[PATCH 1/3] docs/conf.py: Treat mathjax as fallback math renderer

[Akira Yokosawa <akiyks@gmail.com>]

Currently, math expressions using the "math::" directive or
the ":math:" role of Sphinx need the imgmath extension for proper
rendering in html and epub builds.
imgmath requires dvipng (and latex).
Otherwise, "make htmldocs" will complain of missing commands.

As a matter of fact, the mathjax extension is loaded by default since
Sphinx v1.8 and it is good enough for html docs without any dependency
on texlive packages.

Stop loading the imgmath extension for html docs unless requirements
for imgmath are met.

For epub docs, keep the same behavior of always loading imgmath.

Signed-off-by: Akira Yokosawa <akiyks@gmail.com>
Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
---
 Documentation/conf.py | 34 +++++++++++++++++++++++++++++++++-
 1 file changed, 33 insertions(+), 1 deletion(-)

diff --git a/Documentation/conf.py b/Documentation/conf.py
index 934727e23e0e..3ec1f845c839 100644
--- a/Documentation/conf.py
+++ b/Documentation/conf.py
@@ -15,6 +15,23 @@
 import sys
 import os
 import sphinx
+from subprocess import check_output
+
+# helper
+# ------
+
+def have_command(cmd, ver_opt, str_in_ver):
+    """Run ```cmd`` with ``ver_opt`` and see if ``str_in_ver`` is found
+    or not.
+    """
+
+    try:
+        ver_str = check_output([cmd, ver_opt]).decode('utf-8', 'ignore')
+        have_cmd = str_in_ver in ver_str
+    except:
+        have_cmd = False
+
+    return have_cmd
 
 # Get Sphinx version
 major, minor, patch = sphinx.version_info[:3]
@@ -106,7 +123,22 @@ else:
 autosectionlabel_prefix_document = True
 autosectionlabel_maxdepth = 2
 
-extensions.append("sphinx.ext.imgmath")
+# Load math renderer:
+# For html builder, load imgmath only when its dependencies are met.
+# mathjax is the default math renderer since Sphinx 1.8.
+have_latex =  have_command('latex', '--version', 'pdfTeX')
+have_dvipng = have_command('dvipng', '--version', 'dvipng')
+load_imgmath = ((have_latex and have_dvipng)
+                or (major == 1 and minor < 8)
+                or 'epub' in sys.argv)
+
+if load_imgmath:
+    extensions.append("sphinx.ext.imgmath")
+    math_renderer = 'imgmath'
+else:
+    math_renderer = 'mathjax'
+
+sys.stderr.write("math_renderer: %s\n" % math_renderer)
 
 # Add any paths that contain templates here, relative to this directory.
 templates_path = ['_templates']
-- 
2.25.1