Bug 110213

Summary: Used pwm value is lesser than the requested value
Product: DRI Reporter: jpalaciosdev
Component: DRM/AMDgpuAssignee: Default DRI bug account <dri-devel>
Status: RESOLVED NOTABUG QA Contact:
Severity: normal    
Priority: medium    
Version: unspecified   
Hardware: x86-64 (AMD64)   
OS: Linux (All)   
Whiteboard:
i915 platform: i915 features:
Attachments:
Description Flags
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Description jpalaciosdev 2019-03-20 16:50:24 UTC 
Created attachment 143743 [details]
dmesg

On manual fan control through pwm interface, the used value on pwm1 is lesser that the requested value (except for maximum).
For example:
echo 1 > /sys/class/drm/card0/device/hwmon/hwmon1/pwm1_enable

echo 10 > /sys/class/drm/card0/device/hwmon/hwmon1/pwm1
cat /sys/class/drm/card0/device/hwmon/hwmon1/pwm1
5

echo 100 > /sys/class/drm/card0/device/hwmon/hwmon1/pwm1
cat /sys/class/drm/card0/device/hwmon/hwmon1/pwm1
96

echo 254 > /sys/class/drm/card0/device/hwmon/hwmon1/pwm1
cat /sys/class/drm/card0/device/hwmon/hwmon1/pwm1
249

The maximum value works as expected:
echo 255 > /sys/class/drm/card0/device/hwmon/hwmon1/pwm1
cat /sys/class/drm/card0/device/hwmon/hwmon1/pwm1
255

Other info:
Hardware: MSI RX480 Gaming (Polaris)
Kernel: linux 5.0.2

Please, let me know if more info is needed.
Thanks.
Comment 1 Alex Deucher 2019-03-21 02:36:06 UTC 
The hwmon API specifies the range as 0-255, but the hw actually has a different range.  The conversion between ranges for read and writes accounts for the difference.  See smu7_fan_ctrl_get_fan_speed_percent() and smu7_fan_ctrl_set_fan_speed_percent() smu7_thermal.c and amdgpu_hwmon_get_pwm1() and amdgpu_hwmon_set_pwm1() amdgpu_pm.c.

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