HelenOS regression in QEMU implies OpenBIOS VGA issue

[Mark Cave-Ayland]

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 
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.

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.

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?

ATB,

Mark.

[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 
> 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.

[Mark Cave-Ayland]

On 16/03/12 20:59, Benjamin Herrenschmidt wrote:
> 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
> > 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?

For those people not on the HelenOS/qemu-ppc lists, I can confirm that 
the bug is actually in HelenOS which currently doesn't work with 
non-8-bit displays. In the meantime, you can boot HelenOS with my QEMU 
PPC modified interrupt patch and a recent OpenBIOS by forcing the 
bit-depth to 8 using "-g 800x600x8" on the QEMU command line.

ATB,

Mark.