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tf: aarch64 fix for cases where CNTFRQ_EL0 returns bogus value #2929

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merged 1 commit into from
Mar 1, 2024
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tf: aarch64 fix for cases where CNTFRQ_EL0 returns bogus value #2929

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3 changes: 3 additions & 0 deletions pxr/base/arch/testenv/testTiming.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -32,6 +32,9 @@ PXR_NAMESPACE_USING_DIRECTIVE

int main()
{
ARCH_AXIOM(ArchGetNanosecondsPerTick() > 0.0);
// If you're not even doing 1 tick a second, it's probably a bogus value
ARCH_AXIOM(ArchGetNanosecondsPerTick() < 1e9);
// Verify conversions for many tick counts.
for (size_t ticks = 0ul; ticks != 1ul << 24u; ++ticks) {
ARCH_AXIOM( (uint64_t) ArchTicksToNanoseconds(ticks) ==
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20 changes: 17 additions & 3 deletions pxr/base/arch/timing.cpp
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Expand Up @@ -115,8 +115,23 @@ Arch_ComputeNanosecondsPerTick()
#if defined(ARCH_CPU_ARM)
uint64_t counter_hz;
__asm __volatile("mrs %0, CNTFRQ_EL0" : "=&r" (counter_hz));
Arch_NanosecondsPerTick = double(1e9) / double(counter_hz);
#else

// As noted in this commit in the linux kernel:
//
// https://github.com/torvalds/linux/commit/c6f97add0f2ac83b98b06dbdda58fa47638ae7b0
//
// ...the value of CNTFRQ_EL0 is sometimes unreliable. The linux kernel
// instead reads the tick rate from the device tree, and if that fails,
// only then falls back on CNTFRQ_EL0.
//
// Since we already have measurement-based code, and reading from the device
// tree seemed tricky, we instead check if CNTFRQ_EL0 seems "sane"
// (ie, > 1Hz), and if not, fall back on the measurement code used in all
// other linux flavors.
if (counter_hz > 1) {
return double(1e9) / double(counter_hz);
}
#endif

// Measure how long it takes to call ::now().
uint64_t nowDuration =
Expand All @@ -135,7 +150,6 @@ Arch_ComputeNanosecondsPerTick()
// as the overhead to call now() one time.
return clockNanoSecs /
double(ticks - ArchGetIntervalTimerTickOverhead() - nowDuration);
#endif
}
#elif defined(ARCH_OS_WINDOWS)

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