linux-kernel - Re: [PATCH RFC 1/3] dt-bindings: hwmon: Add tachometer interrupt to pwm-fan

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Message-ID: <20190131171947.GA14844@roeck-us.net>
Date:   Thu, 31 Jan 2019 09:19:47 -0800
From:   Guenter Roeck <linux@...ck-us.net>
To:     Stefan Wahren <stefan.wahren@...e.com>
Cc:     Kamil Debski <kamil@...as.org>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
        Jean Delvare <jdelvare@...e.com>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        linux-hwmon@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC 1/3] dt-bindings: hwmon: Add tachometer interrupt to
 pwm-fan

On Thu, Jan 31, 2019 at 09:02:33AM +0100, Stefan Wahren wrote:
> Hi Guenter,
> 
> Am 30.01.19 um 21:35 schrieb Guenter Roeck:
> > On Wed, Jan 30, 2019 at 09:23:31PM +0100, Stefan Wahren wrote:
> >> Hi Guenter,
> >>
> >>> Guenter Roeck <linux@...ck-us.net> hat am 30. Januar 2019 um 18:28 geschrieben:
> >>>
> >>>
> >>> On Wed, Jan 30, 2019 at 04:07:05PM +0100, Stefan Wahren wrote:
> >>>> This adds the tachometer interrupt to the pwm-fan binding, which is
> >>>> necessary for RPM support.
> >>>>
> >>>> Signed-off-by: Stefan Wahren <stefan.wahren@...e.com>
> >>>> ---
> >>>>  Documentation/devicetree/bindings/hwmon/pwm-fan.txt | 3 +++
> >>>>  1 file changed, 3 insertions(+)
> >>>>
> >>>> diff --git a/Documentation/devicetree/bindings/hwmon/pwm-fan.txt b/Documentation/devicetree/bindings/hwmon/pwm-fan.txt
> >>>> index 49ca5d8..7f69b0b 100644
> >>>> --- a/Documentation/devicetree/bindings/hwmon/pwm-fan.txt
> >>>> +++ b/Documentation/devicetree/bindings/hwmon/pwm-fan.txt
> >>>> @@ -8,6 +8,9 @@ Required properties:
> >>>>  
> >>>>  Optional properties:
> >>>>  - fan-supply    : phandle to the regulator that provides power to the fan
> >>>> +- interrupts    : contains a single interrupt specifier which describes the
> >>>> +                  tachometer pin output of a 2 pulse-per-revolution fan.
> >>>> +                  See interrupt-controller/interrupts.txt for the format.
> >>> So a hypothetical {1,4} pulse-per-revolution fan would explicitly not be
> >>> supported ? Why ?
> >> i could add an additional property to specify the pulse per revolution and use the 2 as default (according to the Intel spec for 4 pin pwm fan) which should fit in most cases.
> >>
> > That would be a possibility and make sense, but that is not
> > the point here.  The "interrupts" property does not and should
> > not care how many pulses per revolution the fan provides.
> 
> sorry, i'm not sure what's the problem about. Do you want me to use a
> GPIO instead of interrupt?
> 
> Or is it the wording here?
> 

You wording limits the use of interrupts with pwm fans to fans with
2 pulses per revolution. You do not explain why that restriction would
be required or even make sense, or why it should be associated with
the 'interrupts' property.

Logically I assume that is because you expect an interrupt per pulse,
but that is not explained (or what the interrupt is expected to be
used for in the first place - it might, after all, be some kind of
error interrupt).

Guenter