[microcolonel]

Most UEFI in the wild is based on Tianocore/EDK-II, which has about 1.5 million SLOC, plus about 400,000 source lines in header files. Some subset of this would actually be running on the device, but this doesn't include the drivers and the very base of the firmware. Let's say maybe two million SLOC is running on a typical UEFI firmware blob.

Linux also lacks the most basic level of the firmware (which is most likely handled Coreboot? I'm not fully clear on what NERF is overall). A much smaller proportion of Linux codebase would be running on a given device, as compared to the EDK-II, though. Who knows. /kernel and /arch/x86 together make about 440k SLOC.

Funnily enough, this is actually the second time a "Linux BIOS" has been tried. The first time was called LinuxBIOS (later renamed to Coreboot), and the idea was quite literally to put a kernel on ROM and have as little firmware as possible to get to a booted system.