I have recently started manually suspending1 my computer2 (Dell Precision T5500 - yes, it's a bit old...), and I have started noticing something strange when it wakes up again.

The motherboard fan starts "doing its own thing". Only the motherboard fan - all the others are normal.

Before suspending it's all nice and calm, and the fans react nicely to the cooling requirements. However after resume the motherboard fan starts running with a sinewave pattern, at about 0.25Hz. Audibly it's an "ooooEEEEooooEEEEoooo" sound.

Graphing that fan using lm-sensors and gnuplot shows the almost perfect sinewave form of the speed changes:

enter image description here

I've seen plenty of posts that talk about fans being full on after resume, but never anything with this bizarre kind of pattern. It's like the hysteresis of the fan control goes completely out of whack and it's reacting almost immediately to the smallest of changes, and effectively oscillating.

So, has anyone ever heard of anything like this before? Is there some way I can set the fan control hysteresis to prevent this kind of oscillation?

Ideally I'd like to be able to run a simple command that undoes whatever has been done by suspending - be that setting something in /sys or whatever.

By the way, fancontrol doesn't work well on my system - it can't get the fan speeds, even though the fan speeds are obviously changing when it tries them out, so it defaults to 100% speed on all fans, which makes it sound like a wind tunner.

17.10 lacks the pwmX_enable entries in /sys to be able to set the fan mode. Whether that's a change in Linux, or just something that's particular to the dell_smm hwmon I don't know. (Flicking through the source to dell-smm-hwmon.c it looks like it doesn't implement that interface).

  1. systemctl suspend
  2. Linux dell 4.13.0-32-generic #35-Ubuntu SMP Thu Jan 25 09:13:46 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux -- DISTRIB_DESCRIPTION="Ubuntu 17.10"
  • Are there any options in the BIOS to control this fan ? You say "motherboard fan" - so its directly on the mainboard (southbridge?) ? Do you see any strange temperatures with lm-sensors ? You can try to control it from ubuntu as well as the BIOS. – Robert Riedl Feb 14 '18 at 12:42
  • I shall do another graph in a moment of the mobo temperature to compare. – Majenko Feb 14 '18 at 12:56
  • Actually, I don't have access to that temperature information, only the fan speed. Note that the motherboard fan is distinct from the chipset fan in this system. I have processor, motherboard, and chipset fans. I guess I need to open her up and make sure of which fan is which. – Majenko Feb 14 '18 at 13:11
  • fancontrol doesn't like my system much. It complains it can't get the PWM values for the fans, and ends up ramping the fans all to 100% before exiting. – Majenko Feb 14 '18 at 13:38
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    By the way, when working properly the fan runs at a constant 1145RPM ±1RPM. There's nothing in the BIOS that I can find for configuring fans. – Majenko Feb 14 '18 at 13:45

Same problem with a T5500 BIOS A02, ubuntu 16.04. On wake up, the motherboard fan pulses and wins 500 rpm compared to normal state. enter image description here

On normal use: enter image description here

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