linux-kernel - Re: [PATCH v5 00/11] hwmon: (w83627ehf) Add support for NCT6775F and NCT6776F

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Date:	Fri, 4 Mar 2011 11:56:51 -0800
From:	Guenter Roeck <guenter.roeck@...csson.com>
To:	Andrew Lutomirski <luto@....edu>
CC:	Jean Delvare <khali@...ux-fr.org>,
	Ian Dobson <i.dobson@...net-ian.com>,
	Randy Dunlap <rdunlap@...otime.net>,
	"binximeng@...il.com" <binximeng@...il.com>,
	"lmsensors@...anitarvainen.fi" <lmsensors@...anitarvainen.fi>,
	"andrea.rizzolo@...il.com" <andrea.rizzolo@...il.com>,
	"jeff.sadowski@...il.com" <jeff.sadowski@...il.com>,
	"lm-sensors@...sensors.org" <lm-sensors@...sensors.org>,
	"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5 00/11] hwmon: (w83627ehf) Add support for NCT6775F
 and NCT6776F

On Fri, 2011-03-04 at 13:53 -0500, Andrew Lutomirski wrote:
> On Fri, Mar 4, 2011 at 1:50 PM, Andrew Lutomirski <luto@....edu> wrote:
> > On Fri, Mar 4, 2011 at 12:57 PM, Guenter Roeck
> > <guenter.roeck@...csson.com> wrote:
> >> The following patch series adds support for NCT6775F and NCT6776F to the
> >> w83627ehf driver. It also includes some cleanup and adds support for
> >> the fourth temperature sensor on W83677HG-B.
> >>
> >> No code changes since v4. Resubmitting and widening audience in the hope that
> >> someone may find the time to review the changes prior to the next commit window.
> >
> > I'm not sure my review is worth much, but the driver seems to work.  I've tested
> > fan speed measurement, changing pwm2_enable to 1, changing the fan speed,
> > and changing back to 5.  Everything looks good.
> >
> > The only weird thing I noticed is that, the first time I ran sensors, I got:
> > fan2:                    0 RPM  (min =    0 RPM, div = 64)
> > and thereafter I get
> > fan2:                  136 RPM  (min =    0 RPM, div = 64)  ALARM
> >
> > I suspect (although I haven't checked) that sensors is just reading
> > div after it's already changed to 64.
> 
> I'll add one more weird thing on my NCT6775F.
> 
> [root@...night w83627ehf.656]# echo 3200 >fan2_min
> [root@...night w83627ehf.656]# cat fan2_div
> 8
> [root@...night w83627ehf.656]# cat fan2_input
> 133
> [root@...night w83627ehf.656]# cat fan2_div
> 16
> [root@...night w83627ehf.656]# cat fan2_input
> 133
> [root@...night w83627ehf.656]# cat fan2_div
> 32
> [root@...night w83627ehf.656]# cat fan2_input
> 133
> [root@...night w83627ehf.656]# cat fan2_div
> 64
> [root@...night w83627ehf.656]# cat fan2_input
> 132
> [root@...night w83627ehf.656]# cat fan2_div
> 64

This is actually as expected. rpm calculation is
	rpm = 1350000 / (rpm_reg * div)
In your case, this translates to
	rpm_reg * div ~ 8500
so for div < 64, the value of rpm_reg is larger than 255. If this
happens, the driver automatically increases fan_div until rpm_reg is
smaller than 255.

Reason for this is that while the chip has a 16 bit rpm register, the
min speed register is still only 8 bit wide. Plus, at least some
versions of NCT6775F report an rpm of 0 if the divisor is set too low.

Thanks,
Guenter


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