linux-kernel - RE: [PATCH] asm-generic: make sparse happy with odd-sized put_unaligned

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Message-ID: <12d88da48f6947ab86a845e8d02319ff@AcuMS.aculab.com>
Date: Mon, 8 Jan 2024 11:03:22 +0000
From: David Laight <David.Laight@...LAB.COM>
To: 'Dmitry Torokhov' <dmitry.torokhov@...il.com>, Arnd Bergmann
	<arnd@...db.de>
CC: "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, Linus Torvalds
	<torvalds@...ux-foundation.org>
Subject: RE: [PATCH] asm-generic: make sparse happy with odd-sized
 put_unaligned_*()

From: Dmitry Torokhov
> Sent: 08 January 2024 06:17
> 
> __put_unaligned_be24() and friends use implicit casts to convert
> larger-sized data to bytes, which trips sparse truncation warnings when
> the argument is a constant:
> 
>   CC [M]  drivers/input/touchscreen/hynitron_cstxxx.o
>   CHECK   drivers/input/touchscreen/hynitron_cstxxx.c
> drivers/input/touchscreen/hynitron_cstxxx.c: note: in included file (through
> arch/x86/include/generated/asm/unaligned.h):
> ./include/asm-generic/unaligned.h:119:16: warning: cast truncates bits from constant value (aa01a0
> becomes a0)
> ./include/asm-generic/unaligned.h:120:20: warning: cast truncates bits from constant value (aa01
> becomes 1)
> ./include/asm-generic/unaligned.h:119:16: warning: cast truncates bits from constant value (ab00d0
> becomes d0)
> ./include/asm-generic/unaligned.h:120:20: warning: cast truncates bits from constant value (ab00
> becomes 0)
> 
> To avoid this let's mask off upper bits explicitly, the resulting code
> should be exactly the same, but it will keep sparse happy.

Maybe someone should fix sparse?
I have seen a compiler generate two explicit masks with 0xff
followed by a byte write for:
	*p = (char)(x & 0xff);
but I expect modern gcc is ok.

> Reported-by: kernel test robot <lkp@...el.com>
> Suggested-by: Linus Torvalds <torvalds@...ux-foundation.org>
> Closes: https://lore.kernel.org/oe-kbuild-all/202401070147.gqwVulOn-lkp@intel.com/
> Signed-off-by: Dmitry Torokhov <dmitry.torokhov@...il.com>
> ---
>  include/asm-generic/unaligned.h | 24 ++++++++++++------------
>  1 file changed, 12 insertions(+), 12 deletions(-)
> 
> diff --git a/include/asm-generic/unaligned.h b/include/asm-generic/unaligned.h
> index 699650f81970..a84c64e5f11e 100644
> --- a/include/asm-generic/unaligned.h
> +++ b/include/asm-generic/unaligned.h
> @@ -104,9 +104,9 @@ static inline u32 get_unaligned_le24(const void *p)
> 
>  static inline void __put_unaligned_be24(const u32 val, u8 *p)
>  {
> -	*p++ = val >> 16;
> -	*p++ = val >> 8;
> -	*p++ = val;
> +	*p++ = (val >> 16) & 0xff;
> +	*p++ = (val >> 8) & 0xff;
> +	*p++ = val & 0xff;
>  }

What happens if you implement the as (eg):
	*p = val >> 16;
	put_unaligned_be16(p + 1, val);
I think that should generate better code.
And it may stop sparse bleating.

	David

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