Re: [PATCH V2 3/9] dt-bindings: Tegra186 tachometer device tree bindings

[Rob Herring]

On Mon, Apr 9, 2018 at 9:37 AM, Mikko Perttunen <cyndis@xxxxxxxx> wrote:
>
>
> On 04/09/2018 04:21 PM, Rob Herring wrote:
>>
>> On Mon, Apr 9, 2018 at 12:38 AM, Mikko Perttunen <cyndis@xxxxxxxx> wrote:
>>>
>>> Rob,
>>
>>
>> Please don't top post to lists.
>>
>>> this binding is for a specific IP block (for measuring/aggregating input
>>> pulses) on the Tegra186 SoC, so I don't think it fits into any generic
>>> binding.
>>
>>
>> What is it hooked up to to measure? You only mention "fan" five times
>> in the doc.
>
>
> In practice, fans.
>
>>
>> You have #pwm-cells too, so this block has PWM output as well? If not,
>> then where's the PWM for the fan control because there is no point in
>> having fan tach without some control mechanism.
>
>
> It doesn't provide a PWM output. The (Linux) PWM framework provides
> functionality in both directions - control and capture. But if the device
> tree #pwm-cells/pwms properties are only for control, we may need to
> introduce a new #capture-pwm-cells/capture-pwms or similar.

Yes, perhaps. But there is no point in having
#capture-pwm-cells/capture-pwms if you aren't describing the
connection between the fan and the fan controller.

> The idea is that the generic fan node can then specify two pwms, one for
> control and one for capture, to enable e.g. closed-loop control (I'm not
> personally familiar with the usecase for this but I could imagine something
> like that). The control PWM can be something completely different, maybe not
> a PWM in the first place (e.g. some fixed voltage).

Yes. As you can have different types of fans (3-wire, 4-wire, etc.)
they would have different compatibles and differing properties
associated with them.

>> There's only so many ways to control fans and types of fans, so yes,
>> the interface of control and feedback lines between a fan and its
>> controller should absolutely be generic.
>
>
> I'm not quite getting what you mean by this. Clearly we need a custom
> compatibility string for the tachometer as it's a different hardware block
> with different programming than others.

Yes, of course. It's the interface between fan controllers and fans
that I'm concerned about.

> Or are you complaining about the
> nvidia,pulse-per-rev/capture-window-len properties?

Well, those sound like properties of a fan (at least the first one),
so they belong in a fan node.

The aspeed fan controller is probably the closest thing we have to a
fan binding. Look at that if you haven't already.

Rob