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Date:   Wed, 7 Dec 2022 10:48:44 +0100
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Subject: Re: [PATCH v2 7/9] dt-bindings: spi: mtk-snfi: Add read latch latency
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From:   AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@collabora.com>
In-Reply-To: <e2ccf42f94152ccacad3a2eaec234ae0566474ec.camel@mediatek.com>
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Il 07/12/22 03:00, Xiangsheng Hou (侯祥胜) ha scritto:
> Hi Angelo,
> 
> On Tue, 2022-12-06 at 13:19 +0100, AngeloGioacchino Del Regno wrote:
>>>>> diff --git
>>>>> a/Documentation/devicetree/bindings/spi/mediatek,spi-
>>>>> mtk-snfi.yaml
>>>>> b/Documentation/devicetree/bindings/spi/mediatek,spi-
>>>>> mtk-snfi.yaml
>>>>> index bab23f1b11fd..6e6ff8d73fcd 100644
>>>>> --- a/Documentation/devicetree/bindings/spi/mediatek,spi-mtk-
>>>>> snfi.yaml
>>>>> +++ b/Documentation/devicetree/bindings/spi/mediatek,spi-mtk-
>>>>> snfi.yaml
>>>>> @@ -45,6 +45,13 @@ properties:
>>>>>         description: device-tree node of the accompanying ECC
>>>>> engine.
>>>>>         $ref: /schemas/types.yaml#/definitions/phandle
>>>>>     
>>>>> +  mediatek,rx-latch-latency:
>>>>> +    description: Rx delay to sample data with this value, the
>>>>> value
>>>>> +                 unit is clock cycle.
>>>>
>>>> Can't we use nanoseconds or microseconds as a unit here, instead
>>>> of
>>>> clock cycles?
>>>
>>> The clock cycle will be various with MediaTek SPI NAND controller
>>> which
>>> clock frequency can support 26/52/68/81/104MHz...
>>> It`s may be easy to configure and understand with clock cycle in
>>> unit.
>>>
>>
>> Yes, but whatever clock frequency we use, the target is to always
>> wait for
>> X nanoseconds, right?
>>
>> Waiting for 5 clock cycles at 104MHz is obviously not the same as
>> waiting
>> for the same 5 clock cycles at 26MHz: in that case, expressing the
>> value
>> in nanoseconds or microseconds would make that independent from the
>> controller's clock frequency as the calculation from `time` to
>> `cycles`
>> would be performed inside of the driver.
> 
> There have two rx related timing properties in spi-peripheral-props.
> The rx-sample-delay-ns have been used in Mediatek snfi driver to adjust
> controller sample delay.
> However another spi-rx-delay-us is in microseconds. Take 52MHz for
> example, the clock cycle will be 19.23ns which lower than 1us. This may
> not easy to by one clock cycle.
> 

I agree, but nothing prevents you from adding your own property for that.

I propose "mediatek,rx-latch-latency-ns" or "mediatek,rx-latency-ns", so that
we can specify the delay in nanoseconds: in that case, when we specify 19ns,
the driver will safely round that resulting in 52MHz == 19.23ns => 19ns valid.

Regards,
Angelo