[OpenBIOS] HelenOS regression in QEMU implies OpenBIOS VGA issue

Mark Cave-Ayland mark.cave-ayland at ilande.co.uk
Fri Mar 16 17:44:22 CET 2012

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.

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
     Only enabled for PPC/QEMU and VGA/VBE.

git-svn-id: svn://openbios.org/openbios/trunk@538 

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.

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?



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