This thread is about porting vanilla Debian (not Raspbian) to the Raspberry Pi. My goal is to provide an armel image that boots on all Pi models, an armhf image that boots on the Pi 2 and 3, and an arm64 image that boots on the Pi 3.
Debian has always been able to run on the Pi. Before Raspbian was released, the recommended OS image for the Pi was an Debian Squeeze armel image. This came with the disadvantages of software emulation for floating point tasks which was slower than using hardware for these tasks. When Debian Wheezy was released and had an armhf port (which requires an ARMv7 processor or newer), the RPF started compiling packages for the ARMv6 architecture of the original Pi.
With the release of the Pi 2 which had a quad-core ARM Cortex-A7, it was now possible to run vanilla Debian armhf on the Pi. There are a few blog posts around the internet about getting Debian armhf running on the Pi 2.
With the release of the Pi 3, which had a quad-core ARM Cortex-A53, Debian arm64 can also run on the Pi. A script is available for use with the generic arm64 kernel called raspi3-firmware which copies the kernel and initrd to the boot partition and configures the config.txt to boot from them, but the generic kernel is not yet ready for daily use on the Pi 3.
You can also compile the RPF kernel for ARM64 for use with Debian arm64. For Debian armhf, you can install Raspbian's kernel. For Debian armel, you have to compile the RPF kernel with support for running old ARM EABI binaries.
I provide images on GitHub based on Debian Stretch using the MATE and XFCE desktop and a 64-bit version of the RPF kernel. I haven't updated these images for a while, but I do plan on refreshing them at some point. For those interested, the GitHub page for these images is found here.
I plan on creating Debian armhf and armel images at some point, but don't expect images for each desktop environment for these architectures. I'll probably just create minimal images for Debian armhf and armel and update them every few point releases and every major release.
I'm excited to start creating Debian images for the Pi. Debian tends to have better software support than Raspbian. In Raspbian, Chromium is stuck at v65, whereas in Debian it's at v71.