Re: [PATCH 04/11] drm/panel: merge the drm_kms_helper module into the drm module

["Luca Ceresoli" <luca.ceresoli@bootlin.com>]

Hi Maxime,

On Thu Aug 27, 2026 at 4:17 PM CEST, Maxime Ripard wrote:
> On Mon, Aug 24, 2026 at 04:16:02PM +0200, Luca Ceresoli wrote:
>> Hi Maxime,
>>
>> On Mon Aug 24, 2026 at 9:48 AM CEST, Maxime Ripard wrote:
>> > On Mon, Aug 17, 2026 at 02:27:58PM +0200, Luca Ceresoli wrote:
>> >> On Mon Aug 17, 2026 at 10:31 AM CEST, Maxime Ripard wrote:
>> >> > On Fri, Aug 14, 2026 at 04:05:52PM +0200, Luca Ceresoli wrote:
>> >> >> Work is in progress to make every drm_panel automatically create a
>> >> >> panel_bridge [0][1].
>> >> >>
>> >> >> This requires the panel code to call the drm_panel_bridge APIs. However
>> >> >> this would create a circular dependency loop on modular builds:
>> >> >>
>> >> >> __devm_drm_panel_alloc() [drm]
>> >> >>    -> drm_panel_bridge_add() [drm_kms_helper]
>> >> >>       -> drm_bridge.c APIs [drm]
>> >> >>
>> >> >> Moving just the panel_brige.o file from [drm_kms_helper] to [drm] does not
>> >> >> work because the panel bridge code uses the drm_atomic_helper and
>> >> >> drm_probe_helper which add further dependencies on symbols in the
>> >> >> [drm_kms_helper] module.
>> >> >>
>> >> >> So take a simple approach, and move the entire drm_kms_helper into the
>> >> >> [drm] module.
>> >> >>
>> >> >> Link: https://lore.kernel.org/all/emuj2innmp6zmzd7pyakqzjqpdzhly6qfhakya3ydwmd63pl26@5jwxaidpikjw/ [0]
>> >> >> Link: https://lore.kernel.org/lkml/20250206-hotplug-drm-bridge-v6-8-9d6f2c9c3058@bootlin.com/ [1]
>> >> >> Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
>> >> >
>> >> > So, that's not an option. However, why do we need drm_panel_bridge_add()
>> >> > after this work is done? If we want to create a bridge for every panel,
>> >> > then the bridge implementation can live in drm_panel.c, which is part of
>> >> > the drm module.
>> >> >
>> >> > And we'd essentially move drm_panel_bridge into drm_panel.c, and make it
>> >> > private.
>> >>
>> >> Yes in theory, but the panel_bridge code uses other parts of the
>> >> drm_kms_helper module: drm_atomic_helper and drm_probe_helper, maybe more,
>> >> so we'd have to move them into the drm module too.
>> >
>> > Ah, right. What would happen if we were doing it the other way around
>> > then? Move drm_panel out of the main drm module?
>>
>> Into the drm_kms_helper module?
>>
>> I had a look and did some experiments and I found at least one user in the drm
>> module calling a drm_panel API, and guess who:
>>
>>  drm_of_find_panel_or_bridge() (in drm_of.c, drm module)
>>     -> calls of_drm_find_panel (in drm_panel.c, drm_kms_helper module)
>>
>> Based on our discussion after patch 3, I'm not sure
>> drm_of_find_panel_or_bridge() will disappear soon. If it doesn't, I guess
>> we can try to move drm_of_find_panel_or_bridge() into bridge/panel.c which
>> is in the drm_kms_helper module (and from drm_of.h info
>> drm_bridge.h?). That however might trigger build failures for drivers which
>> currently don't select DRM_KMS_HELPER and which would have to select it.
>>
>> I'll give it a try, and if I see major drawbacks I will get back to moving
>> all the drm_kms_helper code into the main drm module.
>
> No, I meant into its own module.

Looks like a good idea indeed, making the design more modular and
dependencies cleaner.

> Do we have any dependency from the main
> drm module into drm_panel?

As far as I can see there is only the one mentioned above:

  drm_of_find_panel_or_bridge() (in drm_of.c, drm module)
     -> calls of_drm_find_panel (in drm_panel.c, would-be the new drm_panel module)

And while drm_of_find_panel_or_bridge() is possibly going to disappear at
the end of my series, moving drm_panel.c to its own module would make the
series non-build-bisectable, which would be very annoying.

However there seem to be no calls from the drm module to
drm_of_find_panel_or_bridge(), so we could move it out as well. What about
a series doing, in this order:

 - move drm_of_find_panel_or_bridge() from the drm module to the
   drm_kms_helper module
 - move drm_panel.c to a new drm_panel module
 - Main change: embed a drm_bridge into every panel
 - convert drivers to stop using the panel_bridge, hopefully removing all
   calls to drm_of_find_panel_or_bridge()
 - remove drm_of_find_panel_or_bridge()

Looks good?

Luca

--
Luca Ceresoli, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com