I bought a a SPI controlled display 4DPi-32  and compiled a kernel patched with the driver source  provided by the vendor (because the provided deb does not work with RPi2). I had to adapt the Kconfig file in the patch to work with the kernel 3.18.9-v7+ on a RPi2. Here is my 'patched kernel patch' for your convenience .
So I believe a now have a kernel running with the 4dpi driver. The only hint in /var/log/messages is:
[ 1.544568] Console: switching to colour frame buffer device 82x26
I have a device /dev/fb0
Now, the display is attached, but it is either:
a) completly off b) only the backlight off c) defect
Question 1: can I check the driver works?
Question 2: can I check if the display is working at all?
Question 3: how is the backlight of the display controlled?