Ah, I missed that vgahook_setup() checks for NULL. Out of curiosity, is it possible to limit this support to a particular hardware vendor instead of by existence of file?
ATI, AMD, and Nvidia based MXM cards exist. I don't know of a way to probe it to find out if it's an MXM format card. And even if there was a way to tell with 100% reliability, the spec allows for the System Information Structure to be provided by an EEPROM on the mainboard, connected to the MXM GPU via I²C. In that case, we wouldn't have to provide these interrupts.
Despite the declining popularity, new MXM cards are still released. Maybe one day we'd have to add support for Intel Arc cards as well.
Or do you mean CBvendor/CBpart? I fear that list could grow quickly and would just make it more annoying to port new boards. Coreboot already has EliteBook 8770w and HP Compaq Elite 8300 USDT (that's a desktop btw). And Dell Precision ports are in progress.
-- Riku