I have at least a few Raspberry Pi's that are programmed to simply boot and hit the aux out. This could be the difference between my car's white noise system coming online before I even shift the car out of park, vs. when I'm in the roundabout a few hundred feet away. Lifechanging.

Curious: why does your car have a white noise system?

> Come to think of it, doesn't the SD card slot sit off of the USB bus?

It doesn't.

You're probably thinking of the ethernet controller, which is often cited for the Pi 3 and earlier.

Theoretically (I have no idea about the Pi specifically) it's entirely possible for the bootloader to speak USB, load the kernel + initramfs from the device into memory and then then boot a kernel which knows nothing about USB.

This can be done with (for instance) u-boot.

If it did it wouldn't be able to boot, right? The OS/bootloader is on the SD.

The bootloader would still support SD, but the kernel would come up just fine without supporting it, as by the time the kernel is booting it's already in memory. You just wouldn't be able to mount any filesystems from that kernel, so you'd be stuck with whatever userland you packed into your initramfs

Most the drivers can be configured as ko and one can load them later on - such as after GUI is up. It should give impression of fast boot.