tf: aarch64 fix for cases where CNTFRQ_EL0 returns bogus value #2929
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As noted in this commit in the linux kernel: torvalds/linux@c6f97ad ...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. I ran across this issue when running an aarch64 vm running on aarch64 hardware - I'm not sure if the issue was with the actual hardware or just the VMWare vm. In this case, CNTFRQ_EL0 was returning a negative value. Not sure if all "bad" values will be as obvious, but this change is at least an improvement.
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Filed as internal issue #USD-9239 |
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Description of Change(s)
As noted in this commit in the linux kernel:
torvalds/linux@c6f97ad
...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.
I ran across this issue when running an aarch64 vm running on aarch64 hardware - I'm not sure if the issue was with the actual hardware or just the VMWare vm. In this case, CNTFRQ_EL0 was returning a negative value. Not sure if all "bad" values will be as obvious, but this change is at least an improvement.
Fixes Issue(s)