Re: VGA Passthrough

[BALATON Zoltan]

On Fri, 20 Mar 2020, Jd Lyons wrote:
> I’ve been trying to get VGA Passthrough working on a Rage128 PCI Mac addition 
> with system-ppc.
>
> When booting to Mac OS X basic VGA functions work, however trying to 
> enable 2d or 3d acceleration causes display corruption and makes the 
> system unusable.
>
> This also happens with Linux guests, fbdev works for both the tty and 
> the x-server, but trying to enable DRI with the X11 “ati” driver results 
> in display corruption and makes the system unusable.
>
> I’m not sure how to debug this to figure out the corruption issue with VGA(PCI) 
> Passthrough?

(Also cc-ing Gerd Hoffmann who is VGA and USB maintainer so may have more 
knowledge about this.)

I'm not sure either how to debug pci passthrough but maybe you should give 
more details on what did you did (what versions you used, command line, 
other things necessary to reproduce what you did) and what errors you see 
otherwise people probably could only give vague answers at best.

I've found this doc about passing through GPU but don't know how useful or 
accurate is it:

https://github.com/intel/nemu/wiki/Testing-VFIO-with-GPU

You also may need to run the card ROM from OpenBIOS with my patches by 
hand to correctly init the card which is not done automatically. We've 
done this before while testing ati-vga with MacOS so I think you know 
about it and did that. If the ROM is not run, the card may not be in the 
state expected by the OS drivers.

Also the symptoms you get: works with frame buffer but not when enabling 
DRI is similar to what I get with the incomplete ati-vga emulation and the 
reason for that is not being able to use memory buffers accessed via bus 
master DMA:

https://osdn.net/projects/qmiga/ticket/40018

So maybe your case is similar, the real card of course has this capability 
but maybe bus master DMA or accessing the memory buffers is not working 
for some reason but I don't know how to check that. Maybe somebody has an 
idea here?

Regards,
BALATON Zoltan