Re: [PATCH v2 1/4] drm/dp: Add a way to init/add a connector in separate steps

[Jani Nikula <jani.nikula@intel.com>]

On Fri, 29 Nov 2024, Maxime Ripard <mripard@kernel.org> wrote:
> On Fri, Nov 29, 2024 at 04:26:01PM +0200, Imre Deak wrote:
>> Adding more people from DRM core domain. Any comment, objection to this
>> change?
>> 
>> On Tue, Nov 26, 2024 at 06:18:56PM +0200, Imre Deak wrote:
>> > Atm when the connector is added to the drm_mode_config::connector_list,
>> > the connector may not be fully initialized yet. This is not a problem
>> > for user space, which will see the connector only after it's registered
>> > later, it could be a problem for in-kernel users looking up connectors
>> > via the above list.
>
> It could be, or it actually is a problem? If so, in what situation?

It's an actual problem.

While in most cases connectors are created at probe, for DP MST they're
created at runtime via the ->add_connector hook. We want to call
drm_connector_init() on them soon in that hook, so we can pass the
connector around and expect it to have connector->dev etc. initialized
while we continue its initialization.

But there's a race. As soon as we call drm_connector_init(), the
connector gets added to dev->mode_config.connector_list, and any drm
code may discover it. For example connector polling. And we might not be
done with the initialization yet.

>> > To resolve the above issue, add a way to separately initialize the DRM
>> > core specific parts of the connector and add it to the above list. This
>> > will move adding the connector to the list after the properties on the
>> > connector have been added, this is ok since these steps don't have a
>> > dependency.
>> >
>> > v2: (Jani)
>> > - Let initing DDC as well via drm_connector_init_core().
>> > - Rename __drm_connector_init to drm_connector_init_core_and_add().
>> > 
>> > Cc: Jani Nikula <jani.nikula@intel.com>
>> > Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com> (v1)
>> > Signed-off-by: Imre Deak <imre.deak@intel.com>
>
> If we do have an actual problem to solve, we'll need kunit tests too.

That's not an unreasonable request, per se, but what exactly should they
test? That the new init core didn't add stuff to the list, and the new
add connector did?


BR,
Jani.


-- 
Jani Nikula, Intel