kernel-doc: Use c:struct for Sphinx 3.0 and later

The kernel-doc Sphinx plugin and associated script currently emit
'c:type' directives for "struct foo" documentation.

Sphinx 3.0 warns about this:
  /home/petmay01/linaro/qemu-from-laptop/qemu/docs/../include/exec/memory.h:3: WARNING: Type must be either just a name or a typedef-like declaration.
  If just a name:
    Error in declarator or parameters
    Invalid C declaration: Expected identifier in nested name, got keyword: struct [error at 6]
      struct MemoryListener
      ------^
  If typedef-like declaration:
    Error in declarator or parameters
    Invalid C declaration: Expected identifier in nested name. [error at 21]
      struct MemoryListener
      ---------------------^

because it wants us to use the new-in-3.0 'c:struct' instead.

Plumb the Sphinx version through to the kernel-doc script
and use it to select 'c:struct' for newer versions than 3.0.

Fixes: LP:1872113
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>

docs/sphinx/kerneldoc.py

@@ -99,6 +99,7 @@ def run(self):
                 env.note_dependency(os.path.abspath(f))
                 cmd += ['-export-file', f]
 
+        cmd += ['-sphinx-version', sphinx.__version__]
         cmd += [filename]
 
         try:

scripts/kernel-doc

@@ -71,6 +71,8 @@ Output selection (mutually exclusive):
 			DOC: sections. May be specified multiple times.
 
 Output selection modifiers:
+  -sphinx-version VER   Generate rST syntax for the specified Sphinx version.
+                        Only works with reStructuredTextFormat.
   -no-doc-sections	Do not output DOC: sections.
   -enable-lineno        Enable output of #define LINENO lines. Only works with
                         reStructuredText format.
@@ -286,6 +288,7 @@ use constant {
 };
 my $output_selection = OUTPUT_ALL;
 my $show_not_found = 0;	# No longer used
+my $sphinx_version = "0.0"; # if not specified, assume old
 
 my @export_file_list;
 
@@ -436,6 +439,8 @@ while ($ARGV[0] =~ m/^--?(.*)/) {
 	    $enable_lineno = 1;
     } elsif ($cmd eq 'show-not-found') {
 	$show_not_found = 1;  # A no-op but don't fail
+    } elsif ($cmd eq 'sphinx-version') {
+        $sphinx_version = shift @ARGV;
     } else {
 	# Unknown argument
         usage();
@@ -963,7 +968,16 @@ sub output_struct_rst(%) {
     my $oldprefix = $lineprefix;
     my $name = $args{'type'} . " " . $args{'struct'};
 
-    print "\n\n.. c:type:: " . $name . "\n\n";
+    # Sphinx 3.0 and up will emit warnings for "c:type:: struct Foo".
+    # It wants to see "c:struct:: Foo" (and will add the word 'struct' in
+    # the rendered output).
+    if ((split(/\./, $sphinx_version))[0] >= 3) {
+        my $sname = $name;
+        $sname =~ s/^struct //;
+        print "\n\n.. c:struct:: " . $sname . "\n\n";
+    } else {
+        print "\n\n.. c:type:: " . $name . "\n\n";
+    }
     print_lineno($declaration_start_line);
     $lineprefix = "   ";
     output_highlight_rst($args{'purpose'});