It appears when testing my previous fix for some of the legacy
modesetting issues with MST, I misattributed some kernel splats that
started appearing on my machine after a rebase as being from upstream.
But it appears they actually came from my patch series:

[    2.980512] [drm:drm_atomic_helper_check_modeset [drm_kms_helper]] Updating routing for [CONNECTOR:65:eDP-1]
[    2.980516] [drm:drm_atomic_helper_check_modeset [drm_kms_helper]] [CONNECTOR:65:eDP-1] is not registered
[    2.980516] ------------[ cut here ]------------
[    2.980519] Could not determine valid watermarks for inherited state
[    2.980553] WARNING: CPU: 3 PID: 551 at drivers/gpu/drm/i915/intel_display.c:14983 intel_modeset_init+0x14d7/0x19f0 [i915]
[    2.980556] Modules linked in: i915(O+) i2c_algo_bit drm_kms_helper(O) syscopyarea sysfillrect sysimgblt fb_sys_fops drm(O) intel_rapl x86_pkg_temp_thermal iTCO_wdt wmi_bmof coretemp crc32_pclmul psmouse i2c_i801 mei_me mei i2c_core lpc_ich mfd_core tpm_tis tpm_tis_core wmi tpm thinkpad_acpi pcc_cpufreq video ehci_pci crc32c_intel serio_raw ehci_hcd xhci_pci xhci_hcd
[    2.980577] CPU: 3 PID: 551 Comm: systemd-udevd Tainted: G           O      4.19.0-rc7Lyude-Test+ #1
[    2.980579] Hardware name: LENOVO 20BWS1KY00/20BWS1KY00, BIOS JBET63WW (1.27 ) 11/10/2016
[    2.980605] RIP: 0010:intel_modeset_init+0x14d7/0x19f0 [i915]
[    2.980607] Code: 89 df e8 ec 27 02 00 e9 24 f2 ff ff be 03 00 00 00 48 89 df e8 da 27 02 00 e9 26 f2 ff ff 48 c7 c7 c8 d1 34 a0 e8 23 cf dc e0 <0f> 0b e9 7c fd ff ff f6 c4 04 0f 85 37 f7 ff ff 48 8b 83 60 08 00
[    2.980611] RSP: 0018:ffffc90000287988 EFLAGS: 00010282
[    2.980614] RAX: 0000000000000000 RBX: ffff88031b488000 RCX: 0000000000000006
[    2.980617] RDX: 0000000000000007 RSI: 0000000000000086 RDI: ffff880321ad54d0
[    2.980620] RBP: ffffc90000287a10 R08: 000000000000040a R09: 0000000000000065
[    2.980623] R10: ffff88030ebb8f00 R11: ffffffff81416590 R12: ffff88031b488000
[    2.980626] R13: ffff88031b4883a0 R14: ffffc900002879a8 R15: ffff880319099800
[    2.980630] FS:  00007f475620d180(0000) GS:ffff880321ac0000(0000) knlGS:0000000000000000
[    2.980633] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[    2.980636] CR2: 00007f9ef28018a0 CR3: 000000031b72c001 CR4: 00000000003606e0
[    2.980639] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[    2.980642] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[    2.980645] Call Trace:
[    2.980675]  i915_driver_load+0xb0e/0xdc0 [i915]
[    2.980681]  ? kernfs_add_one+0xe7/0x130
[    2.980709]  i915_pci_probe+0x46/0x60 [i915]
[    2.980715]  pci_device_probe+0xd4/0x150
[    2.980719]  really_probe+0x243/0x3b0
[    2.980722]  driver_probe_device+0xba/0x100
[    2.980726]  __driver_attach+0xe4/0x110
[    2.980729]  ? driver_probe_device+0x100/0x100
[    2.980733]  bus_for_each_dev+0x74/0xb0
[    2.980736]  driver_attach+0x1e/0x20
[    2.980739]  bus_add_driver+0x159/0x230
[    2.980743]  ? 0xffffffffa0393000
[    2.980746]  driver_register+0x70/0xc0
[    2.980749]  ? 0xffffffffa0393000
[    2.980753]  __pci_register_driver+0x57/0x60
[    2.980780]  i915_init+0x55/0x58 [i915]
[    2.980785]  do_one_initcall+0x4a/0x1c4
[    2.980789]  ? do_init_module+0x27/0x210
[    2.980793]  ? kmem_cache_alloc_trace+0x131/0x190
[    2.980797]  do_init_module+0x60/0x210
[    2.980800]  load_module+0x2063/0x22e0
[    2.980804]  ? vfs_read+0x116/0x140
[    2.980807]  ? vfs_read+0x116/0x140
[    2.980811]  __do_sys_finit_module+0xbd/0x120
[    2.980814]  ? __do_sys_finit_module+0xbd/0x120
[    2.980818]  __x64_sys_finit_module+0x1a/0x20
[    2.980821]  do_syscall_64+0x5a/0x110
[    2.980824]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
[    2.980826] RIP: 0033:0x7f4754e32879
[    2.980828] Code: 00 c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 8b 0d f7 45 2c 00 f7 d8 64 89 01 48
[    2.980831] RSP: 002b:00007fff43fd97d8 EFLAGS: 00000246 ORIG_RAX: 0000000000000139
[    2.980834] RAX: ffffffffffffffda RBX: 0000559a44ca64f0 RCX: 00007f4754e32879
[    2.980836] RDX: 0000000000000000 RSI: 00007f475599f4cd RDI: 0000000000000018
[    2.980838] RBP: 00007f475599f4cd R08: 0000000000000000 R09: 0000000000000000
[    2.980839] R10: 0000000000000018 R11: 0000000000000246 R12: 0000000000000000
[    2.980841] R13: 0000559a44c92fd0 R14: 0000000000020000 R15: 0000000000000000
[    2.980881] WARNING: CPU: 3 PID: 551 at drivers/gpu/drm/i915/intel_display.c:14983 intel_modeset_init+0x14d7/0x19f0 [i915]
[    2.980884] ---[ end trace 5eb47a76277d4731 ]---

The cause of this appears to be due to the fact that if there's
pre-existing display state that was set by the BIOS when i915 loads, it
will attempt to perform a modeset before the driver is registered with
userspace. Since this happens before the driver's registered with
userspace, it's connectors are also unregistered and thus-states which
would turn on DPMS on a connector end up getting rejected since the
connector isn't registered.

These bugs managed to get past Intel's CI partially due to the fact it
never ran a full test on my patches for some reason, but also because
all of the tests unload the GPU once before running. Since this bug is
only really triggered when the drivers tries to perform a modeset before
it's been fully registered with userspace when coming from whatever
display configuration the firmware left us with, it likely would never
have been picked up by CI in the first place.

After some discussion with vsyrjala, we decided the best course of
action would be to just move the unregistered connector checks out of
update_connector_routing() and into drm_atomic_set_crtc_for_connector().
The reason for this being that legacy modesetting isn't going to be
expecting failures anywhere (at least this is the case with X), so
ideally we want to ensure that any DPMS changes will still work even on
unregistered connectors. Instead, we now only reject new modesets which
would change the current CRTC assigned to an unregistered connector
unless no new CRTC is being assigned to replace the connector's previous
one.
Signed-off-by: Lyude Paul <lyude@redhat.com>
Reported-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Fixes: 4d802739 ("drm/atomic_helper: Disallow new modesets on unregistered connectors")
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: stable@vger.kernel.org
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181009204424.21462-1-lyude@redhat.com