tools/docs/sphinx-build-wrapper: allow skipping sphinx-build step

Most targets have two steps:
- step 1: run sphinx-build;
- step 2: run a post-build logic.

The second step can be as simple as copying static files like CSS,
but may may also envolve running make. allowing to skip the first
step helps debugging what's broken, and also allows using make
command line arguments like --ignore-errors.

Add an option to skip step 1.

Requested-by: Akira Yokosawa <akiyks@gmail.com>
Link: https://lore.kernel.org/linux-doc/5031e0c4-f17e-41b8-8955-959989e797f2@gmail.com/
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <30dae295606ae1735f3bbeef4ca041a76dcd4540.1758539031.git.mchehab+huawei@kernel.org>

tools/docs/sphinx-build-wrapper

@@ -622,7 +622,8 @@ class SphinxBuilder:
         shutil.rmtree(self.builddir, ignore_errors=True)
 
     def build(self, target, sphinxdirs=None,
-              theme=None, css=None, paper=None, deny_vf=None, rustdoc=False):
+              theme=None, css=None, paper=None, deny_vf=None, rustdoc=False,
+              skip_sphinx=False):
         """
         Build documentation using Sphinx. This is the core function of this
         module. It prepares all arguments required by sphinx-build.
@@ -644,9 +645,10 @@ class SphinxBuilder:
         #
         # Other targets require sphinx-build, so check if it exists
         #
-        sphinxbuild = shutil.which(self.sphinxbuild, path=self.env["PATH"])
-        if not sphinxbuild and target != "mandocs":
-            sys.exit(f"Error: {self.sphinxbuild} not found in PATH.\n")
+        if not skip_sphinx:
+            sphinxbuild = shutil.which(self.sphinxbuild, path=self.env["PATH"])
+            if not sphinxbuild and target != "mandocs":
+                sys.exit(f"Error: {self.sphinxbuild} not found in PATH.\n")
 
         if builder == "latex":
             if not self.pdflatex_cmd and not self.latexmk_cmd:
@@ -732,7 +734,7 @@ class SphinxBuilder:
 
             if target == "mandocs":
                 self.handle_man(kerneldoc, docs_dir, src_dir, output_dir)
-            else:
+            elif not skip_sphinx:
                 try:
                     result = self.run_sphinx(sphinxbuild, build_args,
                                              env=self.env)
@@ -814,6 +816,9 @@ def main():
     parser.add_argument('-i', '--interactive', action='store_true',
                         help="Change latex default to run in interactive mode")
 
+    parser.add_argument('-s', '--skip-sphinx-build', action='store_true',
+                        help="Skip sphinx-build step")
+
     parser.add_argument("-V", "--venv", nargs='?', const=f'{VENV_DEFAULT}',
                         default=None,
                         help=f'If used, run Sphinx from a venv dir (default dir: {VENV_DEFAULT})')
@@ -829,7 +834,8 @@ def main():
 
     builder.build(args.target, sphinxdirs=args.sphinxdirs,
                   theme=args.theme, css=args.css, paper=args.paper,
-                  rustdoc=args.rustdoc, deny_vf=args.deny_vf)
+                  rustdoc=args.rustdoc, deny_vf=args.deny_vf,
+                  skip_sphinx=args.skip_sphinx_build)
 
 if __name__ == "__main__":
     main()