add a make target to generate man pages from kernel-doc

Hi Jon,

Currently, generation of manpages is hacky: one needs to run kernel-doc over c source files and then use an extra script to split results.

The new kernel-doc tool supports multiple files and even dirs, so there's no need to use git ls-files anymore.

Yet, it produces a single output. Change the logic to add, instead a target to docs Makefile to produce them, moving the split and build logic to sphinx-build-wrapper.

That allows honoring SPHINXDIRS when they point to a subdir, while scanning all files by default.

This series comes after:

    https://lore.kernel.org/linux-doc/[email protected]/T/#t

-

It should be noticed that, while Sphinx does have a man pages builder, it requires a completely different set of ReST entries that would need to be designed specially for that:

- for each man page, an entry at conf.py is required;
- the ReST page should be in a format closer to a real man page;
- exactly one .rst file for each man page.

This is completely different than the approach we take where we have, where kernel-doc direcly generates groff output formated data.

So, yeah, this is still a little bit hackish, but to avoid that we would need a completely different setup with, frankly, would be a headache to setup and maintain.

With that in mind, there are a couple of possible alternatives:

1. change kernel-doc to produce one file per man page. A change like that would require adding file directory parameter for output, and/or teaching it about Documentation/output and O=;
2. make sphinx-build-wrapper do that for us. That's IMO the best alternative, which is what I actually implemented;
3. add a man pages Sphinx extension that would be tricking sphinx, making it produce one man page .rst file for each kernel-doc markup entry.
This is probably doable, and could eventually bring some advantages, as we could also have man pages for pages that are only in ReST format, but it will require some extra development, and likely require several changes at the existing .rst files. Also, generating extra output files out of the blue is something that Sphinx doesn't like to do, as it likes to have a 1:1 mapping between source files and output ones.
Not sure if it worth the efforts, and will still be hackish.

I opted for (2), as it means that ones needing to generate docs can benefit from SPHINXDIRS, and we don't need anything extra at docs Makefile. By doing that, we can later improve the man pages logic without needing to touch anything else but kernel-doc and/or the wrapper tool.

There are two points to consider:

- when SPHINXDIRS=. (the default), it searches fror all files treewide, even the ones without kernel-doc entries at docs, which is aligned with the procedure used by the current documentation about how to generate man pages;

- when SHINXDIRS=subdirs, it searches only for the files that come just after kernel-doc markups. It doesn't take into account any kernel-doc markup parameters to filter out internal/external functions or to point to an extra header file.

---

v3:
- fixed a badly-solved conflict on v2;
- improved documentation about manpages generation;
- added SEE ALSO at the man pages.

v2:
- no actual changes, but it was rebased on the top of the actual pacth series, in order to apply cleanly.
*** BLURB HERE ***

Mauro Carvalho Chehab (2):
docs: add support to build manpages from kerneldoc output
tools: kernel-doc: add a see also section at man pages

 Documentation/Makefile                 |  3 +-
 Documentation/doc-guide/kernel-doc.rst | 29 +++++----
 Makefile                               |  2 +-
 scripts/lib/kdoc/kdoc_files.py         |  5 +-
 scripts/lib/kdoc/kdoc_output.py        | 84 ++++++++++++++++++++++++--
 scripts/split-man.pl                   | 28 ---------
 tools/docs/sphinx-build-wrapper        | 81 +++++++++++++++++++++++--
 7 files changed, 178 insertions(+), 54 deletions(-)
 delete mode 100755 scripts/split-man.pl

2.51.0