docs: translations: add translations links when they exist

Add a new Sphinx extension that knows about the translations of kernel
documentation and can insert links to the translations at the top of
the document.

It basically works like this:

1. Register a new node type, LanguagesNode.

2. Register a new transform, TranslationsTransform, that inserts a new
   LanguageNode at the top of every document. The LanguageNode contains
   "pending references" to translations of the document. The key here
   is that these are pending (i.e. unresolved) references that may or
   may not actually exist.

3. Register a 'doctree-resolved' event that iterates over all the
   LanguageNode nodes. Any unresolved references are filtered out; the
   list of resolved references is passed to the 'translations.html'
   template and rendered as an HTML node (if HTML output is selected).

Testing: make htmldocs, make latexdocs with Sphinx v4.3.2 and Firefox.

v2:
- changed bar into a drop-down menu
- fixed language labels
- fixed hysteresis reported by Akira Yokosawa

Cc: Federico Vaga <federico.vaga@vaga.pv.it>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Akira Yokosawa <akiyks@gmail.com>
Cc: Yanteng Si <siyanteng@loongson.cn>
Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20231215123701.2712807-1-vegard.nossum@oracle.com

Documentation/conf.py

@@ -55,7 +55,7 @@ needs_sphinx = '2.4.4'
 extensions = ['kerneldoc', 'rstFlatTable', 'kernel_include',
               'kfigure', 'sphinx.ext.ifconfig', 'automarkup',
               'maintainers_include', 'sphinx.ext.autosectionlabel',
-              'kernel_abi', 'kernel_feat']
+              'kernel_abi', 'kernel_feat', 'translations']
 
 if major >= 3:
     if (major > 3) or (minor > 0 or patch >= 2):

Documentation/sphinx-static/custom.css

@@ -83,3 +83,56 @@ input.kernel-toc-toggle { display: none; }
     h3.kernel-toc-contents { display: inline; }
     div.kerneltoc a { color: black; }
 }
+
+/* Language selection menu */
+
+div.admonition {
+    /*
+     * Make sure we don't overlap notes and warnings at the top of the
+     * document.
+     */
+    clear: both;
+}
+
+div.language-selection {
+    background: #eeeeee;
+    border: 1px solid #cccccc;
+    margin-bottom: 1em;
+    padding: .5em;
+
+    position: relative;
+    float: right;
+}
+
+div.language-selection a {
+    display: block;
+    padding: 0.5em;
+    color: #333333;
+    text-decoration: none;
+}
+
+div.language-selection ul {
+    display: none;
+    position: absolute;
+
+    /* Align with the parent div */
+    top: 100%;
+    right: 0;
+    margin: 0;
+
+    list-style: none;
+
+    background: #fafafa;
+    border: 1px solid #cccccc;
+
+    /* Never break menu item lines */
+    white-space: nowrap;
+}
+
+div.language-selection:hover ul {
+    display: block;
+}
+
+div.language-selection ul li:hover {
+    background: #dddddd;
+}

Documentation/sphinx/templates/translations.html

@@ -0,0 +1,15 @@
+<!-- SPDX-License-Identifier: GPL-2.0 -->
+<!-- Copyright © 2023, Oracle and/or its affiliates. -->
+
+{# Create a language menu for translations #}
+{% if languages|length > 0: %}
+<div class="language-selection">
+{{ current_language }}
+
+<ul>
+{% for ref in languages: %}
+<li><a href="{{ ref.refuri }}">{{ ref.astext() }}</a></li>
+{% endfor %}
+</ul>
+</div>
+{% endif %}

Documentation/sphinx/translations.py

@@ -0,0 +1,101 @@
+# SPDX-License-Identifier: GPL-2.0
+#
+# Copyright © 2023, Oracle and/or its affiliates.
+# Author: Vegard Nossum <vegard.nossum@oracle.com>
+#
+# Add translation links to the top of the document.
+#
+
+import os
+
+from docutils import nodes
+from docutils.transforms import Transform
+
+import sphinx
+from sphinx import addnodes
+from sphinx.errors import NoUri
+
+all_languages = {
+    # English is always first
+    None: 'English',
+
+    # Keep the rest sorted alphabetically
+    'zh_CN': 'Chinese (Simplified)',
+    'zh_TW': 'Chinese (Traditional)',
+    'it_IT': 'Italian',
+    'ja_JP': 'Japanese',
+    'ko_KR': 'Korean',
+    'sp_SP': 'Spanish',
+}
+
+class LanguagesNode(nodes.Element):
+    def __init__(self, current_language, *args, **kwargs):
+        super().__init__(*args, **kwargs)
+
+        self.current_language = current_language
+
+class TranslationsTransform(Transform):
+    default_priority = 900
+
+    def apply(self):
+        app = self.document.settings.env.app
+        docname = self.document.settings.env.docname
+
+        this_lang_code = None
+        components = docname.split(os.sep)
+        if components[0] == 'translations' and len(components) > 2:
+            this_lang_code = components[1]
+
+            # normalize docname to be the untranslated one
+            docname = os.path.join(*components[2:])
+
+        new_nodes = LanguagesNode(all_languages[this_lang_code])
+
+        for lang_code, lang_name in all_languages.items():
+            if lang_code == this_lang_code:
+                continue
+
+            if lang_code is None:
+                target_name = docname
+            else:
+                target_name = os.path.join('translations', lang_code, docname)
+
+            pxref = addnodes.pending_xref('', refdomain='std',
+                reftype='doc', reftarget='/' + target_name, modname=None,
+                classname=None, refexplicit=True)
+            pxref += nodes.Text(lang_name)
+            new_nodes += pxref
+
+        self.document.insert(0, new_nodes)
+
+def process_languages(app, doctree, docname):
+    for node in doctree.traverse(LanguagesNode):
+        if app.builder.format not in ['html']:
+            node.parent.remove(node)
+            continue
+
+        languages = []
+
+        # Iterate over the child nodes; any resolved links will have
+        # the type 'nodes.reference', while unresolved links will be
+        # type 'nodes.Text'.
+        languages = list(filter(lambda xref:
+            isinstance(xref, nodes.reference), node.children))
+
+        html_content = app.builder.templates.render('translations.html',
+            context={
+                'current_language': node.current_language,
+                'languages': languages,
+            })
+
+        node.replace_self(nodes.raw('', html_content, format='html'))
+
+def setup(app):
+    app.add_node(LanguagesNode)
+    app.add_transform(TranslationsTransform)
+    app.connect('doctree-resolved', process_languages)
+
+    return {
+        'parallel_read_safe': True,
+        'parallel_write_safe': True,
+    }