The Android open-source project "AOSP" with its latest code is very close to being able to boot off the mainline Linux kernel when assuming the device drivers are all upstream.
Google's Satya Tangirala and Linaro's Sumit Semwal presented at this week's Linux Plumbers Conference on the state of Android on mainline Linux kernels.
Linux 5.9 moved the mainline tree one step closer to booting with AOSP with the inline encryption patches being merged this cycle. For being able to boot the mainline kernel "to [user interface] on AOSP/Master", for devices with fully upstream hardware support there is just one patch required. The open-source Android kernel tree is currently carrying around 485 patches atop upstream Linux 5.9 due to a lot of hardware/driver-specific work and other patches that haven't yet worked their way through the review and upstreaming process. Among the areas where Android is carrying kernel patches they are looking at upstreaming are for GKI enablement, DMA-BUF HEAPS, DRM core patches, and more.