Kernel 4.4.0-93 Freezes on boot:splash/vt.handoff edit works;but debug fails

[DiagonalArg]

Ubuntu 16.04: has been working fine for a year and a half, now suddenly freezing on bootup, with a permanent spinning logo. The problem appears with the new (4.4.0-93) kernel. (Things were fine in 4.4.0-92.) My system has /boot on a USB and root on LUKS (along with a 2nd data disk on LUKS).

I can boot via a text console if I remove the "splash" kernel switch and change $vt.handoff to "$vt.handoff=7". Even with these changes, adding the "debug" switch causes a freeze, though "debug=vc" boots. Also, for any of the settings that cause a freeze, if I add "break=mountroot", I can mount the LUKS disks by hand and continue with boot.

Note (below): booting in recovery allows me to unlock the disks. Then choosing to continue with normal boot, leads to a blank grey screen. It appears frozen, but there is continuing disk activity for some long time.

---
ApportVersion: 2.20.1-0ubuntu2.10
Architecture: amd64
AudioDevicesInUse:
 USER PID ACCESS COMMAND
 /dev/snd/controlC2: dev 2272 F.... pulseaudio
 /dev/snd/controlC1: dev 2272 F.... pulseaudio
 /dev/snd/controlC0: dev 2272 F.... pulseaudio
CurrentDesktop: GNOME
DistroRelease: Ubuntu 16.04
InstallationDate: Installed on 2016-05-31 (475 days ago)
InstallationMedia: Ubuntu-GNOME 16.04 LTS "Xenial Xerus" - Release amd64 (20160421)
MachineType: LENOVO 23591L9
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 inteldrmfb
 1 nouveaufb
ProcKernelCmdLine: BOOT_IMAGE=/@t530boot/vmlinuz-4.4.0-93-generic root=/username/mapper/luks.root ro rootflags=subvol=@ quiet vt.handoff=7=7
ProcVersionSignature: Ubuntu 4.4.0-93.116-generic 4.4.79
RelatedPackageVersions:
 linux-restricted-modules-4.4.0-93-generic N/A
 linux-backports-modules-4.4.0-93-generic N/A
 linux-firmware 1.157.12
Tags: xenial
Uname: Linux 4.4.0-93-generic x86_64
UpgradeStatus: No upgrade log present (probably fresh install)
UserGroups: adm cdrom dip libvirtd lpadmin plugdev sambashare sudo
_MarkForUpload: True
dmi.bios.date: 09/10/2015
dmi.bios.vendor: LENOVO
dmi.bios.version: G4ETA5WW (2.65 )
dmi.board.asset.tag: Not Available
dmi.board.name: 23591L9
dmi.board.vendor: LENOVO
dmi.board.version: 0B98413 Std
dmi.chassis.asset.tag: No Asset Information
dmi.chassis.type: 10
dmi.chassis.vendor: LENOVO
dmi.chassis.version: Not Available
dmi.modalias: dmi:bvnLENOVO:bvrG4ETA5WW(2.65):bd09/10/2015:svnLENOVO:pn23591L9:pvrThinkPadT530:rvnLENOVO:rn23591L9:rvr0B98413Std:cvnLENOVO:ct10:cvrNotAvailable:
dmi.product.name: 23591L9
dmi.product.version: ThinkPad T530
dmi.sys.vendor: LENOVO

[DiagonalArg (diagonalarg)]

apport <this_bug#> refuses to collect any more information, and I don't know what package/project to report this against. Suggest what I might add and I'll be happy to do it.

[DiagonalArg (diagonalarg)]

Alright, booting in recovery mode, and then "continuing with regular boot", leads to a freeze with a blank grey background. (Disc activity continues for a long time.) So, I am reporting against the kernel.

[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 1718047

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.

[DiagonalArg (diagonalarg)]

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The problem is unchanged in 4.4.0-96.

[DiagonalArg (diagonalarg)]

Right before requesting my LUKS password, while booting without splash and with $vt.handoff=7, I see these errors:

Booting kernel: `7=7` invalid for parameter `vt.handoff'
tpm tpm0: a TPM error (6) occurred attempting to read a pcr value
drm:intel_set_pch_fifo_underrun_resporting [i915] *ERROR* uncleared pch fifo underrun on pch transcoder A
drm:intel_pch_fifo_underrun_irq_handler [i915] *ERROR* PCH transcoder A FIFO underrun

These errors are in the process that does allow me to boot, though.