[Linux-kernel-mentees] [PATCH RFC] 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.

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

Suggested-by: Lukas Bulwahn <lukas.bulwahn at gmail.com>
Signed-off-by: Dwaipayan Ray <dwaipayanray1 at gmail.com>
---
 scripts/checkpatch.pl | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/scripts/checkpatch.pl b/scripts/checkpatch.pl
index 7dc094445d83..47c1017a9973 100755
--- a/scripts/checkpatch.pl
+++ b/scripts/checkpatch.pl
@@ -6570,6 +6570,19 @@ sub process {
 			}
 		}
 
+# check for unnecessary use of %h[xudi] and %hh[xudi]
+		if ($perl_version_ok &&
+		    defined $stat &&
+		    $line =~ /(?:[sd]?print[kfd]|(?:pr_|drm_)(?:info|debug|warn|error))/i) {
+			my $lc = $stat =~ tr@\n@@;
+			$lc = $lc + $linenr;
+			my $stat_real = get_stat_real($linenr, $lc);
+			if ($stat_real =~ /\"[^\"]*%[\d\.\*\-]*h+[idux].*\"/i) {
+				WARN("UNNECESSARY_MODIFIER",
+				     "Unnecessary use of length modifiers %h[xudi] or %hh[xudi]\n" . "$here\n$stat_real\n");
+			}
+		}
+
 # check for naked sscanf
 		if ($perl_version_ok &&
 		    defined $stat &&
-- 
2.27.0