[Bug 111505] third monitor doesn't work since commit "drm/i915: Remove the 8bpc shackles from DP MST"

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=111505

Lakshmi <lakshminarayana.vudum at intel.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Whiteboard|                            |Triaged, ReadyForDev
                 CC|                            |ville.syrjala at linux.intel.c
                   |                            |om
           Priority|not set                     |highest

--- Comment #4 from Lakshmi <lakshminarayana.vudum at intel.com> ---
Considering this as a regression setting the priority to Highest. 

(In reply to Geoffrey Bennett from comment #0)
> Steps to reproduce the issue:
> - Fedora 30, Lenovo ThinkPad T480s laptop, internal + 2x external DisplayPort
> - External monitors: LG 24EB23 1920x1200 (DP-2-2) and LG 32UK550 3840x2160
> (DP-2-3)
> - Boot kernel commit f1477219869cc943ef9e0248b308da7b900c65ec or Fedora
> stock 5.2.9
> - At the LUKS password prompt, only two monitors display it (mirrored); the
> 3840x2160 monitor only displays a text cursor in the top-left
> - After logging in, the third monitor cannot be enabled
> - Boot kernel before the mentioned commit (or Fedora stock 5.1.19); all
> three monitors are working (mirrored password prompt)
> 
> How often do these steps trigger the issue? Always.
> 
> If I did the bisect right, this commit is the issue:
> 
> commit f1477219869cc943ef9e0248b308da7b900c65ec
> Author: Ville Syrjälä <ville.syrjala at linux.intel.com>
> Date:   Tue Mar 26 16:25:53 2019 +0200
> 
>     drm/i915: Remove the 8bpc shackles from DP MST
>     
>     Allow DP MST to output any color depth. This means deep color as
>     well as falling back to 6bpc if we would otherwise require too
>     much bandwidth.
>     
>     TODO: We should probably extend bw_contstrained scheme to force
>     all streams on the link to 6bpc if we can't fit the new stream(s)
>     otherwise.
> 
> Bad commit: f1477219869cc943ef9e0248b308da7b900c65ec
> Good commit: 765bdb0b3948747dda8c9bab300013e34f76fe88
> 
> Comparing dmesg output between good & bad commits with drm.debug=0x1e ...
> 
> Good commit, I see messages like:
> 
> [    7.215513] [drm:intel_atomic_check [i915]] hw max bpp: 30, pipe bpp: 24,
> dithering: 0
> [    7.215977] [drm:intel_dump_pipe_config [i915]] cpu_transcoder: C, pipe
> bpp: 24, dithering: 0 
> [    7.216361] [drm:drm_dp_mst_atomic_check [drm_kms_helper]] [MST
> PORT:000000006766b031] requires 48 vcpi slots
> [    7.216365] [drm:drm_dp_mst_atomic_check [drm_kms_helper]] [MST
> PORT:0000000048ae3b07] requires 14 vcpi slots
> 
> Bad commit, I see messages like:
> 
> [    7.151412] [drm:intel_atomic_check [i915]] hw max bpp: 30, pipe bpp: 30,
> dithering: 0
> [    7.151878] [drm:intel_dump_pipe_config [i915]] cpu_transcoder: C, pipe
> bpp: 30, dithering: 0
> [    7.152272] [drm:drm_dp_mst_atomic_check [drm_kms_helper]] [MST
> PORT:00000000f27944b7] requires 60 vcpi slots
> [    7.152276] [drm:drm_dp_mst_atomic_check [drm_kms_helper]] [MST
> PORT:00000000f43eff2e] requires 14 vcpi slots
> [    7.152281] [drm:drm_dp_mst_atomic_check [drm_kms_helper]] [MST
> PORT:00000000f43eff2e] not enough VCPI slots in mst state 00000000c1ef406a
> (avail=3)
> [    7.152293] [drm:drm_atomic_check_only [drm]] atomic driver check for
> 00000000a9c59397 failed: -28
> 
> I also tested drm-tip, and neither external monitor worked then.

@Ville, can you help here?

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