Re: [PATCH 2/7] dt-bindings: phy: mediatek,tphy: add property to set pre-emphasis

[Chunfeng Yun <chunfeng.yun@mediatek.com>]

On Fri, 2022-09-09 at 10:27 +0200, Krzysztof Kozlowski wrote:
> 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/mediate
> > > > > > > > > > > > k,tp
> > > > > > > > > > > > hy.y
> > > > > > > > > > > > aml
> > > > > > > > > > > > b/Documentation/devicetree/bindings/phy/mediate
> > > > > > > > > > > > k,tp
> > > > > > > > > > > > hy.y
> > > > > > > > > > > > aml
> > > > > > > > > > > > index 848edfb1f677..aee2f3027371 100644
> > > > > > > > > > > > ---
> > > > > > > > > > > > a/Documentation/devicetree/bindings/phy/mediate
> > > > > > > > > > > > k,tp
> > > > > > > > > > > > hy.y
> > > > > > > > > > > > aml
> > > > > > > > > > > > +++
> > > > > > > > > > > > b/Documentation/devicetree/bindings/phy/mediate
> > > > > > > > > > > > k,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.
I couldn't know more about pre-emphasis without your questions, thank
you very much.

>  It's ok for me.
> 
> 
> Best regards,
> Krzysztof


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