Hi everyone i apologize for my bad English, i have installed sierra on my latitude e6420 i5 1366x768 everything works perfectly except the external hdmi port, if i connect the cable the tv becomes all white and i just use the Dell Latitude E6220 with I5-2520M, HD3000 and 1366x768 LCD - Mavericks / Yosemite / El Capitan / Sierra I reinstalled 2 times all but without results I tried the kext patch but nothing I tried various dsdt but nothing. At startup I noticed 3 kext mistakes with this message:
The following symbols are unresolved for this kept: _gIODisplayBrightnessProbKey _gIODisplayLinearBrightnessProbKey
maybe the problem is that but I do not know how to fix it.