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 Tue, Sep 01, 2026 at 04:30:40PM +0200, Luca Ceresoli wrote:
> On Tue Sep 1, 2026 at 2:40 PM CEST, Luca Ceresoli wrote:
> 
> [...]
> 
> >>> >> > 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()
> 
> Ah, no, that won't work. There would be a circular module dependency loop
> later on when we embed a drm_bridge into evern drm_panel: indeed at that
> point drm_panel will use the atomic and probe helpers to implement the
> embedded drm_bridge, resulting in:
> 
>  * the panel_bridge code in bridge/panel.c [drm_kms_helper module]
>    already depends on the drm_panel.c code (it manipulates a drm_panel, OK)
>  * additionally, the drm_panel.c code, in order to create a drm_bridge,
>    will depend on the helpers in drm_atomic_helper and drm_probe_helper
>    code [drm_kms_helper module]
> 
> The loop is only between kernel modules (.ko), not in actual code. So I
> think this revised plan should work (the 2nd bullet is key):
> 
>  - move drm_of_find_panel_or_bridge() from the drm module to
>    bridge/panel.c [currently drm_kms_helper module]
>  - move bridge/panel.o to a new drm_panel_bridge module (NEW)
>  - move drm_panel.c to a new drm_panel module

Looks good on principle, but iirc the starting point of that discussion
was to move bridge panel into the new panel module, so I guess we could:

1) Move drm_of_find_panel_or_bridge() to drm_panel.c. Both are still in
the drm module at this point, so it should be ok.
2) Create a new panel module, with a dependency on bridge
3) Move the bridge/panel.c code into the new panel module to create the
bridge at the same time we create the panel.

Would that work?

Maxime

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