[OpenBIOS] HelenOS regression in QEMU implies OpenBIOS VGA issue

[Benjamin Herrenschmidt]

On Fri, 2012-03-16 at 16:44 +0000, Mark Cave-Ayland wrote:
> Hi all,
> 
> Whilst looking at Jakub's bug report here 
> https://bugs.launchpad.net/qemu/+bug/942299, I've been working on a 
> patch to correct the QEMU interrupt handler on qemu-ppc. Now that this 
> bug is close to being fixed, I've had a look at OpenBIOS to find out why 
> newer versions of OpenBIOS freeze before the HelenOS loading screen.

I observed that as well, it hangs some time during the display of
the boot messages.

> A local git bisect shows that the culprit is actually this commit:
> 
> commit b6de01a59394f1fafbfb2c65cb784c9d23067fa2
> Author: laurent <laurent at f158a5a8-5612-0410-a976-696ce0be7e32>
> Date:   Tue Aug 4 21:08:37 2009 +0000
> 
>      Allows to configure screen size from Qemu command line options 
> using FW_CFG
>      interface.
>      Only enabled for PPC/QEMU and VGA/VBE.

I'm surprised by that. This has worked well for a long time, I think
something else in qemu causes that commit to fail.

> git-svn-id: svn://openbios.org/openbios/trunk@538 
> f158a5a8-5612-0410-a976-696ce0be7e32
> 
> Having stepped through vga_vbe_init() in drivers/vga_vbe.c, the only 
> difference is that reading the display depth from the QEMU FW CFG 
> interface returns 15 instead of the in-built OpenBIOS default of 8.

AFAIK that's been working fine for a while...

> So is a bit-depth of 15 supported by OpenBIOS? This seems quite unusual 
> given that it is not a power of 2; however if I use GDB to override the 
> value to 16 then HelenOS boots but with a blue screen BIOS on VGA so I'm 
> not sure that is the correct solution either. Does anyone know what the 
> default bit-depth is in Apple's firmware?

Cheers,
Ben.