I've installed a SI-PEX40064 SATA controller card http://www.sybausa.com/index.php?route=product/product&product_id=156 in my system that I am trying to use with PCI passthrough via Xen, in order to give a guest VM its own real SATA controller so as to not have to deal with virtual disks. Moreover, I want to boot off of a disk attached to the card.
The card implements (or pretends to implement) AHCI, so SeaBIOS starts up and can see the attached disk. But SeaBIOS gets a timeout communicating with the disk:
|eff9d000| WARNING - Timeout at ahci_port_setup:469! |eff9d000| AHCI/2: device not ready (tf 0x58)
The card has a PCI Option ROM, and I think the Option ROM needs to be loaded for the card to work and/or be bootable. But SeaBIOS refuses to load the Option ROM. (My real physical system with Asus EFI firmware and CSM enabled does load the Option ROM for the card when it is installed.) I dug into the code, and it looks like it's the same issue as is mentioned here:
https://mail.coreboot.org/pipermail/seabios/2017-June/011425.html
Basically, a device is not checked for an option ROM (or an fw_fcg/CBFS-based Option ROM file), and its option ROM is not loaded, if pci->have_driver is set for the device. That field gets set for anything that apparently implements AHCI, so the Option ROMs for AHCI controllers are skipped, even if they are useful. Removing that check causes the option ROM for my card to be loaded, and allows the bootloader stored on the disk attached to the card to start up.
I've attached a patch that implements this change, and adds a few more debug messages to explain what is going on. I haven't observed any problems with my patched SeaBIOS under Xen, but I'm not sure why the check was there in the first place (as it wasn't commented), so I can't speak to the repercussions of disabling it on everyone else's machines.