I tested these kernels from mainline Linux on the Cube:
5.14.0 works
5.15.0-rc3 works
5.15.0-rc4-00019-g5af4055fa813 works
Kernel 5.14.9 from www.kernel.org also works.
Running "apt-get install kernel-source" downloads:
"linux-source-5.14 (5.14.9-2)".
Compiling the downloaded kernel-source using Debian's .config file
"config.powerpc_none_powerpc.xz" resulted in a 200 MB kernel (perhaps
because it included all possible options), and I couldn't get it to
boot
(but I also didn't compile or install any modules). But compiling
using
the same .config file (see attached) that I've been using for testing
5.13 kernels, and that I also used for the above kernels that worked,
results in a working 12 MB kernel, and X is also working:
# uname -a
Linux ppc-cube 5.14.9-pmac #2 SMP Wed Oct 6 21:38:58 MDT 2021 ppc
GNU/Linux
# ls -l vmlinux
-rwxr-xr-x 1 root root 12382136 Oct 6 15:39 vmlinux
So I think the problem with the default kernel in Debian SID
(5.14.0-2)
has already been fixed, or perhaps there was an issue with the options
that were selected for the .config file. Either way, it doesn't appear
that a bisect is needed.