Re: [lm-sensors] Report: fan control for IT8620E

[Guenter Roeck <linux@roeck-us.net>]

On 03/30/2015 12:25 AM, Jean Delvare wrote:
> Hi Dan,
>
> On Thu, 19 Mar 2015 17:44:40 +0200, Dan Moinescu wrote:
>> Here's a report on how I managed to control the CPU fan on a
>> motherboard with the IT8620E chip, as suggested by the lm-sensors
>> wiki: `you can try loading the it87 driver with force_id=0x8728 and
>> see how it goes. If you do, please report, including the output of
>> "sensors"`.
>>
>> First, a quick word on the hardware used. The motherboard is a
>> Gigabyte GA-B85M-DS3H
>> (http://www.gigabyte.com/products/product-page.aspx?pidG31)  and the
>> CPU cooler is an Arctic Cooling Alpine 11 Plus
>> (http://www.arctic.ac/eu_en/alpine-11-plus.html)
>> According to the cooler's technical data, the fan can run at speeds
>> between 500 - 2,000 RPM. However, the BIOS/EFI/UEFI (whatever the
>> proper name is nowadays) would not let me set the PWM value to less
>> than 70, which produces a fan speed of about 1300 RPM (too noisy for
>> my taste).
>> So below is a description of what I did to bring the fan to minimum
>> speed. This was done strictly by trial and error; I do not claim this
>> is the only (or the best) method, it just works for me.
>>
>> sensors-detect reported "Found unknown chip with ID 0x8620".
>> I inserted the it87 module as directed by the wiki page:
>> modprobe it87 force_id=0x8728
>> After this I tried pwmconfig, but it didn't succeed in controlling the
>> fan (it brought it up to full speed and left it in that state). So
>> then I tried a manual approach.
>>
>> Inserting the module creates a directory inside /sys, such as
>> /sys/devices/platform/it87.2608/:
>>      sysdir=$(find /sys/ -name it87.\* -type d)
>> Controlling the fans is possible by modifying various files (mainly
>> pwm1) inside that directory.
>>
>> The first step is to modify the value inside pwm1_enable from 2 to 1.
>> This is needed because if pwm1_enable is left at its initial value
>> (2), pwm1 cannot be changed (attempting to do that produces a "Device
>> or resource busy" error).
>>      echo 1 > $sysdir/pwm1_enable
>
> This is expected, mode 2 is an automatic (hardware-driven) fan control
> mode so the pwm1 file is essentially read-only in that mode.
>
>> Next the desired pwm value can be written into $sysdir/pwm1. Anything
>> from 0-8 puts the fan at full speed, and 9 puts it to around 1600 RPM.
>> Values from 10-255 seem to act as actual pwm values. Initial value was
>> 70 (the value set in BIOS).
>>      echo 10 > $sysdir/pwm1
>
> Now that is weird and this is why pwmconfig got confused. The script
> typically sets pwmN to 0 and checks if any fan speed wnet down. If
> setting pwm1 to 0 puts the fan to full speed then the script
> erroneously concluded that the pwm1 output did not control any fan.
>
> We do not have a datasheet for this part (although Gigabyte offered to
> provide some information about it) and from your results it's hard to
> figure out how the register works. I would understand if values 0-127
> do something and values 128-255 do something else (high bit has a
> special meaning) or if odd values do something and even values do
> something else (low bit has a special meaning) but your findings are
> nothing like that.
>

It might be that this is a fan specialty, ie that the fan acts odd if
presented with low duty cycles. Another might be that the low pwm values
are lower than some minimum programmed into the chip, and that this causes
the odd behavior. No idea which register that might be, though.

Guenter


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