linux-kernel - Re: [PATCH v2 4/4] drm/dp: fallback to maximum when PWM bit count is zero

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Message-ID: <jbowz36pz7es7koc3abi3maw6ytz4auuobhkmkqelvwavilqkr@gxgsdhao3skr>
Date: Wed, 26 Mar 2025 19:17:03 +0200
From: Dmitry Baryshkov <dmitry.baryshkov@....qualcomm.com>
To: Christopher Obbard <christopher.obbard@...aro.org>
Cc: Douglas Anderson <dianders@...omium.org>,
        Neil Armstrong <neil.armstrong@...aro.org>,
        Jessica Zhang <quic_jesszhan@...cinc.com>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
        Rob Herring <robh@...nel.org>,
        Krzysztof Kozlowski <krzk+dt@...nel.org>,
        Conor Dooley <conor+dt@...nel.org>,
        Bjorn Andersson <andersson@...nel.org>,
        Konrad Dybcio <konradybcio@...nel.org>,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        linux-arm-msm@...r.kernel.org, Johan Hovold <johan@...nel.org>,
        Rui Miguel Silva <rui.silva@...aro.org>,
        Abel Vesa <abel.vesa@...aro.org>, devicetree@...r.kernel.org
Subject: Re: [PATCH v2 4/4] drm/dp: fallback to maximum when PWM bit count is
 zero

On Wed, Mar 26, 2025 at 03:08:30PM +0000, Christopher Obbard wrote:
> Hi Dmitry,
> 
> On Tue, 25 Mar 2025 at 22:53, Dmitry Baryshkov
> <dmitry.baryshkov@....qualcomm.com> wrote:
> >
> > On 25/03/2025 21:21, Christopher Obbard wrote:
> > > Some eDP devices report DP_EDP_PWMGEN_BIT_COUNT as 0, but still provide
> > > valid non-zero MIN and MAX values. This patch reworks the logic to
> > > fallback to the max value in such cases, ensuring correct backlight PWM
> > > configuration even when the bit count value is not explicitly set.
> >
> > I don't think this matches the eDP standard. It tells to use MIN if
> > BIT_COUNT is less than MIN, if I understand it correctly.
> 
> Thanks for your comment; that's a good point.
> 
> I need to re-read this section of the spec; but at least on this
> hardware I printed the values of the registers and it seems like
> MIN and MAX are the same, so I could switch the patch around to use
> MIN in the next version.

SGTM.

-- 
With best wishes
Dmitry