Public bug reported:

Symptom: booting 18.04 kernel reduces resolution available while 17.10
kernel (on 18.04) does not

I have a 3 identical monitor setup - all at 2560x1600 native resolution.
If I boot the current 18.04 kernel 4.15.0-13-generic then the third
monitor does not have the native resolution available.  If I reboot with
the last 17.10 kernel 4.13.0-37-generic then all monitors have the
native resolution available.

On the video card the monitors are connected to HDMI, DisplayPort and
DVI.  On the monitors themselves the connectors are HDMI, DisplayPort
and HDMI.  (3rd monitor is using DVI to HDMI cable.)

Xorg logs show identical EDID information between the kernels.  18.04's
just decides to not make 2560x1600 available, having 2048x1080 or
1920x1200 as the highest.

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: xorg 1:7.7+19ubuntu6
ProcVersionSignature: Ubuntu 4.15.0-13.14-generic 4.15.10
Uname: Linux 4.15.0-13-generic x86_64
ApportVersion: 2.20.9-0ubuntu4
Architecture: amd64
CompizPlugins: No value set for 
CompositorRunning: None
CurrentDesktop: XFCE
Date: Tue Apr 10 12:17:31 2018
DistUpgraded: 2018-03-31 12:24:30,098 DEBUG Running PostInstallScript: 
DistroCodename: bionic
DistroVariant: ubuntu
ExtraDebuggingInterest: Yes
 Advanced Micro Devices, Inc. [AMD/ATI] Baffin [Radeon RX 460] [1002:67ef] (rev 
e5) (prog-if 00 [VGA controller])
   Subsystem: Gigabyte Technology Co., Ltd Baffin [Radeon RX 460/560D / Pro 
450/455/460/560] [1458:230a]
 ** Message: Starting lightdm-gtk-greeter 2.0.1 (Aug  7 2015, 01:24:18)
 ** Message: [Configuration] Reading file: 
 ** Message: [Configuration] Reading file: 
 ** Message: [Configuration] Reading file: /etc/lightdm/lightdm-gtk-greeter.conf
 upstart: indicator-application main process (1456) killed by TERM signal
MachineType: Supermicro C7Z170-SQ
ProcKernelCmdLine: BOOT_IMAGE=/vmlinuz-4.15.0-13-generic 
root=UUID=725712a6-f0e0-4da1-b3d1-746a233eb459 ro rootflags=subvol=@
SourcePackage: xorg
Symptom: display
UpgradeStatus: Upgraded to bionic on 2018-03-31 (9 days ago) 12/19/2016
dmi.bios.vendor: American Megatrends Inc.
dmi.bios.version: 2.0
dmi.board.asset.tag: Default string C7Z170-SQ
dmi.board.vendor: Supermicro
dmi.board.version: 1.01A
dmi.chassis.asset.tag: Default string
dmi.chassis.type: 17
dmi.chassis.vendor: Supermicro
dmi.chassis.version: 0123456789
dmi:bvnAmericanMegatrendsInc.:bvr2.0:bd12/19/2016:svnSupermicro:pnC7Z170-SQ:pvr0123456789:rvnSupermicro:rnC7Z170-SQ:rvr1.01A:cvnSupermicro:ct17:cvr0123456789: Default string C7Z170-SQ
dmi.product.version: 0123456789
dmi.sys.vendor: Supermicro
version.compiz: compiz N/A
version.libdrm2: libdrm2 2.4.91-2
version.libgl1-mesa-dri: libgl1-mesa-dri 18.0.0~rc5-1ubuntu1
version.libgl1-mesa-glx: libgl1-mesa-glx 18.0.0~rc5-1ubuntu1
version.xserver-xorg-core: xserver-xorg-core 2:1.19.6-1ubuntu3
version.xserver-xorg-input-evdev: xserver-xorg-input-evdev 1:2.10.5-1ubuntu1
version.xserver-xorg-video-ati: xserver-xorg-video-ati 1:18.0.1-1
version.xserver-xorg-video-intel: xserver-xorg-video-intel 
version.xserver-xorg-video-nouveau: xserver-xorg-video-nouveau 1:1.0.15-2

** Affects: xorg (Ubuntu)
     Importance: Undecided
         Status: New

** Tags: amd64 apport-bug bionic resolution ubuntu

** Attachment added: "Xorg log when booting with 17.10 kernel"

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  Regression: Upgrading from 17.10 to 18.04 reduces resolution available
  on monitor

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