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Message-ID: <7775eb70-692f-3f1b-f226-f7e0fad47e37@collabora.com>
Date:   Mon, 4 Apr 2022 10:40:34 +0200
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Subject: Re: [PATCH 2/2] dt-bindings: mfd: syscon: Add support for regmap
 fast-io
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To:     Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>,
        lee.jones@linaro.org
Cc:     robh+dt@kernel.org, krzk+dt@kernel.org, arnd@arndb.de,
        matthias.bgg@gmail.com, devicetree@vger.kernel.org,
        linux-kernel@vger.kernel.org, nfraprado@collabora.com,
        kernel@collabora.com
References: <20220401135048.23245-1-angelogioacchino.delregno@collabora.com>
 <20220401135048.23245-3-angelogioacchino.delregno@collabora.com>
 <8588a941-6d3e-9e14-cb21-d7af29b4b2bd@linaro.org>
From:   AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@collabora.com>
In-Reply-To: <8588a941-6d3e-9e14-cb21-d7af29b4b2bd@linaro.org>
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Il 02/04/22 13:38, Krzysztof Kozlowski ha scritto:
> 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.
> 

Hello Krzysztof,

well yes, this property is changing how software behaves - specifically,
as you've correctly understood, what regmap does.

It's true that most of MMIOs inside a SoC are considered fast.. the word "most" is
the exact reason why I haven't proposed simply hardcoding '.fast_io = true' in
syscon, or in regmap-mmio...
There are too many different SoCs around, and I didn't want to end up breaking
anything (even if it should be unlikely, since MMIO is fast by principle).


>>
>> Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.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)."
> 

That's a very good point. I didn't think about any future in which the
implementation would be changed from regmap to *new-name-here*... but
anyway it makes a lot more sense to "speak generic".

I'll change the description to match your proposal, thank you!

Regards,
Angelo