Q: Can I modify PCH soft straps from bootblock?

All,

A use case for the title is beginning to emerge for a number of Asus
P8x7x boards. For example:
https://review.coreboot.org/c/coreboot/+/85413 and
https://review.coreboot.org/c/coreboot/+/85768

Many standard ATX boards in the family have more peripherals than the
8 PCIe lanes available out of the PCH, so some are structured to share
3 of the 4 PCIe lanes originally earmarked for an x4 slot. They do
this with a number of 2-way PCIe switches (specifically ASM1440 - no
datasheet, but I figured out the pinout, only thing I don't know is
the polarity of the control signal) controlled by GPIOs either on the
super I/O or the PCH. With vendor firmware this is an option that can
be configured from UEFI setup itself.

I can handle GPIOs fine, but stealing lanes from an x4 slot requires
changing a soft strap to reconfigure lanes 1-4 from one x4 port to
four x1 ports.

The only strategy I can see work is flash a modified soft strap to the
descriptor, during bootblock before PCI bus scan happens and before
SPI is locked down, trigger a full platform reset (eg. outb(0xe,
0xcf9)) , then program the GPIOs to match the updated soft strap.

Has this been done before in coreboot so there is code I can lift?

Thanks
Keith
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