Re: [PATCH] [v2] coresight: etm4x: avoid build failure with unrolled loops

[Suzuki K Poulose]

Hi Arnd

On 29/04/2021 15:57, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@xxxxxxxx>
>
> clang-12 fails to build the etm4x driver with -fsanitize=array-bounds,
> where it decides to unroll certain loops in a way that result in a
> C variable getting put into an inline assembly
>
> <instantiation>:1:7: error: expected constant expression in '.inst' directive
> .inst (0xd5200000|((((2) << 19) | ((1) << 16) | (((((((((((0x160 + (i * 4))))) >> 2))) >> 7) & 0x7)) << 12) | ((((((((((0x160 + (i * 4))))) >> 2))) & 0xf)) << 8) | (((((((((((0x160 + (i * 4))))) >> 2))) >> 4) & 0x7)) << 5)))|(.L__reg_num_x8))
>        ^
> drivers/hwtracing/coresight/coresight-etm4x-core.c:702:4: note: while in macro instantiation
>                          etm4x_relaxed_read32(csa, TRCCNTVRn(i));
>                          ^
> drivers/hwtracing/coresight/coresight-etm4x.h:403:4: note: expanded from macro 'etm4x_relaxed_read32'
>                   read_etm4x_sysreg_offset((offset), false)))
>                   ^
> drivers/hwtracing/coresight/coresight-etm4x.h:383:12: note: expanded from macro 'read_etm4x_sysreg_offset'
>                          __val = read_etm4x_sysreg_const_offset((offset));       \
>                                  ^
> drivers/hwtracing/coresight/coresight-etm4x.h:149:2: note: expanded from macro 'read_etm4x_sysreg_const_offset'
>          READ_ETM4x_REG(ETM4x_OFFSET_TO_REG(offset))
>          ^
> drivers/hwtracing/coresight/coresight-etm4x.h:144:2: note: expanded from macro 'READ_ETM4x_REG'
>          read_sysreg_s(ETM4x_REG_NUM_TO_SYSREG((reg)))
>          ^
> arch/arm64/include/asm/sysreg.h:1108:15: note: expanded from macro 'read_sysreg_s'
>          asm volatile(__mrs_s("%0", r) : "=r" (__val));                  \
>                       ^
> arch/arm64/include/asm/sysreg.h:1074:2: note: expanded from macro '__mrs_s'
> "       mrs_s " v ", " __stringify(r) "\n"                      \
>   ^
>
> This only happened in a few loops in which the array bounds sanitizer
> added a special case for an array overflow that clang determined to be
> possible, but any compiler is free to unroll any of the loops in the
> same way that breaks the sysreg macros.
>
> Introduce helper functions that perform a sysreg access with a
> non-constant register number and use them in each call that passes
> a loop counter.

You don't need to add this special helper. We have the exact 
infrastructure already. So these could simply be replaced with:

csdev_access_xxx(csa, ...)

see :

include/linux/coresight.h

Cheers
Suzuki