linux-kernel - Re: [PATCH] arm64: dts: qcom: trogdor: Add no-hpd to DSI bridge node

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Message-ID: <CAD=FV=VfM10w60VUuTW5yXdykC8oXuxqq=FLfZYDP2aUh0P0_g@mail.gmail.com>
Date:   Wed, 24 Mar 2021 08:32:00 -0700
From:   Doug Anderson <dianders@...omium.org>
To:     Stephen Boyd <swboyd@...omium.org>
Cc:     Andy Gross <agross@...nel.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        Laurent Pinchart <laurent.pinchart+renesas@...asonboard.com>
Subject: Re: [PATCH] arm64: dts: qcom: trogdor: Add no-hpd to DSI bridge node

Hi,

On Tue, Mar 23, 2021 at 7:55 PM Stephen Boyd <swboyd@...omium.org> wrote:
>
> We should indicate that we're not using the HPD pin on this device, per
> the binding document. Otherwise if code in the future wants to enable
> HPD in the bridge when this property is absent we'll be wasting power
> powering hpd when we don't use it on trogdor boards. We didn't notice
> this before because the kernel driver blindly disables hpd, but that
> won't be true for much longer.
>
> Cc: Laurent Pinchart <laurent.pinchart+renesas@...asonboard.com>
> Cc: Douglas Anderson <dianders@...omium.org>
> Fixes: 7ec3e67307f8 ("arm64: dts: qcom: sc7180-trogdor: add initial trogdor and lazor dt")
> Signed-off-by: Stephen Boyd <swboyd@...omium.org>
> ---
>  arch/arm64/boot/dts/qcom/sc7180-trogdor.dtsi | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/arch/arm64/boot/dts/qcom/sc7180-trogdor.dtsi b/arch/arm64/boot/dts/qcom/sc7180-trogdor.dtsi
> index 07c8b2c926c0..298af6d7fb4a 100644
> --- a/arch/arm64/boot/dts/qcom/sc7180-trogdor.dtsi
> +++ b/arch/arm64/boot/dts/qcom/sc7180-trogdor.dtsi
> @@ -595,6 +595,8 @@ sn65dsi86_bridge: bridge@2d {
>                 clocks = <&rpmhcc RPMH_LN_BB_CLK3>;
>                 clock-names = "refclk";
>
> +               no-hpd;
> +

Sigh. I can't believe that I added this for cheza in commit
0d1ce0d14bd7 ("arm64: dts: sdm845: Add "no-hpd" to sn65dsi86 on
cheza") but forgot trogdor. Thanks.

Reviewed-by: Douglas Anderson <dianders@...omium.org>

NOTE: if you were feeling charitable you might consider sending a
patch for "sdm850-lenovo-yoga-c630.dts" as well.  I don't personally
know if HPD is hooked up on that system, but presumably even if it is
it's just as useless as it is on other systems where the bridge is
used for eDP. If nothing else setting it preserves existing behavior.
Someone (I forget who) requested that I word the bindings specifically
to say that "no-hpd" was OK to specify in cases like that, since the
bindings say:

>      Set if the HPD line on the bridge isn't hooked up to anything or is
>      otherwise unusable.

-Doug