Recently, I saw a patch[1] on the ext4 mailing list regarding
the correction of a macro definition error. Jan mentioned
that "The bug in the macro is a really nasty trap...".
Because existing compilers are unable to detect
unused parameters in macro definitions. This inspired me
to write a script to check for unused parameters in
macro definitions and to run it.

Surprisingly, the script uncovered numerous issues across
various subsystems, including filesystems, drivers, and sound etc.

Some of these issues involved parameters that were accepted
but never used, for example:
	#define	XFS_DAENTER_DBS(mp,w)	\
	(XFS_DA_NODE_MAXDEPTH + (((w) == XFS_DATA_FORK) ? 2 : 0))
where mp was unused.

While others are actual bugs.
For example:
	#define HAL_SEQ_WCSS_UMAC_CE0_SRC_REG(x) \
		(ab->hw_params.regs->hal_seq_wcss_umac_ce0_src_reg)
	#define HAL_SEQ_WCSS_UMAC_CE0_DST_REG(x) \
		(ab->hw_params.regs->hal_seq_wcss_umac_ce0_dst_reg)
	#define HAL_SEQ_WCSS_UMAC_CE1_SRC_REG(x) \
		(ab->hw_params.regs->hal_seq_wcss_umac_ce1_src_reg)
	#define HAL_SEQ_WCSS_UMAC_CE1_DST_REG(x) \
		(ab->hw_params.regs->hal_seq_wcss_umac_ce1_dst_reg)
where x was entirely unused, and instead, a local variable ab was used.

I have submitted patches[2-5] to fix some of these issues,
but due to the large number, many still remain unaddressed.
I believe that the kernel and matainers would benefit from
this script to check for unused parameters in macro definitions.

It should be noted that it may cause some false positives
in conditional compilation scenarios, such as
	#ifdef DEBUG
	static int debug(arg) {};
	#else
	#define debug(arg)
	#endif
So the caller needs to manually verify whether it is a true
issue. But this should be fine, because Maintainers should only
need to review their own subsystems, which typically results
in only a few reports.

[1]: https://patchwork.ozlabs.org/project/linux-ext4/patch/1717652596-58760-1-git-send-email-carrionbent@linux.alibaba.com/
[2]: https://lore.kernel.org/linux-xfs/20240721112701.212342-1-sunjunchao2870@gmail.com/
[3]: https://lore.kernel.org/linux-bcachefs/20240721123943.246705-1-sunjunchao2870@gmail.com/
[4]: https://sourceforge.net/p/linux-f2fs/mailman/message/58797811/
[5]: https://sourceforge.net/p/linux-f2fs/mailman/message/58797812/

[sunjunchao2870@gmail.com: reduce false positives]
  Link: https://lkml.kernel.org/r/20240726031310.254742-1-sunjunchao2870@gmail.com
Link: https://lkml.kernel.org/r/20240723091154.52458-1-sunjunchao2870@gmail.comSigned-off-by: Julian Sun <sunjunchao2870@gmail.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Darrick J. Wong <djwong@kernel.org>
Cc: Jan Kara <jack@suse.cz>
Cc: Junchao Sun <sunjunchao2870@gmail.com>
Cc: Kalle Valo <kvalo@kernel.org>
Cc: Masahiro Yamada <masahiroy@kernel.org>
Cc: Miguel Ojeda <ojeda@kernel.org>
Cc: Nicolas Schier <n.schier@avm.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>