linux-kernel - Re: [v2 1/3] dt-bindings: msm/dsi: Add 10nm dsi phy tuning properties

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Message-ID: <Ydxsz+XSreBKFNcr@robh.at.kernel.org>
Date:   Mon, 10 Jan 2022 11:28:47 -0600
From:   Rob Herring <robh@...nel.org>
To:     Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
Cc:     Rajeev Nandan <quic_rajeevny@...cinc.com>,
        dri-devel@...ts.freedesktop.org, linux-arm-msm@...r.kernel.org,
        freedreno@...ts.freedesktop.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, sean@...rly.run, robdclark@...il.com,
        quic_abhinavk@...cinc.com, quic_kalyant@...cinc.com,
        quic_mkrishn@...cinc.com, jonathan@...ek.ca, airlied@...ux.ie,
        daniel@...ll.ch, swboyd@...omium.org
Subject: Re: [v2 1/3] dt-bindings: msm/dsi: Add 10nm dsi phy tuning properties

On Mon, Jan 10, 2022 at 05:06:03PM +0300, Dmitry Baryshkov wrote:
> On Mon, 10 Jan 2022 at 15:56, Rajeev Nandan <quic_rajeevny@...cinc.com> wrote:
> >
> > In most cases, the default values of DSI PHY tuning registers should be
> > sufficient as they are fully optimized. However, in some cases where
> > extreme board parasitics cause the eye shape to degrade, the override
> > bits can be used to improve the signal quality.
> >
> > The general guidelines for DSI PHY tuning include:
> > - High and moderate data rates may benefit from the drive strength and
> >   drive level tuning.
> > - Drive strength tuning will affect the output impedance and may be used
> >   for matching optimization.
> > - Drive level tuning will affect the output levels without affecting the
> >   impedance.
> >
> > The clock and data lanes have a calibration circuitry feature. The drive
> > strength tuning can be done by adjusting rescode offset for hstop/hsbot,
> > and the drive level tuning can be done by adjusting the LDO output level
> > for the HSTX drive.
> >
> > Signed-off-by: Rajeev Nandan <quic_rajeevny@...cinc.com>
> > ---
> >
> > Changes in v2:
> >  - More details in the commit text (Stephen Boyd)
> >  - Use human understandable values (Stephen Boyd, Dmitry Baryshkov)
> >  - Do not take values that are going to be unused (Dmitry Baryshkov)
> >
> >  .../bindings/display/msm/dsi-phy-10nm.yaml         | 33 ++++++++++++++++++++++
> >  1 file changed, 33 insertions(+)
> >
> > diff --git a/Documentation/devicetree/bindings/display/msm/dsi-phy-10nm.yaml b/Documentation/devicetree/bindings/display/msm/dsi-phy-10nm.yaml
> > index 4399715..d0eb8f6 100644
> > --- a/Documentation/devicetree/bindings/display/msm/dsi-phy-10nm.yaml
> > +++ b/Documentation/devicetree/bindings/display/msm/dsi-phy-10nm.yaml
> > @@ -35,6 +35,35 @@ properties:
> >        Connected to DSI0_MIPI_DSI_PLL_VDDA0P9 pin for sc7180 target and
> >        connected to VDDA_MIPI_DSI_0_PLL_0P9 pin for sdm845 target
> 
> Generic note:
> I think these properties should be prefixed with "qcom," prefix.
> 
> >
> > +  phy-rescode-offset-top:
> > +    $ref: /schemas/types.yaml#/definitions/uint8-array
> > +    minItems: 5
> > +    maxItems: 5
> > +    description:
> > +      Integer array of offset for pull-up legs rescode for all five lanes.
> > +      To offset the drive strength from the calibrated value in an increasing
> > +      or decreasing manner, use 6 bit two’s complement values.
> 
> dtc should support negative values, google hints that <(-2)> should work.

Yes, but the schema checks don't check negative values correctly yet. So 
you can use 'int8-array', but just don't use negative values in the 
examples. I'm working on changes that will fix this issue.

What does 6-bit mean? 0x3f is negative? Just sign extend the values and 
specify the valid range instead:

minimum: -32
maximum: 31

Rob