linux-kernel - Re: [REGRESSION] drm/panel/panel-simple v6.17 WARNING regression

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Message-ID: <kcunz2b2usmvadgrnnu65op5oi5ttblrc463twgxp5gqhnufav@eze53y23avel>
Date: Tue, 14 Oct 2025 13:25:53 +0200
From: Ernest Van Hoecke <ernestvanhoecke@...il.com>
To: Luca Ceresoli <luca.ceresoli@...tlin.com>
Cc: Anusha Srivatsa <asrivats@...hat.com>, 
	Maxime Ripard <mripard@...nel.org>, Neil Armstrong <neil.armstrong@...aro.org>, 
	Andrzej Hajda <andrzej.hajda@...el.com>, Jessica Zhang <quic_jesszhan@...cinc.com>, 
	Robert Foss <rfoss@...nel.org>, Laurent Pinchart <Laurent.pinchart@...asonboard.com>, 
	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, Thomas Zimmermann <tzimmermann@...e.de>, 
	Thomas Petazzoni <thomas.petazzoni@...tlin.com>, David Airlie <airlied@...il.com>, 
	Simona Vetter <simona@...ll.ch>, Hui Pu <Hui.Pu@...ealthcare.com>, 
	Dmitry Baryshkov <dmitry.baryshkov@....qualcomm.com>, dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org, 
	imx@...ts.linux.dev, regressions@...ts.linux.dev
Subject: Re: [REGRESSION] drm/panel/panel-simple v6.17 WARNING regression

Hi Luca,

Thanks for your fast reply and all your work here.

On Tue, Oct 14, 2025 at 12:31:35PM +0200, Luca Ceresoli wrote:
> Let me have a look at the DRM_IMX driver, I'll try to send a series
> converting it to the new API within today.

I will gladly test, thanks!

> I recently sent a series proposing to make drm_bridge_add() mandatory
> before drm_bridge_attach() in the docs and warn if that is violated [1]. If
> you apply patch 4 of that series you should see the warning.

I gave it a quick try and did not see the warning. Some printk debugging
told me that `list_empty(&bridge->list)`, inside drm_bridge_attach, is
returning 0. 

> > However, later on, another regression seems to be introduced by
> > commit 8fa5909400f3 ("drm/bridge: get the bridge returned by drm_bridge_chain_get_first_bridge()")
> > so reverting 94d50c1a2ca3 on top of drm-misc-next does not solve
> > everything. This was tested by rebasing drm-misc-next onto (260f6f4fda93
> > plus the revert of 94d50c1a2ca3) and then bisecting.
> >
> > So in v6.18-rc1, both regressions are present.
> >
> > There, I get the following additional warnings:
> >
> > [    9.732278] ------------[ cut here ]------------
> > [    9.732336] WARNING: CPU: 0 PID: 38 at lib/refcount.c:22 drm_bridge_get+0x10/0x18
> > [    9.744608] refcount_t: saturated; leaking memory.
> 
> Not sure here, but it may well be another symptom of the same bug: the
> refcount was not initialized correctly, so it is found inconsistent later
> when trying to increase it. Let's fix the known issue and then we'll see.

Makes sense to me.

Kind regards,
Ernest