hi Vadim,
   i am trying to flashing the gigabyte dual bios at this moment, have you ever tried to boot coreboot from the main bios?
i flashed the main bios, but the board always did the checksum failed, the restore the bios to the legacy bios.
Have you ever tried that? if flash the backup chip, how to make it boot first?

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Wang Qing Pei
Phone: 86+13426369984