linux-kernel - Re: [PATCH] arm64: dts: qcom: c630: Add no-hpd to DSI bridge node

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Date:   Wed, 24 Mar 2021 15:43:39 -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>,
        Steev Klimaszewski <steev@...i.org>
Subject: Re: [PATCH] arm64: dts: qcom: c630: Add no-hpd to DSI bridge node

Hi,

On Wed, Mar 24, 2021 at 12:15 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.

It's not really about wasting power. It's really more about:

a) If HPD is actually hooked up on the board, it's actually _slower_
to use it than to just assume the worst case time.

b) If HPD isn't hooked up but we try to use it then everything will just fail.

I don't know which of a) or b) is true, but I'd imagine that one or
the other is.


> Presumably this board isn't using hpd
> on the bridge.
>
> Cc: Laurent Pinchart <laurent.pinchart+renesas@...asonboard.com>
> Cc: Douglas Anderson <dianders@...omium.org>

Cc: Steev Klimaszewski <steev@...i.org>


> Fixes: 7ec3e67307f8 ("arm64: dts: qcom: sc7180-trogdor: add initial trogdor and lazor dt")

Wrong Fixes?


> Signed-off-by: Stephen Boyd <swboyd@...omium.org>
> ---
>  arch/arm64/boot/dts/qcom/sdm850-lenovo-yoga-c630.dts | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/arch/arm64/boot/dts/qcom/sdm850-lenovo-yoga-c630.dts b/arch/arm64/boot/dts/qcom/sdm850-lenovo-yoga-c630.dts
> index 140db2d5ba31..c2a709a384e9 100644
> --- a/arch/arm64/boot/dts/qcom/sdm850-lenovo-yoga-c630.dts
> +++ b/arch/arm64/boot/dts/qcom/sdm850-lenovo-yoga-c630.dts
> @@ -376,6 +376,8 @@ sn65dsi86: bridge@2c {
>                 clocks = <&sn65dsi86_refclk>;
>                 clock-names = "refclk";
>
> +               no-hpd;
> +
>                 ports {
>                         #address-cells = <1>;
>                         #size-cells = <0>;
>
> base-commit: a38fd8748464831584a19438cbb3082b5a2dab15
> --
> https://chromeos.dev
>