How can I read the system timer in AArch64 assembly?
I want to read a global timer, not a timer local to each processor.
What are you trying to achieve exactly ? With ARMv8 there are per-core generic timers, system (linux) relies on these timers to track time (cf kernel/time/timekeeping.c). At user-space level you can typically retrieve ‘time’ with the gettimeofday function (cf arch/arm64/kernel/vdso/gettimeofday.S). You can also directly read the value of the timer (CNTVCT_EL0 register).
I’m writing a bare-metal program.
I want to get elapsed time between a core executes an instruction, and other core executes another.
This can be achieved if I can read the global timer.
Per-core timers are not suitable for this usage because I guess they have some gap.
I guessed I could read the system-wide timer via some memory-mapped registers.
So I wanted to know to which address the system timer is mapped.
If I’m not wrong timer counter is a system-wide64-bit count, which counts up from zero, giving a consistent view of time across cores. Each core can then manage its own comparators, typically to generate per-core timer interrupt.
The CNTFRQ_EL0 register reports the frequency of the system timer.
The CNTPCT_EL0 register reports the current count value.
CNTKCTL_EL1 controls whether EL0 can access the system timer.
I suggest you to have a look at the programmer’s Guide for ARMv8-A.
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I have already read the Architecture Reference Manual for ARMv8-A, but I misunderstood that the value of CNTFRQ_EL0 and CNTPCT_EL0 is per-core.
Now I understand that they have system-wide values.
Thank you for your quick response!
And sorry for that my question is not relevant to HiKey960.
What do you mean, you’re not supposed to change system timer frequency.
