regression(?) - hauppage quad card tuning

[media@arewethere.net]

Hi

I need some help determining if there's been a regression for
one of my DVB TV cards. Details below.

I have a Hauppage QuadHD DVB card
which has two multimedia controllers:

 Conexant Systems, Inc. CX23887/8 PCIe Broadcast Audio and Video
   Decoder with 3D Comb (rev 04)

i.e. PCI ID 14f1:8880 (rev 04)

With subvendor:subdevice 0700:0x6a28.

The cards have SiLabs Si2157 tuners.

I had a working system running Debian Bullseye (linux 5.10.197-1).

After I upgraded to Debian Bookworm (running 6.1.52-1)
the tuners in this card no longer get a lock.

I also tried a 'backports' kernel, 6.5.10-1~bpo12+1.
This is based on the upstream version 6.1.66-1.
This kernel also fails to tune and get a lock.

The other symptom, which may or may not be relevant,
is 'i2cdetect -l' returns nothing, though there are
various i2c device detections on dmesg.

There is a second card in the system that tunes and locks on
just fine, with both the 6.1 and 6.5 kernels.
It is a  Philips Semiconductors SAA7164 (rev 81)
marketed as a (Hauppauge WinTV HVR-2200,
i.e. PCI ID 1131:7164, subbvendor:subdevice 0070:8953.

So I can rule out lack of signal; the signal is routed into
the quad card first and then to the working dual-tuner card.

Things I have tried

 - checking the firmware blobs are all present and loading
 - setting debug=6 on the cx23885 module
 - setting tuner_lock_debug=1 on the si2157 module
 - enabling some dynamic debug, e.g.
   echo 'file  si2157.c +p' >/sys/kernel/debug/dynamic_debug/control
 - running w_scan under strace
 - review the kernel commit logs for the cx23885 and si2157 modules

So far I am not seeing any helpful error messages or commits that
could be causing the problem.
Are there some test programs I could run to debug this further?

Any help would be welcome