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

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Message-ID: <aM0o-phEMI0wSPA7@linaro.org>
Date: Fri, 19 Sep 2025 11:57:14 +0200
From: Stephan Gerhold <stephan.gerhold@...aro.org>
To: Luca Weiss <luca.weiss@...rphone.com>
Cc: Bjorn Andersson <andersson@...nel.org>,
	Konrad Dybcio <konradybcio@...nel.org>,
	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

On Fri, Sep 19, 2025 at 11:49:20AM +0200, Luca Weiss wrote:
> 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.

It's not a drm/msm change, the change was done in the pmdomain core:
https://lore.kernel.org/r/20250701114733.636510-1-ulf.hansson@linaro.org/

> 
> 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.

You might have had multiple different problems in the past. Perhaps
keeping on the power domain during boot actually helps ensure a clean
shutdown of the MDSS/DPU and then the reset is enough to bring it into a
clean state for the new configuration. But that's just guessing.

Thanks,
Stephan