linux-kernel - Re: [PATCH v3 1/2] arm64: dts: qcom: x1e78100-t14s: add hpd gpio to eDP panel

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Message-ID: <Z-pJP4PMwPo3L3Og@hovoldconsulting.com>
Date: Mon, 31 Mar 2025 09:50:23 +0200
From: Johan Hovold <johan@...nel.org>
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>,
	Dmitry Baryshkov <dmitry.baryshkov@....qualcomm.com>,
	dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
	linux-arm-msm@...r.kernel.org,
	Rui Miguel Silva <rui.silva@...aro.org>,
	Abel Vesa <abel.vesa@...aro.org>, devicetree@...r.kernel.org,
	Bryan O'Donoghue <bryan.odonoghue@...aro.org>
Subject: Re: [PATCH v3 1/2] arm64: dts: qcom: x1e78100-t14s: add hpd gpio to
 eDP panel

On Thu, Mar 27, 2025 at 04:56:53PM +0000, Christopher Obbard wrote:
> The eDP panel has an HPD GPIO. Describe it in the device tree
> for the generic T14s model, as the HPD GPIO property is used in
> both the OLED and LCD models which inherit this device tree.

AFAICT, this patch is not correct as the hotplug detect signal is
connected directly to the display controller on (these) Qualcomm SoCs
and is already handled by its driver.

Describing it as you do here leads to less accurate delays, see commits:
	
	2327b13d6c47 ("drm/panel-edp: Take advantage of wait_hpd_asserted() in struct drm_dp_aux").
	3b5765df375c ("drm/panel: atna33xc20: Take advantage of wait_hpd_asserted() in struct drm_dp_aux")

Perhaps you lose some other functionality too.
 
> Reviewed-by: Bryan O'Donoghue <bryan.odonoghue@...aro.org>
> Signed-off-by: Christopher Obbard <christopher.obbard@...aro.org>
> ---
>  arch/arm64/boot/dts/qcom/x1e78100-lenovo-thinkpad-t14s.dtsi | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/arch/arm64/boot/dts/qcom/x1e78100-lenovo-thinkpad-t14s.dtsi b/arch/arm64/boot/dts/qcom/x1e78100-lenovo-thinkpad-t14s.dtsi
> index 962fb050c55c4fd33f480a21a8c47a484d0c82b8..46c73f5c039ed982b553636cf8c4237a20ba7687 100644
> --- a/arch/arm64/boot/dts/qcom/x1e78100-lenovo-thinkpad-t14s.dtsi
> +++ b/arch/arm64/boot/dts/qcom/x1e78100-lenovo-thinkpad-t14s.dtsi
> @@ -980,8 +980,12 @@ &mdss_dp3 {
>  	aux-bus {
>  		panel: panel {
>  			compatible = "edp-panel";
> +			hpd-gpios = <&tlmm 119 GPIO_ACTIVE_HIGH>;
>  			power-supply = <&vreg_edp_3p3>;
>  
> +			pinctrl-0 = <&edp_hpd_n_default>;
> +			pinctrl-names = "default";
> +
>  			port {
>  				edp_panel_in: endpoint {
>  					remote-endpoint = <&mdss_dp3_out>;
> @@ -1286,6 +1290,13 @@ hall_int_n_default: hall-int-n-state {
>  		bias-disable;
>  	};
>  
> +	edp_hpd_n_default: edp-hpd-n-state {
> +		pins = "gpio119";
> +		function = "gpio";
> +		drive-strength = <2>;
> +		bias-pull-up;
> +	};

I checked the firmware configuration for this pin on my T14s, which
does not match what you have here. Instead the function is set to
"edp0_hot" which forwards the signal to the display controller which
already handles the signal on panel power on. (And there is also no
internal pull up enabled).

We may want to describe this pin configuration somewhere, but that's a
separate issue.

Johan