linux-kernel - Re: [PATCH 1/3] arm64: dts: qcom: msm8916: Add missing MDSS reset

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Message-Id: <DCWOLHPCYG3X.32KTGBE4SYMDV@fairphone.com>
Date: Fri, 19 Sep 2025 11:49:20 +0200
From: "Luca Weiss" <luca.weiss@...rphone.com>
To: "Stephan Gerhold" <stephan.gerhold@...aro.org>, "Bjorn Andersson"
 <andersson@...nel.org>, "Konrad Dybcio" <konradybcio@...nel.org>
Cc: "Vincent Knecht" <vincent.knecht@...loo.org>, "Bryan O'Donoghue"
 <bryan.odonoghue@...aro.org>, "Rob Herring" <robh@...nel.org>, "Krzysztof
 Kozlowski" <krzk+dt@...nel.org>, "Conor Dooley" <conor+dt@...nel.org>,
 <linux-arm-msm@...r.kernel.org>, <devicetree@...r.kernel.org>,
 <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/3] arm64: dts: qcom: msm8916: Add missing MDSS reset

Hi Stephan,

On Mon Sep 15, 2025 at 3:28 PM CEST, Stephan Gerhold wrote:
> On most MSM8916 devices (aside from the DragonBoard 410c), the bootloader
> already initializes the display to show the boot splash screen. In this
> situation, MDSS is already configured and left running when starting Linux.
> To avoid side effects from the bootloader configuration, the MDSS reset can
> be specified in the device tree to start again with a clean hardware state.
>
> The reset for MDSS is currently missing in msm8916.dtsi, which causes
> errors when the MDSS driver tries to re-initialize the registers:
>
>  dsi_err_worker: status=6
>  dsi_err_worker: status=6
>  dsi_err_worker: status=6
>  ...
>
> It turns out that we have always indirectly worked around this by building
> the MDSS driver as a module. Before v6.17, the power domain was temporarily
> turned off until the module was loaded, long enough to clear the register
> contents. In v6.17, power domains are not turned off during boot until
> sync_state() happens, so this is no longer working. Even before v6.17 this
> resulted in broken behavior, but notably only when the MDSS driver was
> built-in instead of a module.

Do you have a link to the patch that causes this behavior? I've tried
looking through the git log for drivers/gpu/drm/msm/ but couldn't find
anything that looks relevant.

FWIW a similar change to this was also necessary for sc7280 (as done by
Bjorn) and for sm6350 (will send the patches very soon).

And happily enough for me, with v6.17 and that reset, a long-standing
issue on sm7225-fairphone-fp4 has been resolved that the display init
seems to somehow fail the first time after bootup, with the screen
needing to be turned off once and back on to work. I traced this back
to some power domain behavior as well back then.

> "mdss_gdsc needs to be off before mdss/dpu probe, this can happen with
> genpd_power_off_unused but not guaranteed"

Anyways, I'm hoping this is not just a coincidence it works now but
will stay working on my device. Just the reset in the past didn't seem
to affect anything.

Regards
Luca

>
> Cc: stable@...r.kernel.org
> Fixes: 305410ffd1b2 ("arm64: dts: msm8916: Add display support")
> Signed-off-by: Stephan Gerhold <stephan.gerhold@...aro.org>
> ---
>  arch/arm64/boot/dts/qcom/msm8916.dtsi | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/arch/arm64/boot/dts/qcom/msm8916.dtsi b/arch/arm64/boot/dts/qcom/msm8916.dtsi
> index b50c7e6e0bfcd35ab4f8b84aeabe214fd60e8d7c..de0c10b54c86c7795b7a0d1ecd80652e60e117b6 100644
> --- a/arch/arm64/boot/dts/qcom/msm8916.dtsi
> +++ b/arch/arm64/boot/dts/qcom/msm8916.dtsi
> @@ -1562,6 +1562,8 @@ mdss: display-subsystem@...0000 {
>  
>  			interrupts = <GIC_SPI 72 IRQ_TYPE_LEVEL_HIGH>;
>  
> +			resets = <&gcc GCC_MDSS_BCR>;
> +
>  			interrupt-controller;
>  			#interrupt-cells = <1>;
>