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@xxxxxxxxx> > Suggested-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> > Closes: https://lore.kernel.org/oe-kbuild-all/202401070147.gqwVulOn-lkp@xxxxxxxxx/ > Signed-off-by: Dmitry Torokhov <dmitry.torokhov@xxxxxxxxx> > --- > 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 - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
