Messages in this thread | | | From | Arnd Bergmann <> | Date | Thu, 29 Apr 2021 19:50:46 +0200 | Subject | Re: [PATCH] [v2] coresight: etm4x: avoid build failure with unrolled loops |
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On Thu, Apr 29, 2021 at 7:37 PM Suzuki K Poulose <suzuki.poulose@arm.com> wrote: > On 29/04/2021 15:57, Arnd Bergmann wrote: > > From: Arnd Bergmann <arnd@arndb.de> > > > > 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
Ah, nice!
Do you mean replacing only the ones that use a nonconstant offset, or all of them? I guess changing all would avoid some really ugly magic macros, but the indirect function call and the switch() adds a few cycles of overhead every time and the code looks like it is micro-optimized for fast register access here.
Arnd
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