Re: [PATCH v8 1/3] dt-bindings: hwmon: fan: Add fan binding to schema

[Billy Tsai]

On Thu, Sep 07, 2023 at 07:17:55AM +0000, Billy Tsai wrote:
> > On Wed, Aug 30, 2023 at 08:32:00PM +0800, Billy Tsai wrote:
> > >> From: Naresh Solanki <naresh.solanki@xxxxxxxxxxxxx>
> > >> 
> > >> Add common fan properties bindings to a schema.
> > >> 
> > >> Bindings for fan controllers can reference the common schema for the
> > >> fan


> > >> +properties:
> > >> +  max-rpm:
> > >> +    description:
> > >> +      Max RPM supported by fan.
> > >> +    $ref: /schemas/types.yaml#/definitions/uint32
> > 
> > > Physics will limit this to something much less than 2^32. Add some 
> > > constraints. 10000?
> > 
> > 
> > >> +
> > >> +  min-rpm:
> > >> +    description:
> > >> +      Min RPM supported by fan.
> > >> +    $ref: /schemas/types.yaml#/definitions/uint32
> > 
> > > ditto
> > 
> > >> +
> > >> +  pulses-per-revolution:
> > >> +    description:
> > >> +      The number of pulse from fan sensor per revolution.
> > >> +    $ref: /schemas/types.yaml#/definitions/uint32
> > 
> > >Needs constraints. I assume this is never more than 4 (or 2 even)?
> > 
> > Do you think we should add the contraint in the common binding?
> > In my option, the limit of the max/min rpm should be declared by
> > the binding if necessary, because the usage of each fan monitor is
> > based on the connection of the tach pin.

> Yes, I think we should have default limits.

> Unless we go as far as a schema for every specific fan model, then there 
> is actually no way we can have specific limits unless the fan 
> controllers have some limits.

> The most I see in tree for pulses-per-revolution is 2. There's no value 
> in more. So set the max to 4 and then if anyone needs more they can bump 
> the value.

> Or maybe there's some electrical/mechanical design reason fans are 1 or 
> 2 pulses and we'll never see anything else? This document[1] seems to 
> indicate that is indeed the case. (First hit googling "fan tach signal 
> pulses")

OK, I will add the maximum value for the max-rpm, min-rpm and pulses-per-revolution.

> > 
> > 
> > >> +  div:
> > 
> > > Too generic of a name.
> > 
> > >> +    description:
> > >> +      Fan clock divisor
> > 
> > > But what is a fan clock?
> > 
> > This is the divisor for the tachometer sampling clock, which determines the sensitivity of the tach pin.
> > So, if the name of the property changes to 'tach-div,' is it acceptable to you?

> That sounds like a property of the controller, not the fan, so it 
> belongs in the controller binding. Is this really a common thing?

Yes, I believe this is a common feature for fans. You can refer to the Documentation/hwmon/sysfs-interface.rst,
where the fan divisor is defined for users, determining the fan's sensitivity.
 
> > >> +    $ref: /schemas/types.yaml#/definitions/uint32
> > >> +
> > >> +  target-rpm:
> > >> +    description:
> > >> +      Target RPM the fan should be configured during driver probe.
> > 
> > > What driver? By the time the OS driver runs, a bunch of other boot 
> > > software has already run on modern systems. So this value would likely 
> > > be used much earlier. The point is that when exactly is outside the 
> > > scope of DT. This is "what RPM do I use in case of no other information 
> > > (e.g. temperature)".
> > 
> > So, the description should be changed to 'The default desired fan speed in RPM,'
> > and we shouldn't mention the timing of the property's operation in the DT, is that correct?

> Correct.

> > 
> > >> +    $ref: /schemas/types.yaml#/definitions/uint32
> > >> +
> > >> +  mode:
> > 
> > > Too generic.
> > 
> > >> +    description:
> > >> +      Select the operational mode of the fan.
> > 
> > > What are modes? Spin and don't spin?
> > 
> > The mode is used to indicate the driving mode of the fan (DC, PWM and so on).
> > So, if the name of the property changes to 'fan-driving-mode,' is it acceptable to you?

> I tend to think that should be implied from the parent node and/or other 
> properties. PWM if "pwms" property is present. DC if the supply is 
> variable. We could also use compatible strings in the fan nodes if 
> there's a need.

So, it looks that this property isn't necessary for the fan. And it should be determined by the
present of the driving source. is that correct?

> That reminds me, both of these modes probably need a table of 
> voltage/duty-cycle to RPMs. I imagine it's not always a linear response.  
> Naresh also privately sent me (don't do that) an updated common binding 
> which we discussed the need for this. I expect him to comment further 
> with details.

For this purpose, we should add the speed-map like the usage of the gpio-fan, right?
https://github.com/torvalds/linux/blob/master/Documentation/devicetree/bindings/hwmon/gpio-fan.txt


> > >> +    $ref: /schemas/types.yaml#/definitions/uint32
> > >> +
> > >> +  pwms:
> > >> +    description:
> > >> +      PWM provider.
> > 
> > > maxItems: 1
> > 
> > > I don't think there are fans with more than 1 PWM input?
> > 
> > Ok, I will add the constraint for the pwm input.
> > 
> > >> +
> > >> +  tach-ch:
> > >> +    description:
> > >> +      The tach channel used for the fan.
> > >> +    $ref: /schemas/types.yaml#/definitions/uint32
> > 
> > > The existing ASpeed version of this property allows more than 1 entry. I 
> > > don't understand how a fan would have 2 tach signals, but if so, the 
> > > generic property should allow for that.
> > 
> > Ok, I will modify it to the uint32-array

> Perhaps uint8-array to align with existing versions of the property.

Ok, I will modify it to the uint8-array.

> > 
> > > Perhaps 'reg' should be defined in here with some text saying 'reg' 
> > > corresponds to the fan controller specific id which may be the PWM+TACH 
> > > channel, PWM channel (deprecated), or TACH channel. I think there are 
> > > examples of all 3 of these cases.
> > 
> > I don't think it's necessary for the 'reg' because the case you mentioned is
> > already covered by the property 'tach-ch' and the 'pwms'.

> Yes, but when we have N child nodes of the same thing, we usually have 
> "reg" and its value corresponds to how the parent identifies each child. 
> We already have a mixture using PWM or tach channel. Yes, this can all 
> just be in the fan controllers binding, but putting it here would just 
> document the options.

Ok, I will add reg property for the option.

> Rob


> [1] http://www.comairrotron.com/methods-monitoring-fan-performance