Re: [Qemu-devel] CONFIG_FB_CIRRUS for Linux guests

[Jan Kiszka <jan.kiszka@siemens.com>]

Anthony Liguori wrote:
> On 01/26/2010 08:44 AM, Jan Kiszka wrote:
>> Hi,
>>
>> just received some Linux guest kernel that refuses to properly use its
>> framebuffer console under QEMU (probably any version, tried down to some
>> 0.10.x). It turned out that it has CONFIG_FB_CIRRUS enabled and fails to
>> initialize the display when this is actually used. Anyone came across
>> this before or has some idea what goes wrong?
>>    
> 
> Looks like it worked at some point:
> 
> commit ca896ef389fc44c7a673d1bd2b94823e28da37a1
> Author: aurel32 <aurel32@c046a42c-6fe2-441c-8c8c-71466251a162>
> Date:   Thu May 8 12:21:27 2008 +0000
> 
>      cirrusfb: proper "Attribute Controller Toggle Readback" register 
> behaviour
> 
>      (Marcelo Tosatti)
> 
> 
>      git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@4386 
> c046a42c-6fe2-441c
> 
> The cirrusfb Linux driver uses the toggle readback register to determine
> whether to flip 0xC30 to data/address mode.
> 
> Without proper implementation the VGA mode will be erroneously set to
> blank resulting in crashes when starting BITBLT.
> 
> 3d4h index 24h (R):  Attribute Controller Toggle Readback Register
> bit    7  If set the Attribute Controller will next access a data register, if
>            clear it'll access the index register
> 
> 
> Signed-off-by: Marcelo Tosatti<mtosatti@redhat.com>
> 

Yep, and 2bec46dc97571a3c34b18fe4ca198e7bfbdca41f broke it again.

Unless Glauber immediately confesses the crime and removes the bug he
left behind, I will try to understand the reason later.

Jan

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