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Subject: Re: [PATCH] VGA arbitration: draft of kernel side
From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
To: Jon Smirl <jonsmirl@gmail.com>
Cc: xorg@freedesktop.org, Egbert Eich <eich@freedesktop.org>,
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Date: Wed, 09 Mar 2005 17:00:40 +1100
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On Wed, 2005-03-09 at 00:58 -0500, Jon Smirl wrote:
> On Wed, 09 Mar 2005 16:37:13 +1100, Benjamin Herrenschmidt
> <benh@kernel.crashing.org> wrote:
> > On Tue, 2005-03-08 at 23:35 -0500, Jon Smirl wrote:
> > > This is from /linux-2.5/Documentation/filesystems/sysfs-pci.txt. It
> > > describes how ia64 is achieving legacy IO.  The VGA control code
> > > probably needs to be coordinated with this.
> > 
> > This is a different thing, and I will implement it on ppc one of these
> > days. This is for issuing the IO cycles on the bus. It has nothing
> > to do with the actual arbitration work.
> 
> Each one of these legacy spaces corresponds to an allowable
> simultaneous VGA use. There should be one arbiter per legacy space.

Ugh ? They are or they are not independant, that's a platform thing and
has nothing to do with arbitration. They aren't VGA specific neither.

The arbitrer uses the vga_conflicts() callback for that purpose. It is
defined to always return 1 by default, but the platform can override it
if it has separate PCI domains, in order to tell the arbitrer wether 2
cards can conflict or not.

Based on that, the arbitrer will, or will not, let you lock the VGA
legacy resources simultaneously.

Ben.