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

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Date: Mon, 8 Jan 2024 09:45:28 -0800
From: 'Dmitry Torokhov' <dmitry.torokhov@...il.com>
To: David Laight <David.Laight@...lab.com>
Cc: Arnd Bergmann <arnd@...db.de>,
	"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_*()

On Mon, Jan 08, 2024 at 11:03:22AM +0000, David Laight wrote:
> 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 proposed doing this in
https://lore.kernel.org/oe-kbuild-all/ZZnzd3s2L-ZwGOlz@google.com/ but
the idea was not welcome.

> 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.

This is rarely in a hot path (typically you do this with a "slow"
device), and while being faster it looks more complex. But if that's
what people prefer...

Thanks.

-- 
Dmitry