Subject: [PATCH] ARM: DT: Add binding for GIC virtualization extentions (VGIC)

From: [email-masked] (Grant Likely)
Date: Fri, 06 Apr 2012 18:35:31 -0700

On Thu, 05 Apr 2012 13:59:56 +0100, Marc Zyngier <[email-masked]> wrote:
> On 03/04/12 16:35, Grant Likely wrote:
> 
> Hi Grant,
> 
> > On Tue, 03 Apr 2012 10:53:44 +0100, Marc Zyngier <[email-masked]> wrote:
> >> On 03/04/12 10:22, David Vrabel wrote:
> >>
> >> Hi David,
> >>
> >>> On 02/04/12 17:30, Marc Zyngier wrote:
> >>>> The GICv2 can have virtualization extension support, consisting
> >>>> of an additional set of registers and interrupts. Add the necessary
> >>>> binding to the GIC DT documentation.
> >>>
> >>> The Xen hypervisor's device tree support is very much incomplete so I've
> >>> not looked into this is much detail.
> >>>
> >>> Would it make more sense to extend the existing gic binding with the the
> >>> additional information rather than adding a new node?
> >>
> >> I'm actually torn between the two approaches. On one side, the VGIC is
> >> part of the GIC spec, hence should be part of the GIC node. On the other
> >> hand, it is logically handled by a different piece of software (the
> >> hypervisor), and would normally be probed separately. Having a separate
> >> node makes the probing more sensible.
> > 
> > Don't get too hung up on the software side of things.  Describe it in
> > a way that makes sense for the hardware.  There is lots of precidence
> > for two hunks of software initializating from the same node; either by
> > probe kicking off two init hooks, or by early init code going looking
> > for the node manually.
> 
> What I'm trying to avoid is a royal mess in the future if we get some
> other extension to the GIC.
> 
> Let's say we implement the following:
> 
> 	gic: interrupt-controller at 2c001000 {
> 		compatible = "arm,cortex-a15-gic";
> 		#interrupt-cells = <3>;
> 		#address-cells = <1>;
> 		interrupt-controller;
> 		reg = <0x2c001000 0x1000>,
> 		      <0x2c002000 0x100>,
> 		      <0x2c004000 0x2000>,
> 		      <0x2c006000 0x2000>;
> 		interrupts = <1 9 0xf04>;
> 	};
> 
> It's all fine (the two last regions and the interrupt are for VGIC),
> until someone comes up with extension FOO which requires two new regions
> and am interrupt. It is then impossible to distinguish between the two,
> short of adding more attributes.
> 
> How about this?
> 
> 	gic: interrupt-controller at 2c001000 {
> 		compatible = "arm,cortex-a15-gic";
> 		#interrupt-cells = <3>;
> 		#address-cells = <1>;
> 		#size-cells = <1>;
> 		interrupt-controller;
> 		reg = <0x2c001000 0x1000>,
> 		      <0x2c002000 0x100>;
> 
> 		vgic at 2c004000 {
> 			compatible = "arm,cortex-a15-vgic", "arm,vgic";
> 			reg = <0x2c004000 0x2000>,
> 			      <0x2c006000 0x2000>;
> 			interrupts = <1 9 0xf04>;
> 		};
> 	};
> 
> It cleanly separate the extension from the core GIC, and still make it
> part of the GIC node.

I think that the compatible property already supports what you need to
do.  If an extension is added to the core binding, then suppliment it
with a new compatible value:

	gic: interrupt-controller at 2c001000 {
		compatible = "arm,cortex-a15-vgic", "arm,cortex-a15-gic";
		#interrupt-cells = <3>;
		#address-cells = <1>;
		#size-cells = <1>;
		interrupt-controller;
		reg = <0x2c001000 0x1000>,
		      <0x2c002000 0x100>,
		      <0x2c004000 0x2000>,
		      <0x2c006000 0x2000>;
		interrupts = <1 9 0xf04>;
	};

Or, if it can be done in a backwards compatible way, then just extend
the existing binding.  In you're example above, the 'vgic' extension
only gets enabled if the 3rd and 4th 'reg' tuples are added.

g.