checkpatch: try to avoid mask and shift errors

Shift has a higher precedence that mask so warn when a mask then shift
operation is done without parentheses around the mask.

This test works well for a right shift, but the left shift is pretty
commonly done correctly so only warn on the right shift.
Signed-off-by: NJoe Perches <joe@perches.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

scripts/checkpatch.pl

@@ -4482,6 +4482,14 @@ sub process {
 			}
 		}
 
+# check for mask then right shift without a parentheses
+		if ($^V && $^V ge 5.10.0 &&
+		    $line =~ /$LvalOrFunc\s*\&\s*($LvalOrFunc)\s*>>/ &&
+		    $4 !~ /^\&/) { # $LvalOrFunc may be &foo, ignore if so
+			WARN("MASK_THEN_SHIFT",
+			     "Possible precedence defect with mask then right shift - may need parentheses\n" . $herecurr);
+		}
+
 # check for bad placement of section $InitAttribute (e.g.: __initdata)
 		if ($line =~ /(\b$InitAttribute\b)/) {
 			my $attr = $1;