SeaBIOS (and Grub) use PIO instead of DMA with IDE mode, so AHCI is indeed a better default IMO. The speed difference is easily a order of magnitude faster with AHCI (on ICH7 at least).

IIRC SeaBIOS needs explicit:
CONFIG_ATA_DMA=y
CONFIG_ATA_PIO32=y
passed as a SeaBIOS config file in order to use PIO. Maybe should I add a patch for default PIO in SeaBIOS Makefile?

AFAIK, it's not the default because it breaks compatibility
with actual PATA drives. So it would have to be decided per
board.

Patch set 4:Code-Review +1

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