linux-kernel - Re: arch/alpha/include/asm/xchg.h:234:32: sparse: sparse: cast truncates bits from constant value (5f0ecafe becomes fe)

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Message-ID: <20200727091905.GA17835@xiangao.remote.csb>
Date:   Mon, 27 Jul 2020 17:19:05 +0800
From:   Gao Xiang <hsiangkao@...hat.com>
To:     kernel test robot <lkp@...el.com>
Cc:     Gao Xiang <hsiangkao@....com>, kbuild-all@...ts.01.org,
        linux-kernel@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: arch/alpha/include/asm/xchg.h:234:32: sparse: sparse: cast
 truncates bits from constant value (5f0ecafe becomes fe)

Hi,

On Sat, Jul 25, 2020 at 03:15:39PM +0800, kernel test robot wrote:
> tree:   https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master
> head:   23ee3e4e5bd27bdbc0f1785eef7209ce872794c7
> commit: 47e4937a4a7ca4184fd282791dfee76c6799966a erofs: move erofs out of staging
> date:   11 months ago
> config: alpha-randconfig-s031-20200725 (attached as .config)
> compiler: alpha-linux-gcc (GCC) 9.3.0
> reproduce:
>         wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
>         chmod +x ~/bin/make.cross
>         # apt-get install sparse
>         # sparse version: v0.6.2-93-g4c6cbe55-dirty
>         git checkout 47e4937a4a7ca4184fd282791dfee76c6799966a
>         # save the attached .config to linux build tree
>         COMPILER_INSTALL_PATH=$HOME/0day COMPILER=gcc-9.3.0 make.cross C=1 CF='-fdiagnostic-prefix -D__CHECK_ENDIAN__' ARCH=alpha 
> 
> If you fix the issue, kindly add following tag as appropriate
> Reported-by: kernel test robot <lkp@...el.com>
> 
> 
> sparse warnings: (new ones prefixed by >>)
> 
>    fs/erofs/zdata.c: note: in included file (through arch/alpha/include/asm/cmpxchg.h, arch/alpha/include/asm/atomic.h, include/linux/atomic.h, ...):
> >> arch/alpha/include/asm/xchg.h:234:32: sparse: sparse: cast truncates bits from constant value (5f0ecafe becomes fe)
> >> arch/alpha/include/asm/xchg.h:236:32: sparse: sparse: cast truncates bits from constant value (5f0ecafe becomes cafe)
> >> arch/alpha/include/asm/xchg.h:234:32: sparse: sparse: cast truncates bits from constant value (5f0ecafe becomes fe)
> >> arch/alpha/include/asm/xchg.h:234:32: sparse: sparse: cast truncates bits from constant value (5f0edead becomes ad)
> >> arch/alpha/include/asm/xchg.h:236:32: sparse: sparse: cast truncates bits from constant value (5f0ecafe becomes cafe)
> >> arch/alpha/include/asm/xchg.h:236:32: sparse: sparse: cast truncates bits from constant value (5f0edead becomes dead)

I'm not sure what's wrong here? What should I proceed with it
(it seems relating with Alpha architecture? since I didn't notice the sparse warning in x86 and arm)

Z_EROFS_PCLUSTER_TAIL is a pointer with magic value to indicate
the end of opening decompressing chain, whose function is likewise
the function of POISON_POINTER_DELTA and CONFIG_ILLEGAL_POINTER_VALUE.

Thanks,
Gao Xiang