linux-kernel - Re: [PATCH v2 05/20] drm/tilcdc: Convert legacy panel binding via DT overlay at boot time

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Message-ID: <20260105181838.1f307964@kmaincent-XPS-13-7390>
Date: Mon, 5 Jan 2026 18:18:38 +0100
From: Kory Maincent <kory.maincent@...tlin.com>
To: Herve Codina <herve.codina@...tlin.com>
Cc: "Luca Ceresoli" <luca.ceresoli@...tlin.com> (by way of Kory Maincent
 <kory.maincent@...tlin.com>), "Jyri Sarha" <jyri.sarha@....fi>, "Tomi
 Valkeinen" <tomi.valkeinen@...asonboard.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>, "Russell King" <linux@...linux.org.uk>,
 "Bartosz Golaszewski" <brgl@...ev.pl>, "Tony Lindgren" <tony@...mide.com>,
 "Andrzej Hajda" <andrzej.hajda@...el.com>, "Neil Armstrong"
 <neil.armstrong@...aro.org>, "Robert Foss" <rfoss@...nel.org>, "Laurent
 Pinchart" <Laurent.pinchart@...asonboard.com>, "Jonas Karlman"
 <jonas@...boo.se>, "Jernej Skrabec" <jernej.skrabec@...il.com>, "Markus
 Schneider-Pargmann" <msp@...libre.com>, "Bajjuri Praneeth"
 <praneeth@...com>, "Louis Chauvet" <louis.chauvet@...tlin.com>, "Thomas
 Petazzoni" <thomas.petazzoni@...tlin.com>, "Miguel Gazquez"
 <miguel.gazquez@...tlin.com>, <dri-devel@...ts.freedesktop.org>,
 <devicetree@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
 <linux-arm-kernel@...ts.infradead.org>, <linux-omap@...r.kernel.org>
Subject: Re: [PATCH v2 05/20] drm/tilcdc: Convert legacy panel binding via
 DT overlay at boot time

On Mon, 5 Jan 2026 17:22:20 +0100
Herve Codina <herve.codina@...tlin.com> wrote:

> Hi Luca, Kory,
> 
> On Wed, 17 Dec 2025 15:23:26 +0100
> "Luca Ceresoli" <luca.ceresoli@...tlin.com> (by way of Kory Maincent
> <kory.maincent@...tlin.com>) wrote:
> 
> > Hi,
> > 
> > Cc: Hervé, can you review the DT overlay aspects?  
> 
> Yes sure.
> 
> Here is my global review.
> 
> Depending on the discussion on things I have spotted, I will go deeper in
> patch details.
> 
> ...
> 
> > > +
> > > +static void __init
> > > +tilcdc_panel_update_prop(struct device_node *node, char *name,
> > > +			 void *val, int length)
> > > +{
> > > +	struct property *prop;
> > > +
> > > +	prop = kzalloc(sizeof(*prop), GFP_KERNEL);
> > > +	if (!prop)
> > > +		return;
> > > +
> > > +	prop->name = kstrdup(name, GFP_KERNEL);
> > > +	prop->length = length;
> > > +	prop->value = kmemdup(val, length, GFP_KERNEL);
> > > +	of_update_property(node, prop);  
> 
> I would use OF changesets to perform the modification.
> 
> OF changesets are kind of atomic. You first prepare all modifications in a
> changeset and then you apply the changeset.
> If something goes wrong, the changeset is removed.
> 
> Also, if something goes wrong during the changeset preparation, you can abort
> without any modification on the live device-tree.

Ok, I will take a look at changeset to use it to copy all the properties to new
the panel-dpi node.

...

> > > +
> > > +	ret = of_overlay_fdt_apply(dtbo_start, dtbo_size, &ovcs_id,
> > > NULL);
> > > +	if (ret)
> > > +		goto out;  
> 
> As soon as the overlay is applied, the driver handling the panel-dti node
> can be probed.
>
> Modifying some properties after applying the overlay could be not seen by the
> driver.

I disagree here. This driver is loaded at subsys_initcall (initcall 4) while
the panel is loaded at device_initcall (initcall 6) so the panel driver won't
probe in-between.

...

> > > +subsys_initcall(tilcdc_panel_legacy_init);  
> 
> IMHO, the call to tilcdc_panel_legacy_init() will be too late.
> 
> subsys initcalls are called after arch initcalls.
> 
> During arch initcalls, of_platform_populate_init() is called
> https://elixir.bootlin.com/linux/v6.19-rc3/source/drivers/of/platform.c#L599
> 
> The root node is populated and handled by the platform bus.
> 
> Later at subsys initcall, the tilcdc_panel_legacy_init() function is called.
> This function starts by applying the overlay and so a new node (panel-dpi)
> is added at the root node.
> 
> This trigger an OF_RECONFIG_CHANGE_ADD event handled by the platform bus.
> https://elixir.bootlin.com/linux/v6.19-rc3/source/drivers/of/platform.c#L731
> 
> If the "panel-dpi" compatible driver is available, its probe() is called but
> the panel-dpi DT node is not fully correct. Indeed, tilcdc_panel_copy_props()
> has not be called yet.

As said before, I don't think this will happen due to the initcall difference
value.

> Also, the legacy compatible string is removed after the
> of_platform_populate_init() call. The legacy driver could have been already
> probed.

Same here.

Regards,
-- 
Köry Maincent, Bootlin
Embedded Linux and kernel engineering
https://bootlin.com