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

From: [email-masked] (Marc Zyngier)
Date: Thu, 05 Apr 2012 15:07:22 +0100

On 05/04/12 14:34, Rob Herring wrote:
> 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.

Yes, probably.

>> 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.

In this case we don't have an interrupt parent property, so the GIC is
properly identified as the top-level interrupt controller. This is
admittedly a bit fragile, and contradicts the current wording of the
binding.

>> 	};
>>
>> 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.

The first I posted (vgic node outside of the gic), or the one with
everything in the same node?

	M.
-- 
Jazz is not dead. It just smells funny...