Please add drm.debug=14 module parameter, and attach dmesg all the way from boot to reproducing the problem.

Created attachment 127516 [details]
DMESG DRM.DEBUG=14

as requested

Async fbdev initial config calls back to intel_dp_detect before we've done the first proper modeset and set the lane_count.

There are a couple of commits that are the *messenger* here that this will bisect to, but must not be reverted:

commit d4cb3fd9b548b8bfe2a712ec920b9ebabd3547ab
Author: Matthew Auld <matthew.auld@intel.com>
Date:   Wed Oct 19 22:29:53 2016 +0100

    drm/i915/dp: add lane_count check in intel_dp_check_link_status

and related

commit dd788090822300a66ff469ae9e50f6d28d124eb8
Author: Ville Syrjälä <ville.syrjala@linux.intel.com>
Date:   Thu Jul 28 17:55:04 2016 +0300

    drm/i915: Warn about aux msg buffer vs. size mismatch

Apparently there was a commit to mitigate related problems in

commit d626f34573551479f62b09d4df09efc4af88136d
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Tue Jun 21 09:16:53 2016 +0100

    drm/i915/fbdev: Perform async fbdev initialisation much later

Possibly the problem wasn't introduced not so much by async fbdev but rather changes in intel_dp_detect.

Created attachment 127844 [details]
dmesg-t460s-drm-intel-next-2016-11-08-979-gaa74f4d

I'm getting a similar error during boot with drm-intel-next-2016-11-08-979-gaa74f4d on SKL Thinkpad T460s.

Here's one way to fix it: https://patchwork.freedesktop.org/patch/101473/
But looks like Daniel wants something fancier.

It's not just async fbdev. It's just complete garbage in

commit 7d23e3c37bb3fc6952dc84007ee60cb533fd2d5c
Author: Shubhangi Shrivastava <shubhangi.shrivastava@intel.com>
Date:   Wed Mar 30 18:05:23 2016 +0530

    drm/i915: Cleaning up intel_dp_hpd_pulse
    
    Current DP detection has DPCD operations split across
    intel_dp_hpd_pulse and intel_dp_detect which contains
    duplicates as well. Also intel_dp_detect is called
    during modes enumeration as well which will result
    in multiple dpcd operations. So this patch tries
    to solve both these by bringing all DPCD operations
    in one single function and make intel_dp_detect
    use existing values instead of repeating same steps.

Created attachment 128148 [details]
dmesg with drm.debug=14 drm-intel-nightly: 2016y-11m-21d-18h-22m-22s

on Broxton maniboard with converter chip on mainboard DP1.2 to HDMI 2.0

@Chris

> It's not just async fbdev. It's just complete garbage in … drm/i915: Cleaning up intel_dp_hpd_pulse

Any plans to fix this?
Have there fixes landed already in this regard?

*** Bug 99098 has been marked as a duplicate of this bug. ***

Thank you for adding me to this bug. I'm including HSW in the i915 platform list, That is where I'm seeing this problem.

*** Bug 99113 has been marked as a duplicate of this bug. ***

The WARN_ON has been removed and replaced by a conditional check so this warning should not be seen any more. Also I am working on a patch for properly fixing this.

Regards
Manasi

*** Bug 99655 has been marked as a duplicate of this bug. ***

Hello, 
I was trying to reproduce this issue and I could not find any message in dmesg and kern.log.

Configuration:
======================================
        Graphic stack
======================================

======================================
             Software
======================================
kernel version              : 4.12.0-rc3-drm-tip-ww22-commit-187376e+
architecture                : x86_64
os version                  : Ubuntu 17.04
os codename                 : zesty
kernel driver               : i915
bios revision               : 4.6
bios release date           : 03/02/2017

======================================
        Graphic drivers
======================================
mesa                        : 17.0.3
modesetting                 : modesetting_drv.so
xorg-xserver                : 1.19.3
libdrm                      : 2.4.81
cairo                       : 1.14.8
xserver                     : X.Org X Server 1.19.99.1
intel-gpu-tools (tag)       : intel-gpu-tools-1.18-211-g00ce341b
intel-gpu-tools (commit)    : 00ce341b

======================================
             Hardware
======================================
platform                   : HSW-Nuc
motherboard id             : D54250WYK
form factor                : Desktop
cpu family                 : Core i5
cpu family id              : 6
cpu information            : Intel(R) Core(TM) i5-4250U CPU @ 1.30GHz
gpu card                   : Intel Corporation Haswell-ULT Integrated Graphics Controller (rev 09) (prog-if 00 [VGA controller])
memory ram                 : 3.79 GB
max memory ram             : 16 GB
display resolution         : 1600x900
cpu thread                 : 4
cpu core                   : 2
cpu model                  : 69
cpu stepping               : 1
socket                     : Socket LGA1150
signature                  : Type 0, Family 6, Model 69, Stepping 1
hard drive                 : 223GiB (240GB)
current cd clock frequency : 450000 kHz
maximum cd clock frequency : 450000 kHz
displays connected         : DP-1

======================================
             Firmware
======================================

======================================
             kernel parameters
======================================
quiet splash fastboot drm.debug=0xe


Regards.