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

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Date:   Thu, 8 Mar 2018 11:36:24 +0530
From:   Laxman Dewangan <ldewangan@...dia.com>
To:     Guenter Roeck <linux@...ck-us.net>,
        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 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?