Status of VGA Arbiter and Legacy VGA I/O?

Adam Jackson ajax at
Fri Nov 20 13:36:39 PST 2009

On Fri, 2009-11-20 at 13:23 -0800, Mike Travis wrote:
> Hi Adam, Jesse,
> We here at SGI are getting ready to deploy the SGI Ultraviolet
> system (see refs below) and the question came up about supporting
> legacy I/O for multiple VGA cards.  The UV architecture requires
> some additional setup when changing the "owner" of the legacy
> VGA I/O space.
> I've been trying to follow the progress of the VGA Arbiter, and
> it's use by X, as a means of providing the kernel hook to execute
> this setup.  Can you (or someone) enlighten me as to whether
> a.) X still requires the use of legacy I/O [or are all accesses
> being done with MMIO operations]? and b.) is the plan still to use
> the VGA Arbiter as currently implemented in linux-2.6.32?

It depends on the driver, really.  X doesn't try to poke the VGA space
on its own anymore (pretty sure I got rid of all that), but if your
driver uses the vgahw module or does I/O directly, then that needs

My understanding is, by default the VGA arbiter support code in X will
assume the driver _does_ need VGA arbitration, unless the driver opts
out of it, and will wrap device access for you.  So whatever extra setup
your platform needs should Just Work if you hook it up to the
arbitration grant in the kernel.

- ajax
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