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Date: Tue, 26 Jan 2010 10:54:17 -0600
From: Anthony Liguori <anthony@codemonkey.ws>
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Subject: Re: [Qemu-devel] CONFIG_FB_CIRRUS for Linux guests
References: <4B5EFFBC.8020703@siemens.com>
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To: Jan Kiszka <jan.kiszka@siemens.com>
Cc: Marcelo Tosatti <mtosatti@redhat.com>, qemu-devel <qemu-devel@nongnu.org>

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>

Regards,

Anthony Liguori

> Jan
>
>