Date:   Tue, 5 Sep 2023 13:57:57 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Lorenzo Pieralisi <lpieralisi@...nel.org>
Cc:     linux-kernel@...r.kernel.org, Rob Herring <robh@...nel.org>,
        linux-arm-kernel@...ts.infradead.org, devicetree@...r.kernel.org,
        Mark Rutland <mark.rutland@....com>,
        Rob Herring <robh+dt@...nel.org>,
        Fang Xiang <fangxiang3@...omi.com>,
        Marc Zyngier <maz@...nel.org>
Subject: Re: [PATCH 1/2] dt-bindings: interrupt-controller: arm,gic-v3: Add
 dma-noncoherent property

On 05/09/2023 1:22 pm, Lorenzo Pieralisi wrote:
> On Tue, Sep 05, 2023 at 12:17:51PM +0100, Robin Murphy wrote:
>> On 05/09/2023 11:47 am, Lorenzo Pieralisi wrote:
>>> The GIC v3 specifications allow redistributors and ITSes interconnect
>>> ports used to access memory to be wired up in a way that makes the
>>> respective initiators/memory observers non-coherent.
>>>
>>> Add the standard dma-noncoherent property to the GICv3 bindings to
>>> allow firmware to describe the redistributors/ITSes components and
>>> interconnect ports behaviour in system designs where the redistributors
>>> and ITSes are not coherent with the CPU.
>>>
>>> Signed-off-by: Lorenzo Pieralisi <lpieralisi@...nel.org>
>>> Cc: Rob Herring <robh@...nel.org>
>>> ---
>>>    .../bindings/interrupt-controller/arm,gic-v3.yaml         | 8 ++++++++
>>>    1 file changed, 8 insertions(+)
>>>
>>> diff --git a/Documentation/devicetree/bindings/interrupt-controller/arm,gic-v3.yaml b/Documentation/devicetree/bindings/interrupt-controller/arm,gic-v3.yaml
>>> index 39e64c7f6360..0a81ae4519a6 100644
>>> --- a/Documentation/devicetree/bindings/interrupt-controller/arm,gic-v3.yaml
>>> +++ b/Documentation/devicetree/bindings/interrupt-controller/arm,gic-v3.yaml
>>> @@ -106,6 +106,10 @@ properties:
>>>        $ref: /schemas/types.yaml#/definitions/uint32
>>>        maximum: 4096
>>> +  dma-noncoherent:
>>> +    description: |
>>> +      Present if the GIC redistributors are not cache coherent with the CPU.
>>
>> I wonder if it's worth being a bit more specific here, e.g. "if the GIC
>> {redistributors,ITS} permit programming cacheable inner-shareable memory
>> attributes, but are connected to a non-coherent downstream interconnect."
> 
> In my opinion it is and I wanted to elaborate on what I wrote but then I
> thought that this is a standard DT property, I wasn't sure whether we
> really need to explain what it is there for.
> 
> We are using the property to plug a hole so I agree with you, we should
> be as clear as possible in the property definition but I will rely on
> Rob/Marc's opinion, I don't know what's the DT policy for this.
> 
>> That might help clarify why the negative property, which could seem a bit
>> backwards at first glance, and that it's not so important in the cases where
>> the GIC itself is fundamentally non-coherent anyway (which *is*
>> software-discoverable).
> 
> Is it ? Again, see above, are we defining "dma-noncoherent" to fix a bug
> or to fix the specs ? The shareability bits are writeable and even a
> fundamentally non-coherent GIC design could allow writing them, AFAIU.

I mean the case on GIC-500 and earlier where the register bits could be 
hard-wired. I'm not sure a GIC implementation which didn't even *try* to 
honour the programmed attributes in what it emits would be considered 
valid; it certainly couldn't be considered sensible :/

> I would avoid putting ourselves into a corner where we can't use
> this property because the binding itself is too strict on what it is
> solving.

Really I'm just getting at the fact that if you do have a legacy GIC 
with hard-wired attributes then whatever DT says is most likely 
irrelevant anyway (unless the integrator has done something utterly 
bonkers and tied off the interconnect input to *different* attributes, 
but I would consider that beyond the bounds of fair reasoning...)

Cheers,
Robin.

>> Otherwise, this is the same approach that I like and have previously lobbied
>> for, so obviously I approve :)
>>
>> (plus I do think it's the right shape to be able to slot an equivalent field
>> into ACPI MADT entries without *too* much bother)
> 
> We are in agreement, let's see what others think.
> 
> Thanks,
> Lorenzo
> 
>>
>> Thanks,
>> Robin.
>>
>>> +
>>>      msi-controller:
>>>        description:
>>>          Only present if the Message Based Interrupt functionality is
>>> @@ -193,6 +197,10 @@ patternProperties:
>>>          compatible:
>>>            const: arm,gic-v3-its
>>> +      dma-noncoherent:
>>> +        description: |
>>> +          Present if the GIC ITS is not cache coherent with the CPU.
>>> +
>>>          msi-controller: true
>>>          "#msi-cells":
>>
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>> linux-arm-kernel@...ts.infradead.org
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