docs: sphinx/kfigure.py: Use rsvg-convert(1) for DOT -> PDF conversion

On openSUSE, dot(1) command does not support direct PDF output.
On other distros, generated PDF images have unnecessarily wide margins,
especially for small graphs.

By using dot(1) for DOT -> SVG, then rsvg-convert(1) for SVG -> PDF,
more optimal PDF images can be obtained, with the bonus of improved
portability across various distros.

Add rules in kfigure.py so that the above mentioned route is taken
when rsvg-convert(1) is available.

Note that rsvg-convert(1) is recommended by sphinx_pre_install.
So it is most likely that existing systems for building pdfdocs have
rsvg-convert(1) installed.

Note:
    SVG features supported by rsvg-convert(1) vary depending on its
    version and distro config.
    For example, the one found on Ubuntu Bionic (version 2.40.20) does
    poor job in rendering some of SVG files drawn by Inkscape.
    SVG files generated by dot(1) are converted nicely even with such
    old versions of rsvg-convert.

    So this change does not affect the quality of such figures in any
    way.
Signed-off-by: Akira Yokosawa <akiyks@gmail.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
Link: https://lore.kernel.org/r/15b56dd3-081a-2469-c3a4-dfc1ca4c6c2d@gmail.comSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/sphinx/kfigure.py

@@ -31,6 +31,8 @@ u"""
 
     * ``dot(1)``: Graphviz (https://www.graphviz.org). If Graphviz is not
       available, the DOT language is inserted as literal-block.
+      For conversion to PDF, ``rsvg-convert(1)`` of librsvg
+      (https://gitlab.gnome.org/GNOME/librsvg) is used when available.
 
     * SVG to PDF: To generate PDF, you need at least one of this tools:
 
@@ -113,6 +115,9 @@ dot_cmd = None
 # ImageMagick' convert(1) support
 convert_cmd = None
 
+# librsvg's rsvg-convert(1) support
+rsvg_convert_cmd = None
+
 
 def setup(app):
     # check toolchain first
@@ -160,11 +165,12 @@ def setupTools(app):
 
     This function is called once, when the builder is initiated.
     """
-    global dot_cmd, convert_cmd   # pylint: disable=W0603
+    global dot_cmd, convert_cmd, rsvg_convert_cmd   # pylint: disable=W0603
     kernellog.verbose(app, "kfigure: check installed tools ...")
 
     dot_cmd = which('dot')
     convert_cmd = which('convert')
+    rsvg_convert_cmd = which('rsvg-convert')
 
     if dot_cmd:
         kernellog.verbose(app, "use dot(1) from: " + dot_cmd)
@@ -177,6 +183,11 @@ def setupTools(app):
         kernellog.warn(app,
             "convert(1) not found, for SVG to PDF conversion install "
             "ImageMagick (https://www.imagemagick.org)")
+    if rsvg_convert_cmd:
+        kernellog.verbose(app, "use rsvg-convert(1) from: " + rsvg_convert_cmd)
+    else:
+        kernellog.verbose(app, "rsvg-convert(1) not found, "
+                          "falling back to raster image conversion")
 
 
 # integrate conversion tools
@@ -266,7 +277,13 @@ def convert_image(img_node, translator, src_fname=None):
 
             if in_ext == '.dot':
                 kernellog.verbose(app, 'convert DOT to: {out}/' + _name)
-                ok = dot2format(app, src_fname, dst_fname)
+                if translator.builder.format == 'latex':
+                    svg_fname = path.join(translator.builder.outdir, fname + '.svg')
+                    ok1 = dot2format(app, src_fname, svg_fname)
+                    ok2 = svg2pdf_by_rsvg(app, svg_fname, dst_fname)
+                    ok = ok1 and ok2
+                else:
+                    ok = dot2format(app, src_fname, dst_fname)
 
             elif in_ext == '.svg':
                 kernellog.verbose(app, 'convert SVG to: {out}/' + _name)
@@ -319,6 +336,31 @@ def svg2pdf(app, svg_fname, pdf_fname):
         kernellog.warn(app, "Error #%d when calling: %s" % (exit_code, " ".join(cmd)))
     return bool(exit_code == 0)
 
+def svg2pdf_by_rsvg(app, svg_fname, pdf_fname):
+    """Convert SVG to PDF with ``rsvg-convert(1)`` command.
+
+    * ``svg_fname`` pathname of input SVG file, including extension ``.svg``
+    * ``pdf_fname`` pathname of output PDF file, including extension ``.pdf``
+
+    Input SVG file should be the one generated by ``dot2format()``.
+    SVG -> PDF conversion is done by ``rsvg-convert(1)``.
+
+    If ``rsvg-convert(1)`` is unavailable, fall back to ``svg2pdf()``.
+
+    """
+
+    if rsvg_convert_cmd is None:
+        ok = svg2pdf(app, svg_fname, pdf_fname)
+    else:
+        cmd = [rsvg_convert_cmd, '--format=pdf', '-o', pdf_fname, svg_fname]
+        # use stdout and stderr from parent
+        exit_code = subprocess.call(cmd)
+        if exit_code != 0:
+            kernellog.warn(app, "Error #%d when calling: %s" % (exit_code, " ".join(cmd)))
+        ok = bool(exit_code == 0)
+
+    return ok
+
 
 # image handling
 # ---------------------