checkpatch: add support to check 'Fixes:' tag format

index 9b47584..ef34716 100755
@@ -2818,51 +2818,109 @@ sub process {
 		    $line !~ /^\s*(?:Link|Patchwork|http|https|BugLink|base-commit):/i &&
 		    $line !~ /^This reverts commit [0-9a-f]{7,40}/ &&
 		    ($line =~ /\bcommit\s+[0-9a-f]{5,}\b/i ||
+		     $line =~ /\bfixes:\s+[0-9a-f]{5,}\b/i ||
 		     ($line =~ /(?:\s|^)[0-9a-f]{12,40}(?:[\s"'\(\[]|$)/i &&
-		      $line !~ /[\<\[][0-9a-f]{12,40}[\>\]]/i &&
-		      $line !~ /\bfixes:\s*[0-9a-f]{12,40}/i))) {
+		      $line !~ /[\<\[][0-9a-f]{12,40}[\>\]]/i))) {
 			my $init_char = "c";
 			my $orig_commit = "";
+			my $prefix = "commit";
+			my $prefix_case = "[Cc]ommit";
 			my $short = 1;
 			my $long = 0;
 			my $case = 1;
 			my $space = 1;
+			my $space2 = 1;
 			my $hasdesc = 0;
-			my $hasparens = 0;
+			my $has_parens_and_dqm = 0; # Double quotation mark
 			my $hasprefix = 1;
 			my $id = '0123456789ab';
 			my $orig_desc = "commit description";
 			my $description = "";
 			my $sha1_length_min = 12;
+			my $acrosslines = 0;
+			my $title = "title line";
+			my $desc_mismatch = 0;
+			my $diagnostics = "Diagnostics info:\n";
+			my $name = "commit description";
 
-			if ($line =~ /\b(c)ommit\s+([0-9a-f]{5,})\b/i) {
+			if ($line =~ /\b(f)ixes:\s+([0-9a-f]{5,})\b/i) {
+				$init_char = $1;
+				$orig_commit = lc($2);
+				$prefix = "Fixes:";
+				$prefix_case = "Fixes:";
+				$init_char = "F";
+				$title = "a single line title (without line breaks but ellipsis is fine!)";
+				$name = "Fixes: tag";
+			} elsif ($line =~ /\b(c)ommit\s+([0-9a-f]{5,})\b/i) {
 				$init_char = $1;
 				$orig_commit = lc($2);
 			} elsif ($line =~ /\b([0-9a-f]{12,40})\b/i) {
 				$orig_commit = lc($1);
 				$hasprefix = 0;
+				$diagnostics .= "Missing prefix \"$prefix_case\".\n";
+
+			}
+
+			if ($line =~ /\b$prefix\s+[0-9a-f]{41,}/i) {
+				$long = 1;
+			} elsif ($line =~ /\b$prefix\s+[0-9a-f]{$sha1_length_min,40}/i) {
+				$short = 0;
+			} elsif ($hasprefix) {
+				$diagnostics .= "Commit id $orig_commit is too short.\n";
+			}
+
+			if ($line =~ /\b$prefix [0-9a-f]/i) {
+				$space = 0;
+			} elsif ($hasprefix) {
+				$diagnostics .= "Extra whitespace between prefix \"${init_char}" . substr($prefix, 1) . "\" and $orig_commit.\n";
 			}
 
