[coreboot] cmos.layout upgradability

[Vladimir 'φ-coder/phcoder' Serbinenko]

Hello, all. Due to my failue to foresee this problem if one upgrades X60
coreboot, you need to manually reset CMOS config or your trackpad may
not work as option "trackpad_enable" will get a garbage value. While I
admit my part of fault, I feel like we should have a way to update
options safely. I see following possibilities:
1) Hide ourselves and tell that on upgrade you always have to clear
CMOS. I don't think it's really an option as users will continue just
running flashrom and in case of external flashing, CMOS reset may
require full power removal with cell battery sometimes in difficult to
access places inside laptop.
2) Have some version field to check and if it mismatches reset CMOS to
default. To avoid having to maintain version manually I propose to
checksum option table and write 16-bit result to CMOS. 16-bit should
give enough confidence without excessively using CMOS. I've made a
prototype at http://review.coreboot.org/#/c/4790/4 . If we can agree
that it's a right approach, I'll make this prototype into complete patch
(mostly missing is laborous adding of version field)
2a) instead of having separate field we could make XOR layout checksum
into CMOS checksum. This would save 2 bytes but will break any existing
users if they check checksum.

-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 274 bytes
Desc: OpenPGP digital signature
URL: <http://www.coreboot.org/pipermail/coreboot/attachments/20140125/d279496a/attachment.sig>