Enable IVB Cpu in t420 SNB laptop

I’ m trying to enable ivy bridge processors ( lower power consumption, lower tdp in quad core models, better integrated graphic, better overall performance ) in this SNB laptop using a modded bios.
I know that hardware incompatibility stated by lenovo ( and intel ) is not true because IVB processors are been reported working on this laptop using coreboot.
Up to now there are been various tries of doing it by adding IVB cpu microcode and updating vbios to a version with also IVB support, but with no success until now.
Looking at how coreboot managed to get IVB working ( especially in Windows ), seems that a key feature was to intialize his iGPU instead of SNB one, but I’ m not able to identify at which stage that happens in original bios.
I guess that changing that there are good chances of getting all working, also maybe at the cost of losing snb compatibilty.
Any suggestion for where to localize the right stage in bios structure?
I can provide bios files and maybe other useful infos if needed ( I’ ve spent a lot of time analyzing t420 bios, and also t430’s one as a comparison because it has both snb and ivb compatibility ).

Ivy Bridge processor support

Sandy Bridge and Ivy Bridge processors use the same socket, so an Ivy Bridge processor can be installed. To use native graphics init, you should use a patch on gerrit so that coreboot can use the correct code for native graphics init, this change has been merged to upstream and you need to change the Kconfig symbol SANDYBRIDGE_LVDS to SANDYBRIDGE_IVYBRIDGE_LVDS. You can also use a VGA option ROM with SeaBIOS. Manually adding VGA option ROMs for both type of GPUs is recommended (see the build instructions above).

1. I would use the latest bios which has vbios 2170 and supports ivy bridge (modified 1.46: Lenovo_ThinkPad_T420_83ET76WW-83uj28us_NWL_LEN21_ADV_RAM_Speedo_AES_vBios-2170)
2. add option in bios to select ivy bridge microcode.
3. Add option in bios to select intel hd 4000.