Unable to boot kernel 4.15 on MacPro5,1 (mid 2012)

[Scott Talbert]

System: Mac Pro 5,1 (mid-2012).

I'm unable to boot the Ubuntu 18.04 installer ISO on this system (tried both 18.04.0 and 18.04.3). I was able to boot the 16.04.1 ISO (but NOT 16.04.6) and installed 16.04 successfully. Then, I upgraded to 18.04. However, when trying to boot the new kernel (4.15.0-60), it just locks up at the purple screen. Booting back to 4.4.0-161 works.
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ProblemType: Bug
ApportVersion: 2.20.9-0ubuntu7.7
Architecture: amd64
AudioDevicesInUse:
 USER PID ACCESS COMMAND
 /dev/snd/controlC0: stalbert 2072 F.... pulseaudio
 /dev/snd/controlC1: stalbert 2072 F.... pulseaudio
CurrentDesktop: ubuntu:GNOME
DistroRelease: Ubuntu 18.04
HibernationDevice: RESUME=UUID=9c6c2f5f-9303-4f65-955d-30f5db341801
InstallationDate: Installed on 2019-09-05 (0 days ago)
InstallationMedia: Ubuntu 16.04.1 LTS "Xenial Xerus" - Release amd64 (20160719)
IwConfig:
 enp10s0 no wireless extensions.

 lo no wireless extensions.

 enp9s0 no wireless extensions.
MachineType: Apple Inc. MacPro5,1
Package: linux (not installed)
ProcEnviron:
 TERM=xterm-256color
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_US.UTF-8
 SHELL=/bin/bash
ProcFB: 0 radeondrmfb
ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-4.4.0-161-generic root=UUID=c7c8d4ce-3f44-41cd-8ce0-2e835c8d5547 ro quiet splash
ProcVersionSignature: Ubuntu 4.4.0-161.189-generic 4.4.186
RelatedPackageVersions:
 linux-restricted-modules-4.4.0-161-generic N/A
 linux-backports-modules-4.4.0-161-generic N/A
 linux-firmware 1.173.9
RfKill:
 0: hci0: Bluetooth
  Soft blocked: no
  Hard blocked: no
Tags: bionic
Uname: Linux 4.4.0-161-generic x86_64
UpgradeStatus: Upgraded to bionic on 2019-09-05 (0 days ago)
UserGroups: adm cdrom dip lpadmin plugdev sambashare sudo
_MarkForUpload: True
dmi.bios.date: 08/08/17
dmi.bios.vendor: Apple Inc.
dmi.bios.version: MP51.88Z.0084.B00.1708080528
dmi.board.asset.tag: 0
dmi.board.name: Mac-F221BEC8
dmi.board.vendor: Apple Inc.
dmi.chassis.type: 7
dmi.chassis.vendor: Apple Inc.
dmi.chassis.version: Mac-F221BEC8
dmi.modalias: dmi:bvnAppleInc.:bvrMP51.88Z.0084.B00.1708080528:bd08/08/17:svnAppleInc.:pnMacPro5,1:pvr0.0:rvnAppleInc.:rnMac-F221BEC8:rvr:cvnAppleInc.:ct7:cvrMac-F221BEC8:
dmi.product.name: MacPro5,1
dmi.product.version: 0.0
dmi.sys.vendor: Apple Inc.

[Ubuntu Kernel Bot (ubuntu-kernel-bot)]

This bug is missing log files that will aid in diagnosing the problem. While running an Ubuntu kernel (not a mainline or third-party kernel) please enter the following command in a terminal window:

apport-collect 1842965

and then change the status of the bug to 'Confirmed'.

If, due to the nature of the issue you have encountered, you are unable to run this command, please add a comment stating that fact and change the bug status to 'Confirmed'.

This change has been made by an automated script, maintained by the Ubuntu Kernel Team.

[Scott Talbert (swt-techie)]

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[Scott Talbert (swt-techie)]

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[Scott Talbert (swt-techie)]

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[Scott Talbert (swt-techie)]

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[Scott Talbert (swt-techie)]

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[Scott Talbert (swt-techie)]

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[Scott Talbert (swt-techie)]

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[Scott Talbert (swt-techie)]

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[Scott Talbert (swt-techie)]

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[Scott Talbert (swt-techie)]

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[Scott Talbert (swt-techie)]

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[Scott Talbert (swt-techie)]

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[Scott Talbert (swt-techie)]

OK, so I bisected the kernel versions by using the mainline builds. The first kernel where this breaks is v4.8-rc1 (v4.7.x works fine).

I think did a git bisection and found that this commit is the bad one:
27b79027bc112a63ad4004eb83c6acacae08a0de

This seems a bit strange to me, though, as this is a merge commit and both of the two parent commits work. Investigating further.

[Scott Talbert (swt-techie)]

So, I messed up the bisection earlier, probably due to not being careful with the kernel configuration. The actual bad commit is 475fb4e8b2f4444d1d7b406ff3a7d21bc89a1e6f which makes a lot more sense. I reported the problem upstream and a fix is in-process.

[Scott Talbert (swt-techie)]

This is fixed upstream in Linus's tree in c05f8f92b701576b615f30aac31fabdc0648649b. It would be nice if the Ubuntu kernel could pick up this change.

[You-Sheng Yang (vicamo)]

This will be fixed in Bionic kernel version 4.15.0-71.80. Thank you for the great work done to identify this issue as well as the fix.