nl80211: missing minimum TX power attribute causes misleading userspace behavior

[Steffen May <steffen.may@posteo.de>]

This issue was discovered during the analysis of two documented OpenWrt 
bugs. Bug 1 is a type comparison error in LuCI wireless.js that causes 0 
dBm to be displayed incorrectly. Bug 2 is in ucode mac80211.sh where the 
value 0 is treated as falsy, causing the router to transmit at maximum 
power instead. Both bugs are proven and reported.

During verification of Bug 2 on five devices with three different 
chipsets, it became apparent that the hardware floor is device-dependent 
and completely unknown to the stack. This is not a bug but a missing 
feature.

Because the floor is unknown, iwinfo generates selection lists 
containing values that have no real effect on the actual output power. 
Userspace accepts configurations such as 0 dBm even though the hardware 
cannot apply them. The system reports success while the hardware remains 
at its minimum supported level. This creates false assumptions.

Measurements

The deviation between the requested transmit power and the actual 
hardware floor is not constant but depends on the hardware:

Device                Chipset            Requested     Actual floor   
Difference
Buffalo WZR-600DHP    Atheros AR7161     1 dBm         3 dBm          +2 
dB
TP-Link Archer C7     Qualcomm QCA9558   1 dBm         5 dBm          +4 
dB
OpenWrt One           MTK Filogic        1 dBm         1 dBm           0 
dB
Cudy WR3000           MTK Filogic        1 dBm         1 dBm           0 
dB
GL.iNet GL-MT6000     MTK Filogic        1 dBm         1 dBm           0 
dB

Concrete example: Buffalo WZR-600DHP with Atheros AR7161

Because the system does not know the actual floor, the list generated by 
iwinfo contains values with no real effect:

0 dBm (1 mW)  reported, hardware remains at floor
1 dBm (1 mW)  reported, no actual reduction
2 dBm (1 mW)  reported, no actual reduction
3 dBm (1 mW)  actual hardware floor
4 dBm (2 mW)  first real power step above the floor

Proposed solution

Introduction of a new nl80211 attribute NL80211_ATTR_WIPHY_TX_POWER_MIN. 
nl80211 already exposes the maximum transmit power. Minimum and maximum 
are both fixed hardware properties -- it is inconsistent that only one 
of the two values is available.

No patch is provided. This mail is intended to describe the problem and 
to encourage interest in an implementation.