linux-kernel - Re: [PATCH 2/2] dt-bindings: mfd: syscon: Add support for regmap fast-io

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Date:   Sat, 2 Apr 2022 13:38:40 +0200
From:   Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
To:     AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@...labora.com>, lee.jones@...aro.org
Cc:     robh+dt@...nel.org, krzk+dt@...nel.org, arnd@...db.de,
        matthias.bgg@...il.com, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, nfraprado@...labora.com,
        kernel@...labora.com
Subject: Re: [PATCH 2/2] dt-bindings: mfd: syscon: Add support for regmap
 fast-io

On 01/04/2022 15:50, AngeloGioacchino Del Regno wrote:
> The syscon driver now enables the .fast_io regmap configuration when
> the 'fast-io' property is found in a syscon node.
> 
> Keeping in mind that, in regmap, fast_io is checked only if we are
> not using hardware spinlocks, allow the fast-io property only if
> there is no hwlocks reference (and vice-versa).

I have doubts you need a property for this. "fast" is subjective in
terms of hardware, so this looks more like a software property, not
hardware.

I think most of MMIOs inside a SoC are considered fast. Usually also the
syscon/regmap consumer knows which regmap it gets, so knows that it is
fast or not.

> 
> Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@...labora.com>
> ---
>  Documentation/devicetree/bindings/mfd/syscon.yaml | 15 +++++++++++++++
>  1 file changed, 15 insertions(+)
> 
> diff --git a/Documentation/devicetree/bindings/mfd/syscon.yaml b/Documentation/devicetree/bindings/mfd/syscon.yaml
> index 13baa452cc9d..85a2e83b5861 100644
> --- a/Documentation/devicetree/bindings/mfd/syscon.yaml
> +++ b/Documentation/devicetree/bindings/mfd/syscon.yaml
> @@ -83,11 +83,26 @@ properties:
>      $ref: /schemas/types.yaml#/definitions/uint32
>      enum: [1, 2, 4, 8]
>  
> +  fast-io:
> +    description:
> +      Indicates that this bus has a very fast IO, for which
> +      acquiring a mutex would be significant overhead.
> +      When present, regmap will use a spinlock instead.

Regmap is current implementation behind this, but it's not related to
hardware, so how about removing it from the description? Something like:
"..., for which different locking methods should be used to reduce
overhead (e.g. spinlock instead of mutex)."

> +    type: boolean
> +
>    hwlocks:
>      maxItems: 1
>      description:
>        Reference to a phandle of a hardware spinlock provider node.
>  
> +if:
> +  required:
> +    - hwlocks
> +then:
> +  not:
> +    required:
> +      - fast-io
> +
>  required:
>    - compatible
>    - reg


Best regards,
Krzysztof