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

From: [email-masked] (Rob Herring)
Date: Thu, 05 Apr 2012 08:34:18 -0500

On 04/05/2012 07:59 AM, Marc Zyngier 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.
> 

But that would be a new compatible string as is this case.

> 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>;

Does this work having an interrupt within the parent itself? Normally
this would be the connection to the next level up.

> 	};
> 
> 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.
> 
> What do you think?
> 

I prefer the first option.

Rob

> 	M.