Many thanks to Michael Karcher for the useful comments. Here is a new try at this patch using the table-based approach as he suggested.
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
Signed-off-by: Mattias Mattsson firstname.lastname@example.org