Here you will get a list of fan speed devices in linux:

root@simpleminer:~/utils# ls -1 /sys/class/drm/card*/device/hwmon/hwmon*/pwm1
/sys/class/drm/card1/device/hwmon/hwmon0/pwm1
/sys/class/drm/card2/device/hwmon/hwmon1/pwm1
/sys/class/drm/card3/device/hwmon/hwmon2/pwm1
/sys/class/drm/card4/device/hwmon/hwmon3/pwm1
/sys/class/drm/card5/device/hwmon/hwmon4/pwm1
/sys/class/drm/card6/device/hwmon/hwmon5/pwm1
root@simpleminer:~/utils# cat /sys/class/drm/card1/device/hwmon/hwmon0/pwm1
153

If you want to convert this value to % then you need to make simple calculation 153/254 which is 0.6
Multiply that by 100 and you have 60% fan speed.
So the max speed for gpus is 254 and you can inset that speed to the system by using command:
as root: echo "254" > /sys/class/drm/card1/device/hwmon/hwmon0/pwm1
But renember that my script will overwrite fan speed in system.
(acculaty it could be 254, 255 or 256, i dont renember what was top value