[PATCH v3] checkpatch: add warning for unnecessary use of %h[xudi] and %hh[xudi]

[Dwaipayan Ray]

Modifiers %h and %hh should never be used.

Commit cbacb5ab0aa0 ("docs: printk-formats: Stop encouraging use
of unnecessary %h[xudi] and %hh[xudi]") specifies that:

"Standard integer promotion is already done and %hx and %hhx is useless
so do not encourage the use of %hh[xudi] or %h[xudi]."

"The "h" and "hh" things should never be used. The only reason for them
being used if you have an "int", but you want to print it out as a
"char" (and honestly, that is a really bad reason, you'd be better off
just using a proper cast to make the code more obvious)."

Add a new check to emit a warning on finding an unneeded use of %h or
%hh modifier.

Also add a fix option to the check.

Link: https://lore.kernel.org/lkml/4910042649a4f3ab22fac93191b8c1fa0a2e17c3.camel@xxxxxxxxxxx/

Suggested-by: Joe Perches <joe@xxxxxxxxxxx>
Suggested-by: Lukas Bulwahn <lukas.bulwahn@xxxxxxxxx>
Signed-off-by: Dwaipayan Ray <dwaipayanray1@xxxxxxxxx>
---
Changes in v3:
- Change warning message
- Fix regex match to include capture group
- Warn on every unnecesary use of %h on a line
- Add fix option when the format line matches current line

Changes in v2:
- Use $logFunctions instead of the manual list.
- Relocate the check to after logging continuations check.
- Remove perl_version_ok check

 scripts/checkpatch.pl | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/scripts/checkpatch.pl b/scripts/checkpatch.pl
index 7dc094445d83..dc25d32f0c5f 100755
--- a/scripts/checkpatch.pl
+++ b/scripts/checkpatch.pl
@@ -6027,6 +6027,22 @@ sub process {
 			     "Avoid logging continuation uses where feasible\n" . $herecurr);
 		}
 
+# check for unnecessary use of %h[xudi] and %hh[xudi] in logging functions
+		if (defined $stat &&
+		    $line =~ /\b$logFunctions\s*\(/) {
+			my $lc = $stat =~ tr@\n@@;
+			for (my $cur_ln = $linenr; $cur_ln <= $linenr + $lc; $cur_ln++) {
+				my $cur_rawline = raw_line($cur_ln, 0);
+				while ($cur_rawline =~ /^\+.*\"[^\"]*(%[\d\.\*\-]*h+[idux])/g) {
+					if (WARN("INTEGER_PROMOTION",
+						 "Using 'h' in $1 is unnecessary\n" . "$cur_rawline\n") &&
+					    $fix && ($cur_ln == $linenr)) {
+						$fixed[$fixlinenr] =~ s/(\"[^\"]*%[\d\.\*\-]*)h+([idux])/$1$2/;
+					}
+				}
+			}
+		}
+
 # check for mask then right shift without a parentheses
 		if ($perl_version_ok &&
 		    $line =~ /$LvalOrFunc\s*\&\s*($LvalOrFunc)\s*>>/ &&
-- 
2.27.0