[iMac14,1, Cirrus Logic CS4206, Mic, Internal] No sound at all from Mic. Was working in 15.10

The machine is in use since 2014 and has recently been upgraded to 16.04. I am using imac27_122 for the alsa module to get the speakers working, which has worked fine so far. I used to set some hda pins manually with hda_analyzer, but that does not seem necessary any more. I am now not doing that any more.

Since the upgrade to 16.04 (from 15.10), the mic input does not get a signal any more. In particular, pavucontrol does not show a moving bar for the input. No external mic or headphone is connected.
I am happy to provide additional debug information, please just let me know. Apart from the dist upgrade, no other configuration change was done. I currently assume the regression is due to changes in alsa drivers for the Cirrus Logic CS4206.

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: alsa-base 1.0.25+dfsg-0ubuntu5
ProcVersionSignature: Ubuntu 4.4.0-21.37-generic 4.4.6
Uname: Linux 4.4.0-21-generic x86_64
NonfreeKernelModules: wl
ApportVersion: 2.20.1-0ubuntu2
Architecture: amd64
AudioDevicesInUse:
 USER PID ACCESS COMMAND
 /dev/snd/controlC1: not 2332 F.... pulseaudio
 /dev/snd/controlC0: not 2332 F.... pulseaudio
CurrentDesktop: XFCE
Date: Tue Apr 26 20:23:46 2016
EcryptfsInUse: Yes
InstallationDate: Installed on 2014-07-10 (656 days ago)
InstallationMedia: Xubuntu 14.04 LTS "Trusty Tahr" - Release amd64 (20140416.2)
PackageArchitecture: all
SourcePackage: alsa-driver
Symptom: audio
Symptom_AlsaRecordingTest: ALSA recording test through plughw:PCH failed
Symptom_AlsaRecordingTestStderr: a r e c o r d : p c m _ r e a d : 2 0 3 2 : r e a d e r r o r : I n p u t / o u t p u t e r r o r
Symptom_Card: Built-in Audio - HDA Intel PCH
Symptom_DevicesInUse:
 USER PID ACCESS COMMAND
 /dev/snd/pcmC1D0p: not 2332 F...m pulseaudio
 /dev/snd/controlC1: not 2332 F.... pulseaudio
 /dev/snd/controlC0: not 2332 F.... pulseaudio
Symptom_Jack: Mic, Internal
Symptom_Type: No sound at all
Title: [iMac14,1, Cirrus Logic CS4206, Mic, Internal] No sound at all
UpgradeStatus: Upgraded to xenial on 2016-04-22 (4 days ago)
dmi.bios.date: 01/29/2014
dmi.bios.vendor: Apple Inc.
dmi.bios.version: IM141.88Z.0118.B01.1401291047
dmi.board.asset.tag: Base Board Asset Tag#
dmi.board.name: Mac-031B6874CF7F642A
dmi.board.vendor: Apple Inc.
dmi.board.version: iMac14,1
dmi.chassis.type: 13
dmi.chassis.vendor: Apple Inc.
dmi.chassis.version: Mac-031B6874CF7F642A
dmi.modalias: dmi:bvnAppleInc.:bvrIM141.88Z.0118.B01.1401291047:bd01/29/2014:svnAppleInc.:pniMac14,1:pvr1.0:rvnAppleInc.:rnMac-031B6874CF7F642A:rvriMac14,1:cvnAppleInc.:ct13:cvrMac-031B6874CF7F642A:
dmi.product.name: iMac14,1
dmi.product.version: 1.0
dmi.sys.vendor: Apple Inc.
mtime.conffile..etc.modprobe.d.alsa-base.conf: 2016-04-26T20:22:32.147562