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Date:   Mon, 4 Apr 2022 10:55:12 +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:     AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@collabora.com>, 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>
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 <7775eb70-692f-3f1b-f226-f7e0fad47e37@collabora.com>
From:   Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
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On 04/04/2022 10:40, AngeloGioacchino Del Regno wrote:
> 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).

What I am proposing, is the regmap consumer knows whether access is fast
or not, so it could call get_regmap() or
syscon_regmap_lookup_by_phandle() with appropriate argument.

Even if we stay with a DT property, I am not sure if this is an
attribute of syscon but rather of a bus.

Best regards,
Krzysztof