linux/Documentation/x86/usb-legacy-support.txt
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   1USB Legacy support
   2~~~~~~~~~~~~~~~~~~
   3
   4Vojtech Pavlik <vojtech@suse.cz>, January 2004
   5
   6
   7Also known as "USB Keyboard" or "USB Mouse support" in the BIOS Setup is a
   8feature that allows one to use the USB mouse and keyboard as if they were
   9their classic PS/2 counterparts.  This means one can use an USB keyboard to
  10type in LILO for example.
  11
  12It has several drawbacks, though:
  13
  141) On some machines, the emulated PS/2 mouse takes over even when no USB
  15   mouse is present and a real PS/2 mouse is present.  In that case the extra
  16   features (wheel, extra buttons, touchpad mode) of the real PS/2 mouse may
  17   not be available.
  18
  192) If CONFIG_HIGHMEM64G is enabled, the PS/2 mouse emulation can cause
  20   system crashes, because the SMM BIOS is not expecting to be in PAE mode.
  21   The Intel E7505 is a typical machine where this happens.
  22
  233) If AMD64 64-bit mode is enabled, again system crashes often happen,
  24   because the SMM BIOS isn't expecting the CPU to be in 64-bit mode.  The
  25   BIOS manufacturers only test with Windows, and Windows doesn't do 64-bit
  26   yet.
  27
  28Solutions:
  29
  30Problem 1) can be solved by loading the USB drivers prior to loading the
  31PS/2 mouse driver. Since the PS/2 mouse driver is in 2.6 compiled into
  32the kernel unconditionally, this means the USB drivers need to be
  33compiled-in, too.
  34
  35Problem 2) can currently only be solved by either disabling HIGHMEM64G
  36in the kernel config or USB Legacy support in the BIOS. A BIOS update
  37could help, but so far no such update exists.
  38
  39Problem 3) is usually fixed by a BIOS update. Check the board
  40manufacturers web site. If an update is not available, disable USB
  41Legacy support in the BIOS. If this alone doesn't help, try also adding
  42idle=poll on the kernel command line. The BIOS may be entering the SMM
  43on the HLT instruction as well.
  44
  45
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