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Message-ID: <8090e2e1-afce-9341-2ae7-3f4e094409b6@linaro.org>
Date:   Fri, 9 Sep 2022 10:27:30 +0200
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Subject: Re: [PATCH 2/7] dt-bindings: phy: mediatek,tphy: add property to set
 pre-emphasis
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To:     Chunfeng Yun <chunfeng.yun@mediatek.com>,
        Vinod Koul <vkoul@kernel.org>, Rob Herring <robh+dt@kernel.org>
Cc:     Kishon Vijay Abraham I <kishon@ti.com>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@linaro.org>,
        Matthias Brugger <matthias.bgg@gmail.com>,
        linux-arm-kernel@lists.infradead.org,
        linux-mediatek@lists.infradead.org, linux-phy@lists.infradead.org,
        devicetree@vger.kernel.org, linux-kernel@vger.kernel.org,
        Eddie Hung <eddie.hung@mediatek.com>
References: <20220819091344.2274-1-chunfeng.yun@mediatek.com>
 <20220819091344.2274-2-chunfeng.yun@mediatek.com>
 <438da392-f419-ad76-7e91-aa4aab36e12c@linaro.org>
 <816ecf6287533137b750c8bde9de5830e4229c56.camel@mediatek.com>
 <bee8abe5-0299-d05e-643c-4810aa33f978@linaro.org>
 <1a16cce9fe164bafc06ae193310be71c6f645d75.camel@mediatek.com>
 <000babd8-5980-3d77-f156-324b3442cbe7@linaro.org>
 <114c357f8d7f049d21ede789a292a8e2d45f4c61.camel@mediatek.com>
 <0a82842d-283c-e266-84f4-6306f29b61da@linaro.org>
 <8dcb4de53a52ab44d40f490099b6ed13e5ef7fe0.camel@mediatek.com>
 <3c180570-ecf9-3db4-c698-39c1b4679c6e@linaro.org>
 <3b18a9c687af38f7299261c9a589ef3dfc5a1aa7.camel@mediatek.com>
From:   Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
In-Reply-To: <3b18a9c687af38f7299261c9a589ef3dfc5a1aa7.camel@mediatek.com>
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On 09/09/2022 05:03, Chunfeng Yun wrote:
> On Wed, 2022-08-31 at 09:03 +0300, Krzysztof Kozlowski wrote:
>> On 31/08/2022 06:00, Chunfeng Yun wrote:
>>> On Tue, 2022-08-30 at 13:08 +0300, Krzysztof Kozlowski wrote:
>>>> On 29/08/2022 05:37, Chunfeng Yun wrote:
>>>>> On Fri, 2022-08-26 at 09:36 +0300, Krzysztof Kozlowski wrote:
>>>>>> On 26/08/2022 08:36, Chunfeng Yun wrote:
>>>>>>> On Tue, 2022-08-23 at 13:24 +0300, Krzysztof Kozlowski
>>>>>>> wrote:
>>>>>>>> On 22/08/2022 10:07, Chunfeng Yun wrote:
>>>>>>>>> On Fri, 2022-08-19 at 15:15 +0300, Krzysztof Kozlowski
>>>>>>>>> wrote:
>>>>>>>>>> On 19/08/2022 12:13, Chunfeng Yun wrote:
>>>>>>>>>>> Add a property to set usb2 phy's pre-emphasis.
>>>>>>>>>>>
>>>>>>>>>>> Signed-off-by: Chunfeng Yun <
>>>>>>>>>>> chunfeng.yun@mediatek.com>
>>>>>>>>>>> ---
>>>>>>>>>>>  Documentation/devicetree/bindings/phy/mediatek,tph
>>>>>>>>>>> y.ya
>>>>>>>>>>> ml |
>>>>>>>>>>> 7
>>>>>>>>>>> +++++++
>>>>>>>>>>>  1 file changed, 7 insertions(+)
>>>>>>>>>>>
>>>>>>>>>>> diff --git
>>>>>>>>>>> a/Documentation/devicetree/bindings/phy/mediatek,tp
>>>>>>>>>>> hy.y
>>>>>>>>>>> aml
>>>>>>>>>>> b/Documentation/devicetree/bindings/phy/mediatek,tp
>>>>>>>>>>> hy.y
>>>>>>>>>>> aml
>>>>>>>>>>> index 848edfb1f677..aee2f3027371 100644
>>>>>>>>>>> ---
>>>>>>>>>>> a/Documentation/devicetree/bindings/phy/mediatek,tp
>>>>>>>>>>> hy.y
>>>>>>>>>>> aml
>>>>>>>>>>> +++
>>>>>>>>>>> b/Documentation/devicetree/bindings/phy/mediatek,tp
>>>>>>>>>>> hy.y
>>>>>>>>>>> aml
>>>>>>>>>>> @@ -219,6 +219,13 @@ patternProperties:
>>>>>>>>>>>          minimum: 1
>>>>>>>>>>>          maximum: 15
>>>>>>>>>>>  
>>>>>>>>>>> +      mediatek,pre-emphasis:
>>>>>>>>>>> +        description:
>>>>>>>>>>> +          The selection of pre-emphasis (U2 phy)
>>>>>>>>>>> +        $ref:
>>>>>>>>>>> /schemas/types.yaml#/definitions/uint32
>>>>>>>>>>> +        minimum: 1
>>>>>>>>>>> +        maximum: 3
>>>>>>>>>>
>>>>>>>>>> Instead of hard-coding register values in bindings,
>>>>>>>>>> you
>>>>>>>>>> should
>>>>>>>>>> rather
>>>>>>>>>> describe here feature/effect. If it is in units, use
>>>>>>>>>> unit
>>>>>>>>>> suffixes.
