linux-kernel - Re: [PATCH 05/10] hwmon: generic-pwm-tachometer: Add generic PWM based tachometer

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Date:   Thu, 8 Mar 2018 06:31:24 -0800
From:   Guenter Roeck <linux@...ck-us.net>
To:     Laxman Dewangan <ldewangan@...dia.com>,
        Rajkumar Rampelli <rrajk@...dia.com>,
        Mikko Perttunen <cyndis@...si.fi>, robh+dt@...nel.org,
        mark.rutland@....com, thierry.reding@...il.com,
        jonathanh@...dia.com, jdelvare@...e.com, corbet@....net,
        catalin.marinas@....com, will.deacon@....com,
        kstewart@...uxfoundation.org, gregkh@...uxfoundation.org,
        pombredanne@...b.com, mmaddireddy@...dia.com,
        mperttunen@...dia.com, arnd@...db.de, timur@...eaurora.org,
        andy.gross@...aro.org, xuwei5@...ilicon.com, elder@...aro.org,
        heiko@...ech.de, krzk@...nel.org, ard.biesheuvel@...aro.org
Cc:     devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-pwm@...r.kernel.org, linux-tegra@...r.kernel.org,
        linux-hwmon@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 05/10] hwmon: generic-pwm-tachometer: Add generic PWM
 based tachometer

On 03/07/2018 10:06 PM, Laxman Dewangan wrote:
> 
> 
> On Wednesday 07 March 2018 07:50 PM, Guenter Roeck wrote:
>> On 03/07/2018 01:47 AM, Rajkumar Rampelli wrote:
>>>
>>>>
>>>> While I am not opposed to ABI changes, the merits of those would need to be
>>>> discussed on the mailing list. But replacing "fan1_input" with "rpm" is
>>>> not an acceptable ABI change, even if it may measure something that turns
>>>> but isn't a fan.
>>>>
>>>> If this _is_ in fact supposed to be used for something else but fans, we
>>>> would have to discuss what that might be, and if hwmon is the appropriate
>>>> subsystem to measure and report it. This does to some degree lead back to
>>>> my concern of having the "fan" part of this patch series in the pwm core.
>>>> I am still not sure if that makes sense.
>>>>
>>>> Thanks,
>>>> Guenter
>>> I am planning to add tachometer support in pwm-fan.c driver (drivers/hwmon/) instead of adding new generic-pwm-tachometer.c driver. Measuring RPM value will be done in pwm-fan driver itself using pwm capture feature and will add new sysfs attributes under this driver to report rpm value of fan.
>>
>> There is an existing attribute to report the RPM of fans. It is called fan[1..n]_input.
>>
>> "replacing "fan1_input" with "rpm" is not an acceptable ABI change"
>>
>> Preemptive NACK.
> 
> The RPM is measured speed via PWM signal capture  which is output from fan.
> So should we have the fan[1..n]_output_rpm?
> 

No. I hear you clearly that you for some reason dislike fan[1..n]_input.
While ABIs are not always to our liking, that doesn't mean that we get
to change them at our whim. If that is not acceptable for you, I can't
help you. And you can't change inX_input to inX_voltage either, sorry.

Guenter