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This patch changes the intel_piix4_gpo_set() function to always check
the GENCFG and XBCS registers for the availability of the
requested GPO line before raising/lowering it and fails otherwise. It
makes no attempt to bypass the values in these configuration
registers.

The old flashrom code did consider it safe to reprogram (multiplexed)
GPO:s 22-26 without checking the value of the controlling register
(GENCFG). I do not really know why.

I have tested this patch on an Asus P2B-N (needs GPO18 low) and MSI
MS-6163 Pro (needs GPO14 high).

The information for these registers are from the Intel "82371AB
PCI-TO-ISA / IDE XCELERATOR (PIIX4)" datasheet available here:
http://www.intel.com/design/intarch/datashts/29056201.pdf



Corresponding to flashrom svn r1142.
Signed-off-by: Mattias Mattsson <vitplister@gmail.com>
Acked-by: Michael Karcher <flashrom@mkarcher.dialup.fu-berlin.de>