Dell XPS 15 9570 - How to control the fans?

I found a solution to my own problem.

To see the fans with lm_sensors

Add dell-smm-hwmon to modules:

sudo su
echo "options dell-smm-hwmon restricted=0 force=1" > /etc/modprobe.d/dell-smm-hwmon.conf
echo "dell-smm-hwmon" > /etc/modules
exit
sudo update-initramfs -u

Reboot, and the fans should be visible with:

sensors
dell_smm-virtual-0
Adapter: Virtual device
fan1:           0 RPM
fan2:           0 RPM

To take over control of your fans

Then follow this guide to install i8kutils:

sudo apt install i8kutils
sudo su 
echo "i8k" > /etc/modules
echo "options i8k force=1" > /etc/modprobe.d/i8k.conf
exit

Reboot:

sudo modprobe i8k force=1

Configure i8kutils:

sudo -H gedit /etc/i8kmon.conf

Finally, disable Dell BIOS Fan Control with this program by TomFreudenberg to let i8kutils take over.

git clone https://github.com/TomFreudenberg/dell-bios-fan-control.git
cd dell-bios-fan-control
make
sudo dell-bios-fan-control 0

Simply i8kmon service will be started and will control your fans. You can also disable i8kmon to try ik8fan to manually control your fans (not recommended as you might turn off your fans and they will never be started putting your CPU and GPU danger!).

i8kfan 2 2

A less intrusive method to get less temperature (and therefore no fans are required) is using powertop.

 cat /sys/bus/pci/devices/0000\:01\:00.0/power/control # (on?. That is bad)
 sudo powertop --auto-tune
 cat /sys/bus/pci/devices/0000\:01\:00.0/power/control # (auto?. That is good)

In order to get the changes permanently, see How do I make Powertop changes permanent? . However, in the help of TLP it is said that 'attempting to apply powertop's --auto-tune settings on each boot will conflict with TLP'. By now, it is working fine for me (XPS 15 9570, bios 1.5).