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How to make the legacy SeaBIOS firmware slot the default on a Haswell/Broadwell based Chromebook

Divulged this evening by Duncan Laurie on the coreboot mailing list:

If you want to boot SeaBIOS by default and you have unlocked the SPI flash write protection you can set flags in the (write protected) “GBB” flash region that will make it boot legacy mode by default.

In Chrome OS there is a script called set_gbb_flags.sh that will do this for you. Run the script with no arguments to get a list of possible flags and then to enable short dev mode screen (1 second timeout) followed by default legacy mode boot you could use these flags:

GBB_FLAG_DEV_SCREEN_SHORT_DELAY 0x00000001
GBB_FLAG_FORCE_DEV_SWITCH_ON 0x00000008
GBB_FLAG_FORCE_DEV_BOOT_LEGACY 0x00000080
GBB_FLAG_DEFAULT_DEV_BOOT_LEGACY 0x00000400

/usr/share/vboot/bin/set_gbb_flags.sh 0x489