checkpatch: Warn on improper usage of Co-developed-by

The purpose of Co-developed-by: is to give attribution to authors who
aren't already attributed by the From: tag, i.e. who aren't the nominal
patch author.  Because Co-developed-by: is essentially a variation of
From:, it must be accompanied by a Signed-off-by: of the associated
co-author.  To ease the burden of determining whether or not co-authors
have signed off, Co-developed-by and Signed-off-by: must be explicitly
paired, i.e. on consecutive lines for a given co-author.
Suggested-by: Joe Perches <joe@perches.com>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

scripts/checkpatch.pl

@@ -2687,6 +2687,24 @@ sub process {
 			} else {
 				$signatures{$sig_nospace} = 1;
 			}
+
+# Check Co-developed-by: immediately followed by Signed-off-by: with same name and email
+			if ($sign_off =~ /^co-developed-by:$/i) {
+				if ($email eq $author) {
+					WARN("BAD_SIGN_OFF",
+					      "Co-developed-by: should not be used to attribute nominal patch author '$author'\n" . "$here\n" . $rawline);
+				}
+				if (!defined $lines[$linenr]) {
+					WARN("BAD_SIGN_OFF",
+                                             "Co-developed-by: must be immediately followed by Signed-off-by:\n" . "$here\n" . $rawline);
+				} elsif ($rawlines[$linenr] !~ /^\s*signed-off-by:\s*(.*)/i) {
+					WARN("BAD_SIGN_OFF",
+					     "Co-developed-by: must be immediately followed by Signed-off-by:\n" . "$here\n" . $rawline . "\n" .$rawlines[$linenr]);
+				} elsif ($1 ne $email) {
+					WARN("BAD_SIGN_OFF",
+					     "Co-developed-by and Signed-off-by: name/email do not match \n" . "$here\n" . $rawline . "\n" .$rawlines[$linenr]);
+				}
+			}
 		}
 
 # Check email subject for common tools that don't need to be mentioned