[Bug 111505] New: 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

            Bug ID: 111505
           Summary: third monitor doesn't work since commit "drm/i915:
                    Remove the 8bpc shackles from DP MST"
           Product: DRI
           Version: unspecified
          Hardware: x86-64 (AMD64)
                OS: Linux (All)
            Status: NEW
          Keywords: bisected, regression
          Severity: major
          Priority: not set
         Component: DRM/Intel
          Assignee: intel-gfx-bugs at lists.freedesktop.org
          Reporter: gmux22 at gmail.com
        QA Contact: intel-gfx-bugs at lists.freedesktop.org
                CC: intel-gfx-bugs at lists.freedesktop.org
     i915 platform: KBL
     i915 features: display/DP MST

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.

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