I had been running with a USB GPS for years and recently purchased a GOOD GNSS device, so I have not had anything on this machine that accesses the pps_api until a few days ago.
FYI I have a Raspberry Pi with Ubuntu where the pps_api works with raspberrypi-kernel 1:1.2010727-1 if that is of any help.

I see two other kernels installed: 5.4.0-54 and 5.4.0-77.
Any suggestion on which to try as it is somewhat of a PITA to do this, would rather do it just once, and will have to schedule a time, hopefully in a day or so.

Also I have had no reason to do this for years and have forgotten how. As I recall, it is just hold down a key while rebooting to get to the menu to select a kernel, but don't remember which key.
    On Wednesday, August 4, 2021, 6:30:46 AM PDT, Kleber Sacilotto de Souza <email address hidden> wrote:  
 
 Hello Jim,

Did it work before with your system and got broken after a kernel
update? If yes, would you be able to identify the kernel version which
was working before? If you didn't run this use case before, would you be
able to boot with an older kernel version and check whether it works?

Thanks.

** Also affects: linux (Ubuntu Focal)
  Importance: Undecided
      Status: New

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report.
https://bugs.launchpad.net/bugs/1938626

Title:
  PPS API broken

Status in linux package in Ubuntu:
  New
Status in linux source package in Focal:
  New

Bug description:
  The kernel PPS API appears to be broken. I have a serial GPS with PPS
  attached to /dev/ttyS4. If I use gpsd to create /dev/pps0, gpsd fails
  to see PPS data. If I disable gpsd and create /dev/pps0 with ldattach,
  any app or tool run against ttyS4 works fine and does see the CTS
  transitions every second, i.e. gpsmon and ppscheck. Anything run
  against pps0 fails, i.e. ppstest, ppswatch. ntpd gets data from ttyS4
  but not pps0.

  There are some issues with stock apparmor with gpsd and ntpd, but I
  have solved those.

  ProblemType: Bug
  DistroRelease: Ubuntu 20.04
  Package: linux-image-5.4.0-80-generic 5.4.0-80.90
  ProcVersionSignature: Ubuntu 5.4.0-80.90-generic 5.4.124
  Uname: Linux 5.4.0-80-generic x86_64
  ApportVersion: 2.20.11-0ubuntu27.18
  Architecture: amd64
  AudioDevicesInUse:
  USER        PID ACCESS COMMAND
  /dev/snd/controlC1:  jimp      6811 F.... pulseaudio
  CasperMD5CheckResult: skip
  CurrentDesktop: GNOME-Flashback:GNOME
  Date: Sat Jul 31 16:41:32 2021
  HibernationDevice: RESUME=UUID=90289c8f-5a00-46c4-8f1f-79dd26128f39
  InstallationDate: Installed on 2016-08-05 (1820 days ago)
  InstallationMedia: Ubuntu-Server 16.04.1 LTS "Xenial Xerus" - Release amd64 (20160719)
  ProcEnviron:
  TERM=xterm-256color
  PATH=(custom, no user)
  XDG_RUNTIME_DIR=<set>
  LANG=en_US.UTF-8
  SHELL=/bin/bash
  ProcFB: 0 i915drmfb
  ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-5.4.0-80-generic root=UUID=389dcb0d-84f2-4340-ad6c-e4f1dbe55b67 ro quiet splash vt.handoff=7
  RelatedPackageVersions:
  linux-restricted-modules-5.4.0-80-generic N/A
  linux-backports-modules-5.4.0-80-generic  N/A
  linux-firmware                            1.187.15
  SourcePackage: linux
  UpgradeStatus: Upgraded to focal on 2020-11-09 (264 days ago)
  dmi.bios.date: 11/26/2009
  dmi.bios.vendor: Intel Corp.
  dmi.bios.version: TYG4110H.86A.0036.2009.1126.2047
  dmi.board.asset.tag: To be filled by O.E.M.
  dmi.board.name: DG41TY
  dmi.board.vendor: Intel Corporation
  dmi.board.version: AAE47335-302
  dmi.chassis.type: 3
  dmi.modalias: dmi:bvnIntelCorp.:bvrTYG4110H.86A.0036.2009.1126.2047:bd11/26/2009:svn:pn:pvr:rvnIntelCorporation:rnDG41TY:rvrAAE47335-302:cvn:ct3:cvr:

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