"Legacy USB Support"So what does it actually do? And what might these ominous "USB Legacy Devices" be?
Many answers are to be found about this one on the net. Reading one of them usually answered half the problem to me and reading two left me just as smart as before. Especially when said option is part of a bigger problem, usually with USB input devices that have mysteriously disappeared after rebooting or just won't be found by a freshly installed computer.
What "USB Legacy Support" really does is -despite the name- this: When enabled, the BIOS will report USB mice and/or keyboards to the operating system -or more important: to the OS' bootloader- as ("legacy") PS/2 devices, so you do not end up without mouse or keyboard if your OS does not have drivers for USB input devices installed yet. This requires, of course, that there are drivers for PS/2 mice and keyboards. So, if you just built a Linux kernel for a recent box and have not built "psmouse" or any other (legacy) PS/2 related modules you might end up with no mouse and keyboard support by GRUB and/or the initramfs or the kernel itself. While the latter can mostly be resolved by loading evdev.ko or usbhid.ko via modprobe, the first two possibilities might easily result in an unbootable system if there is an encrypted root file system or anything else requiring user input to proceed.
To set this straight:
- Activating Legacy USB support in the BIOS creates virtual PS/2 devices for your USB mouse and keyboard in case that your legacy BIOS, boot loader, SCSI-adapter or the like does not provide USB-input support at boot time
- it has nothing to do with USB to PS/2 (or vice versa) adapters, "old mice" or any kind of physically existing legacy hardware
- it might also be helpful if your USB-HID compatible input device is not identified correctly at boot time (especially, when some sort of passphrase or encryption key is required before full HID support is available - think "initrd")
- if you don't need it, try turning it off to make sure that your USB/HID devices are recognized by the OS.