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

[Mikko Perttunen <cyndis@kapsi.fi>]

On 04/09/2018 04:21 PM, Rob Herring wrote:
> On Mon, Apr 9, 2018 at 12:38 AM, Mikko Perttunen <cyndis@kapsi.fi> 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.

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).

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

Thanks,
Mikko

> 
> Rob
> 
>>
>> Thanks,
>> Mikko
>>
>>
>> On 03/27/2018 05:52 PM, Rob Herring wrote:
>>>
>>> On Wed, Mar 21, 2018 at 10:10:38AM +0530, Rajkumar Rampelli wrote:
>>>>
>>>> Supply Device tree binding documentation for the NVIDIA
>>>> Tegra186 SoC's Tachometer Controller
>>>>
>>>> Signed-off-by: Rajkumar Rampelli <rrajk@nvidia.com>
>>>> ---
>>>>
>>>> V2: Renamed compatible string to "nvidia,tegra186-pwm-tachometer"
>>>>       Renamed dt property values of clock-names and reset-names to
>>>> "tachometer"
>>>>       from "tach"
>>>
>>>
>>> Read my prior comments on v1.
>>>
>>> Rob
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