Re: ARM: cmpxchg: support 2-byte xchg() on pre-ARMv6 CPUs

[kernel test robot <lkp@intel.com>]

Hi Karl,

kernel test robot noticed the following build warnings:



url:    https://github.com/intel-lab-lkp/linux/commits/Karl-Mehltretter/ARM-cmpxchg-support-2-byte-xchg-on-pre-ARMv6-CPUs/20261003-113826
base:   
patch link:    https://lore.kernel.org/r/202610081201.nt1PgAFz-lkp%40intel.com
config: arm-randconfig-r2309-20261008 (https://download.01.org/0day-ci/archive/20261008/202610081201.nt1PgAFz-lkp@intel.com/config)
compiler: clang version 17.0.6 (https://github.com/llvm/llvm-project 6009708b4367171ccdbf4b5905cb6a803753fe18)
sparse: v0.6.5-rc1
reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20261008/202610081201.nt1PgAFz-lkp@intel.com/reproduce)

If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <lkp@intel.com>
| Closes: https://lore.kernel.org/oe-kbuild-all/202610081201.nt1PgAFz-lkp@intel.com/

sparse warnings: (new ones prefixed by >>)
   lib/atomic64_test.c: note: in included file (through arch/arm/include/asm/atomic.h, include/linux/atomic.h, include/asm-generic/bitops/lock.h, arch/arm/include/asm/bitops.h, ...):
>> arch/arm/include/asm/cmpxchg.h:114:51: sparse: sparse: cast truncates bits from constant value (deadbeef becomes beef)
>> arch/arm/include/asm/cmpxchg.h:114:51: sparse: sparse: cast truncates bits from constant value (deadbeef becomes beef)
>> arch/arm/include/asm/cmpxchg.h:114:51: sparse: sparse: cast truncates bits from constant value (deadbeef becomes beef)
>> arch/arm/include/asm/cmpxchg.h:114:51: sparse: sparse: cast truncates bits from constant value (deadbeef becomes beef)

vim +114 arch/arm/include/asm/cmpxchg.h

    39	
    40		prefetchw((const void *)ptr);
    41	
    42		switch (size) {
    43	#if __LINUX_ARM_ARCH__ >= 6
    44	#ifndef CONFIG_CPU_V6 /* MIN ARCH >= V6K */
    45		case 1:
    46			asm volatile("@	__xchg1\n"
    47			"1:	ldrexb	%0, [%3]\n"
    48			"	strexb	%1, %2, [%3]\n"
    49			"	teq	%1, #0\n"
    50			"	bne	1b"
    51				: "=&r" (ret), "=&r" (tmp)
    52				: "r" (x), "r" (ptr)
    53				: "memory", "cc");
    54			break;
    55		case 2:
    56			asm volatile("@	__xchg2\n"
    57			"1:	ldrexh	%0, [%3]\n"
    58			"	strexh	%1, %2, [%3]\n"
    59			"	teq	%1, #0\n"
    60			"	bne	1b"
    61				: "=&r" (ret), "=&r" (tmp)
    62				: "r" (x), "r" (ptr)
    63				: "memory", "cc");
    64			break;
    65	#endif
    66		case 4:
    67			asm volatile("@	__xchg4\n"
    68			"1:	ldrex	%0, [%3]\n"
    69			"	strex	%1, %2, [%3]\n"
    70			"	teq	%1, #0\n"
    71			"	bne	1b"
    72				: "=&r" (ret), "=&r" (tmp)
    73				: "r" (x), "r" (ptr)
    74				: "memory", "cc");
    75			break;
    76	#elif defined(swp_is_buggy)
    77	#ifdef CONFIG_SMP
    78	#error SMP is not supported on this platform
    79	#endif
    80		case 1:
    81			raw_local_irq_save(flags);
    82			ret = *(volatile unsigned char *)ptr;
    83			*(volatile unsigned char *)ptr = x;
    84			raw_local_irq_restore(flags);
    85			break;
    86	
    87		case 4:
    88			raw_local_irq_save(flags);
    89			ret = *(volatile unsigned long *)ptr;
    90			*(volatile unsigned long *)ptr = x;
    91			raw_local_irq_restore(flags);
    92			break;
    93	#else
    94		case 1:
    95			asm volatile("@	__xchg1\n"
    96			"	swpb	%0, %1, [%2]"
    97				: "=&r" (ret)
    98				: "r" (x), "r" (ptr)
    99				: "memory", "cc");
   100			break;
   101		case 4:
   102			asm volatile("@	__xchg4\n"
   103			"	swp	%0, %1, [%2]"
   104				: "=&r" (ret)
   105				: "r" (x), "r" (ptr)
   106				: "memory", "cc");
   107			break;
   108	#endif
   109	#if __LINUX_ARM_ARCH__ < 6
   110		case 2:
   111			/* swp has no halfword form. Pre-ARMv6 is never SMP. */
   112			raw_local_irq_save(flags);
   113			ret = *(volatile unsigned short *)ptr;
 > 114			*(volatile unsigned short *)ptr = x;
   115			raw_local_irq_restore(flags);
   116			break;
   117	#endif
   118		default:
   119			/* Cause a link-time error, the xchg() size is not supported */
   120			__bad_xchg(ptr, size), ret = 0;
   121			break;
   122		}
   123	
   124		return ret;
   125	}
   126	

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