linux-kernel - Re: [PATCH v2 1/3] dt-bindings: phy: qcom,usb-snps-femto-v2: Add phy override params bindings

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Date:   Mon, 14 Mar 2022 08:59:52 +0530
From:   Pavan Kondeti <quic_pkondeti@...cinc.com>
To:     Krzysztof Kozlowski <krzysztof.kozlowski@...onical.com>
CC:     Sandeep Maheswaram <quic_c_sanm@...cinc.com>,
        Rob Herring <robh+dt@...nel.org>,
        Andy Gross <agross@...nel.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Kishon Vijay Abraham I <kishon@...com>,
        "Vinod Koul" <vkoul@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Wesley Cheng <wcheng@...eaurora.org>,
        Stephen Boyd <swboyd@...omium.org>,
        Doug Anderson <dianders@...omium.org>,
        Matthias Kaehlcke <mka@...omium.org>,
        <devicetree@...r.kernel.org>, <linux-arm-msm@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <linux-phy@...ts.infradead.org>,
        <linux-usb@...r.kernel.org>, <quic_pkondeti@...cinc.com>,
        <quic_ppratap@...cinc.com>, <quic_kriskura@...cinc.com>
Subject: Re: [PATCH v2 1/3] dt-bindings: phy: qcom,usb-snps-femto-v2: Add phy
 override params bindings

Hi Krzysztof,

On Thu, Mar 03, 2022 at 04:59:22PM +0100, Krzysztof Kozlowski wrote:
> On 03/03/2022 07:13, Sandeep Maheswaram wrote:
> > Add device tree bindings for SNPS phy tuning parameters.
> > 
> > Signed-off-by: Sandeep Maheswaram <quic_c_sanm@...cinc.com>
> > ---
> >  .../bindings/phy/qcom,usb-snps-femto-v2.yaml       | 125 +++++++++++++++++++++
> >  1 file changed, 125 insertions(+)
> > 
> > diff --git a/Documentation/devicetree/bindings/phy/qcom,usb-snps-femto-v2.yaml b/Documentation/devicetree/bindings/phy/qcom,usb-snps-femto-v2.yaml
> > index 0dfe691..227c097 100644
> > --- a/Documentation/devicetree/bindings/phy/qcom,usb-snps-femto-v2.yaml
> > +++ b/Documentation/devicetree/bindings/phy/qcom,usb-snps-femto-v2.yaml
> > @@ -50,6 +50,131 @@ properties:
> >    vdda33-supply:
> >      description: phandle to the regulator 3.3V supply node.
> >  
> > +  qcom,hs-disconnect:
> > +    $ref: /schemas/types.yaml#/definitions/uint32
> > +    description:
> > +      This adjusts the voltage level for the threshold used to
> > +      detect a disconnect event at the host. Possible values are.
> 
> ':', instead of full stop.
> 
> > +      7 -> +21.56%
> > +      6 -> +17.43%
> > +      5 -> +13.32%
> > +      4 -> +9.73%
> > +      3 -> +6.3
> > +      2 -> +3.17%
> > +      1 -> 0, Design default%
> 
> Use "default:" instead. Here and in other places.
> 
> > +      0 -> -2.72%
> 
> In current form this should be an enum... but actually current form is
> wrong. You should not store register values in DT. What if next version
> of hardware has a different meaning of these values?
> 
> Instead, you should store here meaningful values, not register values.
> 

Thanks for the feedback.

The values in % really makes the tuning easy. People look at the eye diagram
and decided whether to increase/decrease the margin. The absolute values
may not be that useful. All we need is an "adjustment" here. The databook
it self does not give any absolute values.

I agree to the "enum" suggestion which we have been following for the
qusb2 driver already. 

The values have not changed in the last 5 years for this hardware block, so
defining enums for the % values would be really helpful. 

> 
> > +
> > +  qcom,squelch-detector:
> > +    $ref: /schemas/types.yaml#/definitions/uint32
> > +    description:
> > +      This adjusts the voltage level for the threshold used to
> > +      detect valid high-speed data. Possible values are
> > +      7-> -20.90%
> > +      6-> -15.60%
> > +      5-> -10.30%
> > +      4-> -5.30%
> > +      3-> 0, Design default%
> > +      2-> +5.30%
> > +      1-> +10.60%
> > +      0-> +15.90%
> > +
> > +  qcom,hs-amplitude:
> > +    $ref: /schemas/types.yaml#/definitions/uint32
> > +    description:
> > +      This adjusts the high-speed DC level voltage.
> > +      Possible values are
> > +      15-> +26.70%
> > +      14-> +24.30%
> > +      13-> +22.20%
> > +      12-> +20.00%
> > +      11-> +17.80%
> > +      10-> +15.60%
> > +      9-> +13.30%
> > +      8-> +11.10%
> > +      7-> +8.90%
> > +      6-> +6.50%
> > +      5-> +4.40%
> > +      4-> +2.30%
> > +      3-> 0, Design default%
> > +      2-> -2.20%
> > +      1-> -4.40%
> > +      0-> -6.60%
> > +
> > +  qcom,pre-emphasis-duration:
> > +    $ref: /schemas/types.yaml#/definitions/uint32
> > +    description:
> > +      This signal controls the duration for which the
> > +      HS pre-emphasis current is sourced onto DP<#> or DM<#>.
> > +      The HS Transmitter pre-emphasis duration is defined in terms of
> > +      unit amounts. One unit of pre-emphasis duration is approximately
> > +      650 ps and is defined as 1X pre-emphasis duration.
> > +      Possible values are
> > +      1-> 1x, short pre-emphasis current duration
> > +      0-> 2x, long pre-emphasis current duration
> 
> I could understand encoding of percentages in way of register value, but
> a boolean flag is too much.
> 

Agreed. This needs to be encoded in % as well (100% or 200%).

Thanks,
Pavan