[OpenBIOS] PIC Passthough( VGA )

Mark Cave-Ayland mark.cave-ayland at ilande.co.uk
Tue Dec 12 09:24:55 CET 2017

On 12/12/17 03:24, Jd Lyons wrote:

>>> I’m assuming that Openbios is not reading the Option Rom of the card, but I’m not sure why.
>> Well... at the moment we actually cheat ;)  Instead of reading the VGA FCode ROM from the virtual display adapter, OpenBIOS embeds its own copy of the FCode ROM and executes it unconditionally when it detects a VGA display device. I must admit I hadn't considered that someone would be interested in legacy device passthough to enable a device that isn't the standard QEMU display adapter.
> Are we talking about the ‘NDRV’ patch?
> I noticed that the ‘NDRV’ for the QEMU,VGA loads to any VGA card in the system. Yet the vga-pci.c loads a PC Video bios for the qemu,vga device, tho I’m pretty sure Openbios doesn’t use it. I’m having trouble locating how the QEMU,VGA device is able to be used by Openbios, how does it “init” the device?

The NDRV driver is loaded into the "driver,AAPL..." property after the 
device node has been created by the FCode device so that occurs later in 
the process.

OpenBIOS creates a basic PCI device node for every device it finds 
during a PCI bus scan and then executes a callback depending upon the 
resulting vendorid/deviceid. The code in OpenBIOS is in vga_config_cb() 
and as you can see, it currently executes the in-built FCode directly 
rather than trying to map the ROM using the process documented at 

>> As a starting point I'd suggest that if you still have the old hardware lying around, you can use the article above to extract the ROM from a real Mac and then run the resulting code through fcode-utils detok which effectively "decompiles" the ROM and therefore give us an idea as to how many Forth words need to be implemented.
> Ok, I’ve done some detok of nVidia Fcode Rom’s before, and I know about words that Openfirmware doesn’t understand, years ago I wanted two get an nVidia card to work in my Old World PM8600, and I figured it was the “map-in” bug, but as it turns out, it was some words in nVidia's FCode Rom that my version of OF didn’t have.
> Atriu Itra was able to get an nVidia card to work in OpenFimware on an Old World machine, but it wouldn’t boot the OS, however if he had stayed at it, I’m sure he’d have figured it out. He did basically what you are preposing. Guy was an Fcode master, we could really use him on this project.

Sounds like someone has already done some of the work here - do you have 
a copy of the detok FCode or any emails related to the subject at all?

Otherwise I would say the first step would be to hack OpenBIOS to expose 
the passthrough ROM and then dump it out from within QEMU.



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