coresight: etm4x: avoid build failure with unrolled loops

coresight: etm4x: avoid build failure with unrolled loops

When the following configs are enabled:
* CORESIGHT
* CORESIGHT_SOURCE_ETM4X
* UBSAN
* UBSAN_TRAP

Clang fails assemble the kernel with the error:
<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"                      \
 ^

Consider the definitions of TRCSSCSRn and TRCCNTVRn:
drivers/hwtracing/coresight/coresight-etm4x.h:56
 #define TRCCNTVRn(n)      (0x160 + (n * 4))
drivers/hwtracing/coresight/coresight-etm4x.h:81
 #define TRCSSCSRn(n)      (0x2A0 + (n * 4))

Where the macro parameter is expanded to i; a loop induction variable
from etm4_disable_hw.

When any compiler can determine that loops may be unrolled, then the
__builtin_constant_p check in read_etm4x_sysreg_offset() defined in
drivers/hwtracing/coresight/coresight-etm4x.h may evaluate to true. This
can lead to the expression `(0x160 + (i * 4))` being passed to
read_etm4x_sysreg_const_offset. Via the trace above, this is passed
through READ_ETM4x_REG, read_sysreg_s, and finally to __mrs_s where it
is string-ified and used directly in inline asm.

Regardless of which compiler or compiler options determine whether a
loop can or can't be unrolled, which determines whether
__builtin_constant_p evaluates to true when passed an expression using a
loop induction variable, it is NEVER safe to allow the preprocessor to
construct inline asm like:
  asm volatile (".inst (0x160 + (i * 4))" : "=r"(__val));
                                 ^ expected constant expression

Instead of read_etm4x_sysreg_offset() using __builtin_constant_p(), use
__is_constexpr from include/linux/const.h instead to ensure only
expressions that are valid integer constant expressions get passed
through to read_sysreg_s().

This is not a bug in clang; it's a potentially unsafe use of the macro
arguments in read_etm4x_sysreg_offset dependent on __builtin_constant_p.

Link: https://github.com/ClangBuiltLinux/linux/issues/1310
Reported-by: Arnd Bergmann <arnd@kernel.org>
Reported-by: Tao Zhang <quic_taozha@quicinc.com>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Link: https://lore.kernel.org/r/20220708231520.3958391-1-ndesaulniers@google.com