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Date:   Tue, 5 Dec 2023 13:24:52 -0800
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Nick Desaulniers <ndesaulniers@google.com>
Cc:     tanzirh@google.com, Kees Cook <keescook@chromium.org>,
        Andy Shevchenko <andy@kernel.org>,
        linux-hardening@vger.kernel.org, linux-kernel@vger.kernel.org,
        Nick DeSaulniers <nnn@google.com>, llvm@lists.linux.dev
Subject: Re: [PATCH] lib/string: shrink lib/string.i via IWYU
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On Tue, 5 Dec 2023 13:14:16 -0800 Nick Desaulniers <ndesaulniers@google.com> wrote:

> >
> > The preferred way to import bit-fiddling stuff is to include
> > <linux/bits.h>.  Under the hood this may include asm/bitsperlong.h.  Or
> > it may not, depending on Kconfig settings (particularly architecture).
> >
> 
> Just triple checking my understanding; it looks like
> include/linux/bits.h unconditionally includes asm/bitsperlong.h (which
> is implemented per arch) most of which seem to include
> asm-generic/bitsperlong.h.
> 
> include/linux/bits.h also defines a few macros (BIT_MASK, BIT_WORD,
> BITS_PER_BYTE, GENMASK, etc).  If lib/string.c is not using any of
> those, why can't we go straight to #including asm/bitsperlong.h?  That
> should resolve to the arch specific impl which may include
> asm-generic/bitsperlong.h?

It's just a general rule.  If the higher-level include is present, use
that.  Because of the above, plus I guess things might change in the
future.

We've been getting better about irregular asm/include files.

But bits.h is a poor example.  A better case to study is spinlock.h. 
If this tool recommended including asm/spinlock.h then that won't work
on any architecture which doesn't implement SMP (there is no
arch/nios2/include/asm/spinlock.h).