linux-kernel - Re: [PATCH v2 1/2] i8k: Autodetect maximal fan speed and fan RPM multiplier

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Message-Id: <201412201354.46435@pali>
Date:	Sat, 20 Dec 2014 13:54:46 +0100
From:	Pali Rohár <pali.rohar@...il.com>
To:	Guenter Roeck <linux@...ck-us.net>
Cc:	Arnd Bergmann <arnd@...db.de>,
	"Greg Kroah-Hartman" <gregkh@...uxfoundation.org>,
	linux-kernel@...r.kernel.org, Valdis.Kletnieks@...edu,
	Steven Honeyman <stevenhoneyman@...il.com>,
	Jean Delvare <jdelvare@...e.de>,
	Gabriele Mazzotta <gabriele.mzt@...il.com>,
	Jochen Eisinger <jochen@...guin-breeder.org>
Subject: Re: [PATCH v2 1/2] i8k: Autodetect maximal fan speed and fan RPM multiplier

On Saturday 20 December 2014 13:44:59 Guenter Roeck wrote:
> On 12/20/2014 04:18 AM, Pali Rohár wrote:
> > On Saturday 20 December 2014 13:04:03 Guenter Roeck wrote:
> >> On 12/20/2014 12:57 AM, Pali Rohár wrote:
> >>> On Friday 19 December 2014 20:28:08 Guenter Roeck wrote:
> >>>> On Fri, Dec 19, 2014 at 07:51:25PM +0100, Pali Rohár 
wrote:
> >>>>> On Friday 19 December 2014 19:32:37 Guenter Roeck wrote:
> >>>>>>> -static int i8k_fan_mult;
> >>>>>>> -static int i8k_pwm_mult;
> >>>>>>> -static int i8k_fan_max = I8K_FAN_HIGH;
> >>>>>>> +static int i8k_fan_mult[2];
> >>>>>>> +static int i8k_pwm_mult[2];
> >>>>>>> +static int i8k_fan_max[2];
> >>>>>> 
> >>>>>> The rationale for this change is not explained in the
> >>>>>> commit log.
> >>>>>> 
> >>>>>> Do you have any indication that those values would ever
> >>>>>> be different for the two fans, ie that you actually
> >>>>>> need arrays here ?
> >>>>> 
> >>>>> I do not know... But if we decide to use only single
> >>>>> value for multiplier and max value which fan to use for
> >>>>> autodetection?
> >>>> 
> >>>> That does not answer my question. That you can not decide
> >>>> which fan to use for auto-detection does not mean that
> >>>> the result of that auto-detection would be different for
> >>>> different fans.
> >>> 
> >>> Really I do not know if some dell products which have more
> >>> fans (some Precision models have 2) and each fan is using
> >>> different multiplier or has different max speed value.
> >> 
> >> "I do not know" is not a reason for introducing such code.
> >> 
> >> Guenter
> > 
> > And having broken fan rpm output in userspace is not good.
> 
> Sure, but only if it is in fact broken.
> 
> > My code introduce fan rpm detection for each fan. I do not
> > see any problem for doing this detection per fan. You
> > suggest to do some global detection and use it for each
> > fan. And for your
> 
> > suggestion I have 2 objections:
> The problem is that there is no evidence that a per-fan
> multiplier is needed. You may not see it that way, but
> unnecessary code _is_ a problem since it increases code size
> for no good reason. Furthermore, people not involved in this
> discussion will be inclined to believe that the code is
> necessary, in the wrong assumption that it would not have
> been introduced unless it was necessary.
> 
> > 1) I do not know if global multiplier will work for all
> > fans. Detection per fan does not have this problem "I do
> > not know".
> 
> It has been working all along, with no evidence that it
> doesn't work.
> 
> > 2) How to do that global multiplier detection? I have no
> > idea how you want to do that.
> 
> A single multiplier worked all along. It doesn't matter which
> fan is used to auto-detect the multiplier, it is still a
> single multiplier.
> 
> Again, introducing code if you don't know that or if it may be
> needed is not an argument for introducing such code. Introduce
> it if and when there is evidence that it is needed.
> 
> Thanks,
> Guenter

Ok, so what you suggest now? We have function which reports on 
supported models if speed level X is supported for fan Y and 
which RPM speed Z is expected for that level X. I have no problem 
to (re)write for fan multiplier detection and maximal fan speed 
level, but I need to know how to do that (which approach you 
accept and which not). What I want to have is correct data in 
userspace and better without hardcoded constants for each laptop 
in driver code.

-- 
Pali Rohár
pali.rohar@...il.com

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