HiDPI on Mac OS X EL Capitan, Dell U3011



El Capitan:

Turn off System Integrity Protection so we can copy file to /System

Step 1:

sudo defaults write /Library/Preferences/com.apple.windowserver DisplayResolutionEnabled -bool YES;
sudo defaults delete /Library/Preferences/com.apple.windowserver DisplayResolutionDisabled;

Step 2:

Create an override file for your display. You may use my script to do that.

sudo ruby hidpi_osx.rb

Step 3:

Reboot, HiDPI mode is available.