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.. SPDX-License-Identifier: GPL-2.0
USB Legacy support
:Author: Vojtech Pavlik <>, January 2004
Also known as "USB Keyboard" or "USB Mouse support" in the BIOS Setup is a
feature that allows one to use the USB mouse and keyboard as if they were
their classic PS/2 counterparts. This means one can use an USB keyboard to
type in LILO for example.
It has several drawbacks, though:
1) On some machines, the emulated PS/2 mouse takes over even when no USB
mouse is present and a real PS/2 mouse is present. In that case the extra
features (wheel, extra buttons, touchpad mode) of the real PS/2 mouse may
not be available.
2) If CONFIG_HIGHMEM64G is enabled, the PS/2 mouse emulation can cause
system crashes, because the SMM BIOS is not expecting to be in PAE mode.
The Intel E7505 is a typical machine where this happens.
3) If AMD64 64-bit mode is enabled, again system crashes often happen,
because the SMM BIOS isn't expecting the CPU to be in 64-bit mode. The
BIOS manufacturers only test with Windows, and Windows doesn't do 64-bit
Problem 1)
can be solved by loading the USB drivers prior to loading the
PS/2 mouse driver. Since the PS/2 mouse driver is in 2.6 compiled into
the kernel unconditionally, this means the USB drivers need to be
compiled-in, too.
Problem 2)
can currently only be solved by either disabling HIGHMEM64G
in the kernel config or USB Legacy support in the BIOS. A BIOS update
could help, but so far no such update exists.
Problem 3)
is usually fixed by a BIOS update. Check the board
manufacturers web site. If an update is not available, disable USB
Legacy support in the BIOS. If this alone doesn't help, try also adding
idle=poll on the kernel command line. The BIOS may be entering the SMM
on the HLT instruction as well.