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

[Maxime Ripard <mripard@kernel.org>]

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

Maxime

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