Generic DT binding for IPIs

[Qais Yousef <qais.yousef-1AXoQHu6uovQT0dZR+AlfA@public.gmane.org>]

Hi,

This is an attempt to revive a discussion on the right list this time 
with all the correct people hopefully on CC.

While trying to upstream a driver, Thomas and Marc Zyngier pointed out 
the need for a generic IPI support in the kernel to allow driver to 
reserve and send ones. Hopefully my latest RFC patch will help to 
clarify what's being done.

     https://lkml.org/lkml/2015/10/13/227

We need a generic DT binding support to accompany that to allow a driver 
to reserve an IPI using this new mechanism.

MarcZ had the following suggestion:

     https://lkml.org/lkml/2015/8/24/628

Which in summary is

	mydevice@f0000000 {
		interrupt-source = <&intc INT_SPEC 2 &inttarg1 &inttarg1>;
	};

	inttarg1: mydevice@f1000000 {
		interrupt-sink = <&intc HWAFFINITY1>;
	};

	inttarg2: cpu@1 {
		interrupt-sink = <&intc HWAFFINITY2>;
	};


interrupt-sink requests to reserve an IPI that it will receive at 
HWAFFINITY cpumask. interrupt-source will not do any reservation. It 
will simply connect an IPI reserved by interrupt-sink to the device that 
will be responsible for generating that IPI. This description should 
allow connecting any 2 devices.
Correct me Marc if I got it wrong please.

I suggested a simplification by assuming that IPIs will only be between 
host OS and a coprocessor which would gives us this form which I think 
is easier to deal with

	coprocessor {
              interrupt-source = <&intc INT_SPEC COP_HWAFFINITY>;
              interrupt-sink = <&intc INT_SPEC CPU_HWAFFINITY>;
	}


interrupt-source here reserves an IPI to be sent from host OS to 
coprocessor at COP_HWAFFINITY. interrupt-sink will reserve an IPI to be 
received by host OS at CPU_HWAFFINITY. Less generic but I don't know how 
important it is for host OS to setup IPIs between 2 external 
coprocessors and whether it should really be doing that.

What do the DT experts think? Any preference or a better suggestion?

I tried to keep this short and simple, please let me know if you need 
more info or if there's anything that needs more clarification.

Thanks,
Qais
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