eDP interface

== Theory ==
Stub article related to the troubleshooting of eDP displays + some modifications made by Apple.
There are 5 main parts of the common eDP connector:
#TCON / LCD Controller Power supply. Depending on panel type it might be 3, 5 or 12V as a main LCD VDD power supply. This supply is always present.
#Service or Auxiliary bidirectional differential pairs to transfer service information.- Standard line is eDP_AUX(N+P), however on macs we would see also a TCON SMBUS connected to SMC(for example, SMC reads LCD temp via this line). On MacBook (see MacBook backlight troubleshooting page) there is also an extra i2c line which connects two parts of backlight driver. Depending on realisation, there might be two AUX lines (on iMacs) each would be connected to Intel / AMD GPU. On MacBooks, there would be one AUX line, which would be later routed by GMUX.
#1-4+ Data lines, depending on refresh rate / max panel resolution
#Backlight control lines, depending on what exact implementation do we have. There might be sync signal to sync backlight with panel refresh rate, there might be EN signal, or as mentioned in nr.2 - i2c line to bind two backlight drivers together. You can also see PWM signal as a secondary backlight control on some MacBooks and most PC laptop screens.
#HPD (Hot Plug Detect) signal, which works as an indicator that panel is on and ready to send/receive data.
There might be some more lines like Microphone or Touchscreen/Camera lines on PC laptop display connectors, but these are not related to eDP itself. 
The common power sequence would be: 
#LCD-VCC provided to Display
#Display initializes and resets LCD panel
#HPD goes high (1.5-3V depending on panel type) on the display side. this tells the mainboard that it is time to communicate with panel
#Display controller (GPU) reads panel parameters via EDP_AUX and also sets its display mode.
#Signal Differential pairs are being trained and display starts to render the image. -- On iMac screens, there is also VIDEO_ON signal which turns on the backlight circuit on the mainboard. This is probably made to avoid any visible artefacts before the display panel is fully reset. This also turns on indicates 4th DEBUG LED. -- If backlight driver is located in display/tcon, Backlight enable will be sent by Mainboard to LCD panel.
#Backlight should be turned on. On most modern panels it only turns on if there was no issue in sending image to the panel.
This is iMac 2017 eDP connector pinout. Pinout is quite different from standard eDP interface, but mostly it works very similar.
== Troubleshooting ==
# Check condition of the screen. Is it recognized (does external screen show login or only background as it was secondary screen)? Does it have image but no backlight? If so, for MacBook backlight troubleshooting refer to this page or iMac Backlight Troubleshooting here.
# If there is no picture, start with checking if there is a power supply provided to the screen.
# Check signal lines in diode mode, separately on screen/mainboard side. N and P lines of the same pair should have very similar readings.
# Check HPD signal. If it's present, there should be communication at least on AUX lines.
# If you have communication, backlight but no image, most likely the display itself is damaged. On MacBook screen, there is an integrity check wire around the panel which disables the image if the controller sees glass as "broken". It happens really often through "security" camera shutter glued on the glass on Macbook screens.