scripts: checkpatch: check unused parameters for function-like macro

If function-like macros do not utilize a parameter, it might result in a
build warning.  In our coding style guidelines, we advocate for utilizing
static inline functions to replace such macros.  This patch verifies
compliance with the new rule.

For a macro such as the one below,

 #define test(a) do { } while (0)

The test result is as follows.

 WARNING: Argument 'a' is not used in function-like macro
 #21: FILE: mm/init-mm.c:20:
 +#define test(a) do { } while (0)

 total: 0 errors, 1 warnings, 8 lines checked

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Xining Xu <[email protected]>
Tested-by: Barry Song <[email protected]>
Signed-off-by: Barry Song <[email protected]>
Acked-by: Joe Perches <[email protected]>
Cc: Chris Zankel <[email protected]>
Cc: Huacai Chen <[email protected]>
Cc: Herbert Xu <[email protected]>
Cc: Guenter Roeck <[email protected]>
Cc: Stephen Rothwell <[email protected]>
Cc: Mark Brown <[email protected]>
Cc: Andy Whitcroft <[email protected]>
Cc: Dwaipayan Ray <[email protected]>
Cc: Jonathan Corbet <[email protected]>
Cc: Lukas Bulwahn <[email protected]>
Cc: Max Filippov <[email protected]>
Cc: Jeff Johnson <[email protected]>
Cc: Charlemagne Lasse <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: Xining Xu

Documentation/dev-tools/checkpatch.rst

@@ -906,6 +906,20 @@ Macros, Attributes and Symbols
 
     See: https://lore.kernel.org/lkml/1399671106.2912.21.camel@joe-AO725/
 
+  **MACRO_ARG_UNUSED**
+    If function-like macros do not utilize a parameter, it might result
+    in a build warning. We advocate for utilizing static inline functions
+    to replace such macros.
+    For example, for a macro such as the one below::
+
+      #define test(a) do { } while (0)
+
+    there would be a warning like below::
+
+      WARNING: Argument 'a' is not used in function-like macro.
+
+    See: https://www.kernel.org/doc/html/latest/process/coding-style.html#macros-enums-and-rtl
+
   **SINGLE_STATEMENT_DO_WHILE_MACRO**
     For the multi-statement macros, it is necessary to use the do-while
     loop to avoid unpredictable code paths. The do-while loop helps to

scripts/checkpatch.pl

@@ -6040,6 +6040,12 @@ sub process {
 					CHK("MACRO_ARG_PRECEDENCE",
 					    "Macro argument '$arg' may be better as '($arg)' to avoid precedence issues\n" . "$herectx");
 				}
+
+# check if this is an unused argument
+				if ($define_stmt !~ /\b$arg\b/) {
+					WARN("MACRO_ARG_UNUSED",
+					     "Argument '$arg' is not used in function-like macro\n" . "$herectx");
+				}
 			}
 
 # check for macros with flow control, but without ## concatenation