linux/Documentation/x86/usb-legacy-support.rst
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   2.. SPDX-License-Identifier: GPL-2.0
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   4==================
   5USB Legacy support
   6==================
   7
   8:Author: Vojtech Pavlik <vojtech@suse.cz>, January 2004
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  10
  11Also known as "USB Keyboard" or "USB Mouse support" in the BIOS Setup is a
  12feature that allows one to use the USB mouse and keyboard as if they were
  13their classic PS/2 counterparts.  This means one can use an USB keyboard to
  14type in LILO for example.
  15
  16It has several drawbacks, though:
  17
  181) On some machines, the emulated PS/2 mouse takes over even when no USB
  19   mouse is present and a real PS/2 mouse is present.  In that case the extra
  20   features (wheel, extra buttons, touchpad mode) of the real PS/2 mouse may
  21   not be available.
  22
  232) If CONFIG_HIGHMEM64G is enabled, the PS/2 mouse emulation can cause
  24   system crashes, because the SMM BIOS is not expecting to be in PAE mode.
  25   The Intel E7505 is a typical machine where this happens.
  26
  273) If AMD64 64-bit mode is enabled, again system crashes often happen,
  28   because the SMM BIOS isn't expecting the CPU to be in 64-bit mode.  The
  29   BIOS manufacturers only test with Windows, and Windows doesn't do 64-bit
  30   yet.
  31
  32Solutions:
  33
  34Problem 1)
  35  can be solved by loading the USB drivers prior to loading the
  36  PS/2 mouse driver. Since the PS/2 mouse driver is in 2.6 compiled into
  37  the kernel unconditionally, this means the USB drivers need to be
  38  compiled-in, too.
  39
  40Problem 2)
  41  can currently only be solved by either disabling HIGHMEM64G
  42  in the kernel config or USB Legacy support in the BIOS. A BIOS update
  43  could help, but so far no such update exists.
  44
  45Problem 3)
  46  is usually fixed by a BIOS update. Check the board
  47  manufacturers web site. If an update is not available, disable USB
  48  Legacy support in the BIOS. If this alone doesn't help, try also adding
  49  idle=poll on the kernel command line. The BIOS may be entering the SMM
  50  on the HLT instruction as well.
  51