drm/msm: DisplayPort regressions in 6.8-rc1

[Johan Hovold]

On Tue, Feb 13, 2024 at 12:42:17PM +0100, Johan Hovold wrote:

> Since 6.8-rc1 the internal eDP display on the Lenovo ThinkPad X13s does
> not always show up on boot.
> 
> The logs indicate problems with the runtime PM and eDP rework that went
> into 6.8-rc1:
> 
> 	[    6.007872] [drm:drm_bridge_attach [drm]] *ERROR* failed to attach bridge /soc at 0/phy at 88eb000 to encoder TMDS-31: -16
	
> and this can also manifest itself as a NULL-pointer dereference:
> 
> 	[    7.339447] Unable to handle kernel NULL pointer dereference at virtual address 0000000000000000
> 	
> 	[    7.643705] pc : drm_bridge_attach+0x70/0x1a8 [drm]

#regzbot ^introduced: 2bcca96abfbf

It looks like it may have been possible to hit this also before commit
2bcca96abfbf ("soc: qcom: pmic-glink: switch to DRM_AUX_HPD_BRIDGE") and
the transparent bridge rework in 6.8-rc1 even if that has not yet been
confirmed.

The above is what made this trigger since 6.8-rc1 however.

Johan