Date:	Sun, 9 Mar 2014 12:44:29 +0000
From:	Laszlo Papp <lpapp@....org>
To:	Guenter Roeck <linux@...ck-us.net>
Cc:	LKML <linux-kernel@...r.kernel.org>, lm-sensors@...sensors.org
Subject: Re: [lm-sensors] Tachometer speed returned rather than absolute fan speed?

On Sun, Mar 9, 2014 at 8:04 AM, Guenter Roeck <linux@...ck-us.net> wrote:
> On 03/08/2014 10:36 PM, Laszlo Papp wrote:
>>
>> On Sat, Mar 8, 2014 at 11:50 PM, Guenter Roeck <linux@...ck-us.net> 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.
>>>
>>> You might also need the 10 uF capacitor on the FB pin.
>>
>>
>> Hmm, interesting, but then I wonder how it works fine when getting the
>> data from userspace with ioctl (i.e .without the driver) through
>> /dev/i2c-1. There must be some trick in that case that I am not yet
>> aware of. I will double check the schematics on Monday.
>>
>
> With i2cdump on my system I can see that the tachometer register
> holds 0 if the resistor is in place, and the sensors command reports
> a fan speed or 0 rpm. If the resistor is not in place, the register
> content is 1 and sensors reports 30 rpm.
>
> sudo i2cdump -y -f 1 0x48
> No size specified (using byte-data access)
>      0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f    0123456789abcdef
> 00: 00 00 0a 0a ff ff 00 00 00 00 00 00 01 01 00 00    ..??........??..
> 10: 01 01 01 01 1f 1f 02 02 91 91 91 91 91 91 91 91    ????????????????
> 20: 00 00 0a 0a ff ff 00 00 00 00 00 00 01 01 00 00    ..??........??..
>
> groeck@...ktop:~$ sensors max6651-i2c-1-48
> max6651-i2c-1-48
> Adapter: i2c-diolan-u2c at bus 003 device 002
> fan1:          30 RPM  (div = 4)
> fan2:           0 RPM
> fan3:          30 RPM
> fan4:          30 RPM
>
> Only fan2 (TACH1 chip input) has the resistor installed.

Hmm, ok. Thank you for the detailed reply!
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