checkpatch.pl: add SPDX license tag check

Add SPDX license tag check based on the rules defined in
Documentation/process/license-rules.rst.  To summarize, SPDX license
tags should be on the 1st line (or 2nd line in scripts) using the
appropriate comment style for the file type.

Link: http://lkml.kernel.org/r/20180202154026.15298-1-robh@kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Joe Perches <joe@perches.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Philippe Ombredanne <pombredanne@nexb.com>
Cc: Andy Whitcroft <apw@canonical.com>
Cc: Joe Perches <joe@perches.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Igor Stoppa <igor.stoppa@huawei.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Author: robherring

scripts/checkpatch.pl

@@ -2257,6 +2257,8 @@ sub process {
 
 	my $camelcase_file_seeded = 0;
 
+	my $checklicenseline = 1;
+
 	sanitise_line_reset();
 	my $line;
 	foreach my $rawline (@rawlines) {
@@ -2448,6 +2450,7 @@ sub process {
 			} else {
 				$check = $check_orig;
 			}
+			$checklicenseline = 1;
 			next;
 		}
 
@@ -2911,6 +2914,30 @@ sub process {
 			}
 		}
 
+# check for using SPDX license tag at beginning of files
+		if ($realline == $checklicenseline) {
+			if ($rawline =~ /^[ \+]\s*\#\!\s*\//) {
+				$checklicenseline = 2;
+			} elsif ($rawline =~ /^\+/) {
+				my $comment = "";
+				if ($realfile =~ /\.(h|s|S)$/) {
+					$comment = '/*';
+				} elsif ($realfile =~ /\.(c|dts|dtsi)$/) {
+					$comment = '//';
+				} elsif (($checklicenseline == 2) || $realfile =~ /\.(sh|pl|py|awk|tc)$/) {
+					$comment = '#';
+				} elsif ($realfile =~ /\.rst$/) {
+					$comment = '..';
+				}
+
+				if ($comment !~ /^$/ &&
+				    $rawline !~ /^\+\Q$comment\E SPDX-License-Identifier: /) {
+					WARN("SPDX_LICENSE_TAG",
+					     "Missing or malformed SPDX-License-Identifier tag in line $checklicenseline\n" . $herecurr);
+				}
+			}
+		}
+
 # check we are in a valid source file if not then ignore this hunk
 		next if ($realfile !~ /\.(h|c|s|S|sh|dtsi|dts)$/);