linux-kernel - Re: [lm-sensors] Tachometer speed returned rather than absolute fan speed?

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Date:	Tue, 8 Apr 2014 11:19:43 +0200
From:	Pavel Machek <pavel@....cz>
To:	Guenter Roeck <linux@...ck-us.net>
Cc:	Laszlo Papp <lpapp@....org>, LKML <linux-kernel@...r.kernel.org>,
	lm-sensors@...sensors.org
Subject: Re: [lm-sensors] Tachometer speed returned rather than absolute
 fan speed?

On Sat 2014-03-08 15:50:49, Guenter Roeck wrote:
> On 03/07/2014 10:17 AM, Guenter Roeck wrote:
> >On Fri, Mar 07, 2014 at 03:47:08PM +0000, Laszlo Papp wrote:
> >>On Fri, Mar 7, 2014 at 3:37 PM, Jean Delvare <jdelvare@...e.de> wrote:
> >>>>>I'm quite confused. While I admit that the term "tachometer speed" is
> >>>>>awkward, the max6650 driver is reporting fan speeds in RPM as every
> >>>>>other hwmon driver. So I really have no idea what you think is wrong.
> >>>>>What did you think "tachometer speed" was, if not the fan speed? Does
> >>>>>the max6650 driver not return correct fan speeds for you?
> >>
> >>That is some strange behavior. If I do "echo 1 > pwm1_enable; echo 0 >
> >>pwm1; cat fan1_input", I still see 30 for the connected fan, whereas I
> >>can see it stopped. Is this an expected behavior? I would expect zero
> >>as a user.
> >>
> >I seem to recall that I had seen that as well, with no fan connected.
> >Maybe the tachometer registers always read at least '1'. I would think
> >it is wrong, but we'll have to understand the chip a bit better
> >to be able to provide a fix. Unless you already have a fix ready,
> >of course. I'll try to re-test tonight if I find the time.
> >
> 
> The reason is (most likely) that your fan input does not have a pull-up
> resistor. Per datasheet, the fan inputs need a 10kOhm pull-up resistor.
> I confirmed this with my test board - with the pull-up resistor,
> inputs read 0, Without pull-up, the reported value is 1, which
> translates to 30 RPM.

While this is hardware problem, it might make sense for kernel to work
around it -- return 0 in case 30 RPM observed.
									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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