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Today I had a look to t420 original/modded ( vbios 2170) bios, and t430 original bios.
First of all I discovered, according to intel bmp, that in modded vbios IVB chipset is selected. Nevertheless that bios works with SNB cpu, not with IVB. After a little investigation, i found some info about intel graphic driver that seems to explain why.
For the uefi boot mode, DXE core loads GOP policy driver, then it locates and loads video bios table into memory, then gop driver start with those settings.
So I started to look in bios for the policy module, looking specifically for the $vbt string, and that' s what I' ve found:
In t420 ( SNB) sainitdxe module has one $vbt reference, in t430 ( SNB & IVB compatible ) two, also t430 has just before ivbgop and snbgop modules, two very similar modules with the $vbt string and that differs for chipset code ( 18-snb,20-ivb ). My idea is that two modules are the actual chipset setting loaded by gopdrivers, so in order to enable IVB on t420 we' ve to find video bios table module in bios and try to work on it.