linux-kernel - Re: [PATCH 1/2] dt-bindings: mailbox : arm,mhuv2: Allow for more RX interrupts

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Date:   Wed, 29 Mar 2023 18:44:31 +0100
From:   Sudeep Holla <sudeep.holla@....com>
To:     Cristian Marussi <cristian.marussi@....com>
Cc:     linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        vincent.guittot@...aro.org, souvik.chakravarty@....com,
        nicola.mazzucato@....com, Tushar.Khandelwal@....com,
        viresh.kumar@...aro.org, jassisinghbrar@...il.com,
        Rob Herring <robh+dt@...nel.org>,
        Sudeep Holla <sudeep.holla@....com>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        devicetree@...r.kernel.org
Subject: Re: [PATCH 1/2] dt-bindings: mailbox : arm,mhuv2: Allow for more RX
 interrupts

On Wed, Mar 29, 2023 at 04:39:35PM +0100, Cristian Marussi wrote:
> The ARM MHUv2 Receiver block can indeed support more interrupts, up to the
> maximum number of available channels, but anyway no more than the maximum
> number of supported interrupt for an AMBA device.
> 
> Signed-off-by: Cristian Marussi <cristian.marussi@....com>
> ---
> Cc: Rob Herring <robh+dt@...nel.org>
> Cc: Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>
> Cc: devicetree@...r.kernel.org
> 
>  .../devicetree/bindings/mailbox/arm,mhuv2.yaml      | 13 +++++++++----
>  1 file changed, 9 insertions(+), 4 deletions(-)
> 
> diff --git a/Documentation/devicetree/bindings/mailbox/arm,mhuv2.yaml b/Documentation/devicetree/bindings/mailbox/arm,mhuv2.yaml
> index a4f1fe63659a..5a57f4e2a623 100644
> --- a/Documentation/devicetree/bindings/mailbox/arm,mhuv2.yaml
> +++ b/Documentation/devicetree/bindings/mailbox/arm,mhuv2.yaml
> @@ -69,10 +69,15 @@ properties:
>  
>    interrupts:
>      description: |
> -      The MHUv2 controller always implements an interrupt in the "receiver"
> -      mode, while the interrupt in the "sender" mode was not available in the
> -      version MHUv2.0, but the later versions do have it.
> -    maxItems: 1
> +      The MHUv2 controller always implements at least an interrupt in the
> +      "receiver" mode, while the interrupt in the "sender" mode was not
> +      available in the version MHUv2.0, but the later versions do have it.
> +      In "receiver" mode, beside a single combined interrupt, there could be
> +      multiple interrupts, up to the number of implemented channels but anyway
> +      no more than the maximum number of interrupts potentially supported by
> +      AMBA.
> +    minItems: 1
> +    maxItems: 9

I am not sure 9 is the correct value here. IIUC it is just what Linux defines
as AMBA_NR_IRQS. Looking at the history it was bumped from 2 to 9 for use
by PL330 DMA driver. I couldn't find anything to relate this 9 in any
AMBA or other related specification.

Ideally I would say we don't know what the max here. We just have a platform
implementing 2 interrupts now. Do we for with 2 for now and change it if some
new users require more in the future ?

I will leave that to the DT maintainers but 9 is simply random based on Linux
code so I would rather choose some other random number with a better reasoning
than 9 as AMBA code in the kernel is limiting it to 9.

-- 
Regards,
Sudeep