systems require fans to enter a defined safe state
>>>> during system shutdown or reboot handoff, until firmware or the next
>>>> boot stage reconfigures the controller.
>>>>
>>>> Add an optional "fan-shutdown-percent" property to fan child nodes
>>>> allowing the shutdown fan speed to be configured per fan output.
>>>>
>>>> Signed-off-by: Florin Leotescu <florin.leotescu at nxp.com>
>>>> ---
>>>>   .../devicetree/bindings/hwmon/microchip,emc2305.yaml      | 8 ++++++++
>>>>   1 file changed, 8 insertions(+)
>>>>
>>>> diff --git a/Documentation/devicetree/bindings/hwmon/microchip,emc2305.yaml b/Documentation/devicetree/bindings/hwmon/microchip,emc2305.yaml
>>>> index d3f06ebc19fa..7bcadfab9fc4 100644
>>>> --- a/Documentation/devicetree/bindings/hwmon/microchip,emc2305.yaml
>>>> +++ b/Documentation/devicetree/bindings/hwmon/microchip,emc2305.yaml
>>>> @@ -54,6 +54,12 @@ patternProperties:
>>>>             The fan number used to determine the associated PWM channel.
>>>>           maxItems: 1
>>>>   
>>>> +      fan-shutdown-percent:
>>>> +        description:
>>>> +          Fan RPM in percent set during shutdown.
>>>
>>> This sounds like something generic, that other devices might need and
>>> should be in fan-common.yaml?
>>>
>> Thanks for the feedback.
>>
>> The property name was reused from pwm-fan.yaml where it already exists
>> with the same meaning. I kept it local to minimize the scope of the change.
>>
>> However, if you prefer it to be modelled as a common property, I can adjust it in the next revision.
> 
> I see. Makes me wonder why half the properties in pwm-fan aren't also in
> fan-common.
> 
> I suppose there's a reason for it, carry on I guess.. Can be lifted if
> there are more users other than you and pwm-fan.
> 
>>>> +        minimum: 0
>>>> +        maximum: 100
>>>> +
>>>>       required:
>>>>         - reg
>>>>   
>>>> @@ -80,12 +86,14 @@ examples:
>>>>               fan at 0 {
>>>>                   reg = <0x0>;
>>>>                   pwms = <&fan_controller 26000 PWM_POLARITY_INVERTED 1>;
>>>> +                fan-shutdown-percent = <100>;
> 
> Also leads to the question of why these devices aren't pwm-fans. Too
> late now to change anything, I just wanna understand why.
> 

They are dedicated fan controllers, with an internal tach -> pwm feedback
loop. I don't mind modeling them as pwm controllers if there is a use case
(such as someone using a PWM output to control a LED), but I don't even know
how modeling them as pwm fans using the pwm-fan compatible would even be
possible since that would exclude the tach -> pwm feedback loop.

Also, if that were to be done, we might end up with two hwmon drivers
for one chip: one to report the fan speed, and one to report pwm values.
On top of that, even if it is not currently supported by the driver,
controlling the tach -> pwm feedback loop would become awkward.

Yes, I understand, there is a strong push to model everything with
a pwm output as pwm controller, but si far no one was able to describe
to me how to model a fan controller, and especially its feedback loop,
in such a scenario. That becomes even more complicated if temperatures
are used to control the feedback loop.

Guenter

>>>>                   #cooling-cells = <2>;
>>>>               };
>>>>   
>>>>               fan at 1 {
>>>>                   reg = <0x1>;
>>>>                   pwms = <&fan_controller 26000 0 1>;
>>>> +                fan-shutdown-percent = <50>;
>>>>                   #cooling-cells = <2>;
>>>>               };
>>>>   
>>>> -- 
>>>> 2.34.1
>>>>
>>
>>