>>>>>>>>>> If
>>>>>>>>>> it is some choice, usually string enum is
>>>>>>>>>> appropriate.
>>>>>>>>>
>>>>>>>>> How about changing description as bellow:
>>>>>>>>>
>>>>>>>>> "The level of pre-emphasis, increases one level, boosts
>>>>>>>>> the
>>>>>>>>> relative
>>>>>>>>> amplitudes of signal's higher frequencies components
>>>>>>>>> about
>>>>>>>>> 4.16%
>>>>>>>>> (U2
>>>>>>>>> phy)"
>>>>>>>>>
>>>>>>>>
>>>>>>>> Still the question is what is the unit. 4.16%?
>>>>>>>
>>>>>>> No unit, it's a level value, like an index of array.
>>>>>>>
>>>>>>
>>>>>> So a value from register/device programming? 
>>>>>
>>>>> Yes
>>>>>> Rather a regular units
>>>>>> should be used if that's possible. If not, this should be
>>>>>> clearly
>>>>>> described here, not some magical number which you encode into
>>>>>> DTS...
>>>>>
>>>>> Ok, I'll add more descriptions.
>>>>
>>>> Better use logical value, e.g.
>>>>
>>>
>>>
> https://urldefense.com/v3/__https://github.com/devicetree-org/dt-schema/blob/main/dtschema/schemas/property-units.yaml*L38__;Iw!!CTRNKA9wMg0ARbw!1e-h0R_uwcaHKfKC9qYfaRWYeuWRq1sLCGy3yupNmkFyuW5s1nmRotL7Y0vFG9ETLLTA$
>>>>  
>>>
>>> Optional unit may be -percent or -bp, but the value 4.16% * X
>>> (X=1,2,3...)is not an exact value, they are variable in a range and
>>> dependent more factors.
>>> So I think use level value is simple enough.
>>
>> Then again explain exactly what are the levels. How you wrote it last
>> time, -bp would do the trick.
> 
> There are many different methods of measuring pre-emphasis.
> The way used in MediaTek USB2 PHY as below:
> 
> pre-emphasis level equation = Vpp/Vs -1;
> Vpp: peak-peak voltage of differential signal;
> Vs : static voltage of differential signal, normal voltage, e.g. 400mV
> for u2 phy;
> 
> The pre-emphasis circuitry within t-phy can be dynamically programmed
> to three different levels of pre-emphasis. The exact value of
> pre-emphasis cannot be predetermined, because each device requires
> a percentage of pre-emphasis that is dependent on the output signal
> strength and transmission path characteristics.
> 
> Below shows three programmable pre-emphasis levels for a differential
> drive signal of 400 mV. The amount of pre-emphasis changes according
> to the transmission path parameters.
> 
> programmable level   typical pre-emphasis level
> 1                    4.16%
> 2                    8.30%
> 3                    12.40%
> 
> The reasons that why prefer to use programmable level in dt-binding as
> following:
> 1. as you said, -bp may do the trick, but the main problem is that
>    pre-emphasis level is variable on different SoC, and is also
>    variable even on different pcb for the same SoC. e.g. for the
>    programmable level 1, pre-emphasis level may be 6% on a platform,
>    but for the programmable level 2, pre-emphasis level may be also
>    6% on another platform;
>    I think use pre-emphasis level in property, e.g. 4.16%, the
>    deviation may be bigger than 40%, may cause confusion for users,
>    due to we can't promise that the actual measurement is about 4.16%,
>    it may be 2%, or 5% when measured;
> 2. the programmable / logical level is flexible when we support more
>    and pre-emphasis level, ans it is easy for us to tune the level
>    due to it's continuous value.
> 3. all other vendor properties that can't provide exact measurable
>    value in this dt-binding make use of logic level, I want to
>    keep them align;

Hm, that's clarifies a lot. Thanks for explanation. It's ok for me.


Best regards,
Krzysztof