Status of VGA Arbiter and Legacy VGA I/O?

Mike Travis travis at
Fri Nov 20 13:44:42 PST 2009

Adam Jackson wrote:
> 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
> wrapping.
> 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

Thanks Adam!  That is good news indeed.

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