I am working on an on the go Retro-pie system. I needed a screen to go onto it so I picked up a KeDei 3.5 inch TFT LCD. However when I received it, I found out that it only works with a specific image of raspbian. Where can i get the drivers for the 6.3 2016/11/1 model of the screen and how do I install them into retropie and allow for simultaneous use of the lcd and HDMI port.
I've got similar KeDei 3.5 TFT LCD touchscreen using the SPI interface.
Regretfully, the device is NOT GOOD for Retropie due to very slow refresh rates.
There is a way to switch from LCD to HDMI, but required a reboot to switch.
As far as dual-monitor use, I had last looked into dual (or split) LCD and HDMI monitor use, but could not get that working. Long story short, I would not recommend wasting too much time on this LCD screen, it is mostly for static displays, not active gaming.
The SPI interface is a very slow framebuffer (fbtft) device. The screen refresh rate across the SPI (serial peripheral interface) is too slow for retropie. It will result in "screen tearing" and choppy video, with a frame rate of about 2-3 per second. This is unacceptable for games.
It's possible, but not overly usable as the LCD is pretty slow. You can get the LCD to mirror what's on the HDMI output.
Install the LCD Show drivers from here: http://kedei.net/raspberry/raspberry.html
Be sure and reboot, then download: https://github.com/tasanakorn/rpi-fbcp . Change output framebuffer device to be /dev/fb0 by changing line 41 of main.c in rpi-fbcp. Build and run as per the Readme.md and Emulationstation will be on the LCD too.