With the DeviceTree addition for the Raspberry Pi 400. No kernel driver changes were needed since it’s basically very close to the Raspberry Pi 4 but the updated DTS configuration is needed for the 1.8GHz clock rate, a different WiFi chip, power off handling via GPIO, and no ACT LED on the 400 model.
Source: Linux 5.14 Mainline Should Work With The Raspberry Pi 400 – Phoronix