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

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

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
 - 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() and all the drm_panel_bridge module

What do you think about it?

Luca

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