[PATCH v4] checkpatch.pl: Add SPDX license tag check

[Rob Herring <robh@kernel.org>]

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.

Cc: Andy Whitcroft <apw@canonical.com>
Cc: Joe Perches <joe@perches.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Philippe Ombredanne <pombredanne@nexb.com>
Signed-off-by: Rob Herring <robh@kernel.org>
---
Thomas, if you are inclined and Joe is happy with this, can you add this 
on top of your series adding license-rules.rst.

v4:
- Reference license-rules.rst
- Add comment style checks based on file types
- Check .rst files

v3:
- Since we specify that the tag goes on the 1st or 2nd line, the logic
  can be greatly simplified compared to v2 because we can just use the
  line number. And now the check is improved too.

 scripts/checkpatch.pl | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

diff --git a/scripts/checkpatch.pl b/scripts/checkpatch.pl
index 31031f10fe56..0324f845011d 100755
--- a/scripts/checkpatch.pl
+++ b/scripts/checkpatch.pl
@@ -2866,6 +2866,31 @@ sub process {
 			}
 		}
 
+# check for using SPDX license tag at beginning of files
+		if ($rawline =~ /^\+/ && !($realline == 1 && $rawline =~ /^[\s\+]#!/)) {
+			my $ln = 1;
+			my $comment = "";
+
+			if ($realfile =~ /\.(h|s|S)$/) {
+				$comment = '/\*';
+			} elsif ($realfile =~ /\.(c|dts|dtsi)$/) {
+				$comment = '//';
+			} elsif ($realfile =~ /\.(sh|pl|py)$/) {
+				if ($prevrawline =~ /^[\s\+]#!/) {
+					$ln = 2;
+				}
+				$comment = '#';
+			} elsif ($realfile =~ /\.rst$/) {
+				$comment = '..';
+			}
+
+			if ($comment !~ /^$/ &&
+			    ($realline == $ln xor $rawline =~ m@^\+$comment SPDX-License-Identifier: @)) {
+				WARN("SPDX_LICENSE_TAG",
+				     "Missing or malformed SPDX-License-Identifier tag in 1st (or 2nd for scripts) line\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)$/);
 
-- 
2.14.1