-			$short = 0 if ($line =~ /\bcommit\s+[0-9a-f]{$sha1_length_min,40}/i);
-			$long = 1 if ($line =~ /\bcommit\s+[0-9a-f]{41,}/i);
-			$space = 0 if ($line =~ /\bcommit [0-9a-f]/i);
-			$case = 0 if ($line =~ /\b[Cc]ommit\s+[0-9a-f]{5,40}[^A-F]/);
-			if ($line =~ /\bcommit\s+[0-9a-f]{5,}\s+\("([^"]+)"\)/i) {
+			if ($line =~ /[0-9a-f]{5,} \S+/i || $line =~ /[0-9a-f]{5,}\s*$/i) {
+				$space2 = 0;
+			} elsif ($hasprefix) {
+				$diagnostics .= "Please use a and only a single whitespace between $orig_commit and title line.\n";
+			}
+
+			if ($line =~ /\b$prefix_case\s+[0-9a-f]{5,40}[^A-F]/) {
+				$case = 0;
+			} elsif ($hasprefix) {
+				$line =~ /(\b$prefix)\s+[0-9a-f]{5,40}[^A-F]/i;
+				$diagnostics .= "The prefix \"$1\" has case error.\n";
+			}
+
+			if ($line =~ /\b$prefix\s+[0-9a-f]{5,}\s+\("(.+)"\)/i) {
 				$orig_desc = $1;
-				$hasparens = 1;
+				$has_parens_and_dqm = 1;
+				# Drop the ellipsis
+				if ($prefix eq "Fixes:" && $orig_desc =~ /(\s*\.{3}\s*$)/) {
+				    $orig_desc = substr($orig_desc, 0, length($orig_desc) - length($1));
+				}
 			} elsif ($line =~ /\bcommit\s+[0-9a-f]{5,}\s*$/i &&
 				 defined $rawlines[$linenr] &&
-				 $rawlines[$linenr] =~ /^\s*\("([^"]+)"\)/) {
+				 $rawlines[$linenr] =~ /^\s*\("(.+)"\)/) {
 				$orig_desc = $1;
-				$hasparens = 1;
-			} elsif ($line =~ /\bcommit\s+[0-9a-f]{5,}\s+\("[^"]+$/i &&
+				$has_parens_and_dqm = 1;
+			} elsif ($line =~ /\b$prefix\s+[0-9a-f]{5,}\s+\(".+$/i &&
 				 defined $rawlines[$linenr] &&
-				 $rawlines[$linenr] =~ /^\s*[^"]+"\)/) {
-				$line =~ /\bcommit\s+[0-9a-f]{5,}\s+\("([^"]+)$/i;
+				 $rawlines[$linenr] =~ /^\s*.+"\)/) {
+				$line =~ /\b$prefix\s+[0-9a-f]{5,}\s+\("(.+)$/i;
 				$orig_desc = $1;
-				$rawlines[$linenr] =~ /^\s*([^"]+)"\)/;
+				$rawlines[$linenr] =~ /^\s*(.+)"\)/;
 				$orig_desc .= " " . $1;
-				$hasparens = 1;
+				$has_parens_and_dqm = 1;
+
+				if ($prefix eq "Fixes:") {
+					$acrosslines = 1;
+					$diagnostics .= "The $name spans across lines.\n";
+				}
+			} elsif (($line =~ /\bcommit\s+[0-9a-f]{5,}\s*$/i &&
+				  defined $rawlines[$linenr] &&
+				  $rawlines[$linenr] =~ /^\s*\("/) ||
+				 $line =~ /\b$prefix\s+[0-9a-f]{5,}\s+\(".+$/i) {
+				$diagnostics .= "Missing right '\")' at the end of title line?\n";
+				$diagnostics .= "The $name spans across more than two lines?\n";
+			} elsif ($hasprefix && !$space2) {
+				$diagnostics .= "No title line in '(\"<$title>\")' format is found.\n";
 			}
 
 			($id, $description) = git_commit_info($orig_commit,

@@ -2879,10 +2937,32 @@ sub process {
 				}
 			}
 
+			if (defined($id) && $has_parens_and_dqm && ($orig_desc ne $description)) {
+			    # Allow short description without too short!
+				if ($prefix eq "Fixes:") {
+						if (length($orig_desc) >= length($description)/2) {
+							my $desc = substr($description, 0, length($orig_desc));
+
+							if ($orig_desc ne $desc) {
+								$desc_mismatch = 1;
+								$diagnostics .= "The title line doesn't match the original commit.\n";
+							}
+						} else {
+							$desc_mismatch = 1;
+							$diagnostics .= "The title line is too abbreviated, at least half of original commit is necessary.\n";
+						}
+				} else {
+						$desc_mismatch = 1;
+						$diagnostics .= "The title line doesn't match the original commit.\n";
+				}
+			}
+
 			if (defined($id) &&
-			   ($short || $space || $case || ($orig_desc ne $description) || !$hasparens)) {
+			   ($short || $space || $space2 || $case || $desc_mismatch || !$hasprefix || !$has_parens_and_dqm || $acrosslines)) {
 				ERROR("GIT_COMMIT_ID",
-				      "Please use git commit description style 'commit <$sha1_length_min+ chars of sha1> (\"<title line>\")' - ie: '${init_char}ommit $id (\"$description\")'\n" . $herecurr);
+				      "Please use git commit description style:\n$prefix <$sha1_length_min+ chars of sha1> (\"<$title>\")\n" .
+				      "i.e.: '${init_char}" . substr($prefix, 1) . " $id (\"$description\")'\n" .
+				      $diagnostics . $herecurr);
 			}
 		}
 

@@ -2982,17 +3062,6 @@ sub process {
 			}
 		}
 
-# check for invalid commit id
-		if ($in_commit_log && $line =~ /^fixes:\s+([0-9a-f]{6,40})\b/i) {
-			my $id;
-			my $description;
-			($id, $description) = git_commit_info($1, undef, undef);
-			if (!defined($id)) {
-				WARN("UNKNOWN_COMMIT_ID",
-				     "Unknown commit id '$1', maybe rebased or not pulled?\n" . $herecurr);
-			}
-		}
-
 # ignore non-hunk lines and lines being removed
 		next if (!$hunk_line || $line =~ /^-/);