For the simplest case, where you have contiguous memory for your framebuffer storage and no MMIO, try the CMA helpers: https://github.com/torvalds/linux/blob/master/drivers/gpu/dr...

I wrote a basic device driver for a platform I worked on. It's since been upstreamed and gotten more eyeballs, but it's still relatively simple. https://github.com/torvalds/linux/blob/master/drivers/gpu/dr...