[PATCH 2/7] drm/dp_mst: Register AUX devices for MST ports

[Li, Sun peng (Leo)]

On 2019-06-03 3:28 p.m., Lyude Paul wrote:
>> I'm reproducing this just by reloading i915 on a machine with some MST
>> displays connected. I uploaded a copy of the script that I use to do this
>> here:
>>
>> https://people.freedesktop.org/~lyudess/archive/06-03-2019/unloadgpumod.sh
> oops-almost forgot to mention. The argument you pass to make it reload instead
> of just unloading is --reload
> 

Thanks for the script!

So, the warning has to do with:

1. Having the aux device as a child of connector device, and
2. During driver unload, drm_connector_unregister() is called before
    drm_dp_mst_topology_put_port()

Which means that connector_unregister() will recursively remove the
child aux device, before put_port() can properly unregister it. Any
further attempts to remove after the first will throw a "not found" warning.

Call-stacks for reference:

   *drm_connector_unregister*+0x37/0x60 [drm]
   drm_connector_unregister_all+0x30/0x60 [drm]
   drm_modeset_unregister_all+0xe/0x30 [drm]
   drm_dev_unregister+0x9c/0xb0 [drm]
   i915_driver_unload+0x73/0x120 [i915]

   drm_dp_aux_unregister_devnode+0xf5/0x180 [drm_kms_helper]
   *drm_dp_mst_topology_put_port*+0x4e/0xf0 [drm_kms_helper]
   drm_dp_mst_topology_put_mstb+0x91/0x160 [drm_kms_helper]
   drm_dp_mst_topology_mgr_set_mst+0x12b/0x2b0 [drm_kms_helper]
   ? __finish_swait+0x10/0x40
   drm_dp_mst_topology_mgr_destroy+0x11/0xa0 [drm_kms_helper]
   intel_dp_encoder_flush_work+0x32/0xb0 [i915]
   intel_ddi_encoder_destroy+0x32/0x60 [i915]
   drm_mode_config_cleanup+0x51/0x2e0 [drm]
   intel_modeset_cleanup+0xc8/0x140 [i915]
   i915_driver_unload+0xa0/0x120 [i915]

A solution is to unregister the aux device immediately before the
connector device is unregistered - if we are to keep the aux device as a
child. Following current scheme with SST, it looks like
drm_connector_funcs->early_unregister() is the right place to do so.
To keep the balance, aux registration will then happen in
drm_connector_funcs->late_register(). This will leave the SDP
transaction handling part in DRM still, but pass the responsibility of
creating and removing remote (fake) aux devices to the driver.

I have a WIP patch here for you to take a look. It should apply on top
of the existing patchset:
https://pastebin.com/1YJZhL4C

I'd like to hear your thoughts, before I go and modify other drivers :)

Thanks,
Leo