As described in

https://lkml.org/lkml/2020/4/8/280

the kernel build system for 'make htmldocs' fails with Sphinx 3. A patch is not yet available.

Extension error: Could not import extension cdomain (exception: cannot import name 'c_funcptr_sig_re' from 'sphinx.domains.c' (/usr/lib/python3.8/site-packages/sphinx/domains/c.py))

The same can be seen in U-Boot.

Downgrading to Sphinx 2 helps but is not a permanent solution.

The latest Sphinx 2 Debian packages are here:

https://snapshot.debian.org/package/sphinx/2.4.3-5/#python3-sphinx_2.4.3-5

Best regards

Heinrich