linux-kernel - Re: [PATCH 03/12] of: Add binding document for MIPS GIC

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Date:	Wed, 3 Sep 2014 16:53:42 -0700
From:	Andrew Bresticker <abrestic@...omium.org>
To:	David Daney <ddaney.cavm@...il.com>
Cc:	Ralf Baechle <ralf@...ux-mips.org>,
	Rob Herring <robh+dt@...nel.org>,
	Pawel Moll <pawel.moll@....com>,
	Mark Rutland <mark.rutland@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>,
	Jeffrey Deans <jeffrey.deans@...tec.com>,
	Markos Chandras <markos.chandras@...tec.com>,
	Paul Burton <paul.burton@...tec.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Jason Cooper <jason@...edaemon.net>,
	Linux-MIPS <linux-mips@...ux-mips.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 03/12] of: Add binding document for MIPS GIC

On Tue, Sep 2, 2014 at 5:50 PM, David Daney <ddaney.cavm@...il.com> wrote:
> On 09/02/2014 12:36 PM, Andrew Bresticker wrote:
>>
>> On Tue, Sep 2, 2014 at 10:27 AM, David Daney <ddaney.cavm@...il.com>
>> wrote:
>>>
>>> On 08/29/2014 03:14 PM, Andrew Bresticker wrote:
>>>>
>>>>
>>>> The Global Interrupt Controller (GIC) present on certain MIPS systems
>>>> can be used to route external interrupts to individual VPEs and CPU
>>>> interrupt vectors.  It also supports a timer and software-generated
>>>> interrupts.
>>>>
>>>> Signed-off-by: Andrew Bresticker <abrestic@...omium.org>
>>>> ---
>>>>    Documentation/devicetree/bindings/mips/gic.txt | 50
>>>> ++++++++++++++++++++++++++
>>>>    1 file changed, 50 insertions(+)
>>>>    create mode 100644 Documentation/devicetree/bindings/mips/gic.txt
>>>>
>>>> diff --git a/Documentation/devicetree/bindings/mips/gic.txt
>>>> b/Documentation/devicetree/bindings/mips/gic.txt
>>>> new file mode 100644
>>>> index 0000000..725f1ef
>>>> --- /dev/null
>>>> +++ b/Documentation/devicetree/bindings/mips/gic.txt
>>>> @@ -0,0 +1,50 @@
>>>> +MIPS Global Interrupt Controller (GIC)
>>>> +
>>>> +The MIPS GIC routes external interrupts to individual VPEs and IRQ
>>>> pins.
>>>> +It also supports a timer and software-generated interrupts which can be
>>>> +used as IPIs.
>>>> +
>>>> +Required properties:
>>>> +- compatible : Should be "mti,global-interrupt-controller"
>>>> +- reg : Base address and length of the GIC registers.
>>>> +- interrupts : Core interrupts to which the GIC may route external
>>>> interrupts.
>>>
>>>
>>>
>>> This doesn't make sense to me.  The GIC can, and does, route interrupts
>>> to
>>> all CPU cores in a SMP system.  How can there be a concept of only
>>> associating it with several interrupt lines on a single CPU in the
>>> system?
>>> That is not what the GIC does, is it?  It is a Global interrupts
>>> controller,
>>> not local.  So specifying device tree bindings that don't show its Global
>>> nature seems wrong.
>>
>>
>> While the GIC can route external interrupts to any HW interrupt vector
>> it may not make sense to actually use all those vectors.  For example,
>> the CP0 timer is usually hooked up to HW vector 5 (it could be treated
>> as a GIC local interrupt, though it may still be fixed to HW vector
>> 5).  BTW, the Malta example about the i8259 I gave before was wrong -
>> it appears that it actually gets chained with the GIC.
>
>
> Your comments don't really make sense to me in the context of my knowledge
> of the GIC.
>
> Of course all the CP0 timer and performance counter interrupts are per-CPU
> and routed directly to the corresponding CP0_Cause[IP7..IP2] bits.  We are
> don't need to give them further consideration.
>
>
> Here is the scenario you should consider:
>
>   o 16 CPU cores.
>   o 1 GIC routing interrupts from external sources to the 16 CPUs.
>   o 2 network controllers each with an interrupt line routed to the GIC.
>
> Q: What would the GIC "interrupts" property look like?
>
> Note that the GIC doesn't have a single "interrupt-parent", as it can route
> interrupts to *all* 16 CPUs.
>
> I propose that the GIC have neither an "interrupt-parent", nor "interrupts".
> The fact that it is an "mti,global-interrupt-controller", means that the
> software drivers for the GIC already know how to route interrupts, and any
> information the device tree could contain is redundant.

Ok, I misunderstood your opposition to the binding.

My intention for the "interrupt-parent" and "interrupts" property of
the GIC was to express that GIC interrupts are routed to the CPU
interrupt vectors and that a certain set of these vectors are
available for use by the GIC.  I would agree that these are mostly
redundant (obviously the GIC routes interrupts to CPU interrupt
vecotrs) and that it is not the most accurate description of the
GIC-CPU relationship (the CPU interrupt controller are per-CPU, not
global, and the GIC can route interrupts to any of them), though I'm
not sure that there's a better way of describing it in DT.

So that leaves us with something like this:

interrupt-controller@...c0000 {
        compatible = "mti,global-interrupt-controller";

        interrupt-controller;
        #interrupt-cells = <2>;

        available-cpu-vectors = <2>, <3>, ...
};

DT folks, thoughts?
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