checkpatch: add constant comparison on left side test

"CONST <comparison> variable" checks like:

        if (NULL != foo)
and
        while (0 < bar(...))

where a constant (or what appears to be a constant like an upper case
identifier) is on the left of a comparison are generally preferred to be
written using the constant on the right side like:

        if (foo != NULL)
and
        while (bar(...) > 0)

Add a test for this.

Add a --fix option too, but only do it when the code is immediately
surrounded by parentheses to avoid misfixing things like "(0 < bar() +
constant)"

Signed-off-by: Joe Perches <[email protected]>
Cc: Nicolas Morey Chaisemartin <[email protected]>
Cc: Viresh Kumar <[email protected]>
Cc: Dan Carpenter <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

scripts/checkpatch.pl

@@ -4231,6 +4231,35 @@ sub process {
 			}
 		}
 
+# comparisons with a constant or upper case identifier on the left
+#	avoid cases like "foo + BAR < baz"
+#	only fix matches surrounded by parentheses to avoid incorrect
+#	conversions like "FOO < baz() + 5" being "misfixed" to "baz() > FOO + 5"
+		if ($^V && $^V ge 5.10.0 &&
+		    $line =~ /^\+(.*)\b($Constant|[A-Z_][A-Z0-9_]*)\s*($Compare)\s*($LvalOrFunc)/) {
+			my $lead = $1;
+			my $const = $2;
+			my $comp = $3;
+			my $to = $4;
+			my $newcomp = $comp;
+			if ($lead !~ /$Operators\s*$/ &&
+			    $to !~ /^(?:Constant|[A-Z_][A-Z0-9_]*)$/ &&
+			    WARN("CONSTANT_COMPARISON",
+				 "Comparisons should place the constant on the right side of the test\n" . $herecurr) &&
+			    $fix) {
+				if ($comp eq "<") {
+					$newcomp = ">";
+				} elsif ($comp eq "<=") {
+					$newcomp = ">=";
+				} elsif ($comp eq ">") {
+					$newcomp = "<";
+				} elsif ($comp eq ">=") {
+					$newcomp = "<=";
+				}
+				$fixed[$fixlinenr] =~ s/\(\s*\Q$const\E\s*$Compare\s*\Q$to\E\s*\)/($to $newcomp $const)/;
+			}
+		}
+
 # Return of what appears to be an errno should normally be negative
 		if ($sline =~ /\breturn(?:\s*\(+\s*|\s+)(E[A-Z]+)(?:\s*\)+\s*|\s*)[;:,]/) {
 			my $name = $1;