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ARM v8 PMU Cycle counter


When I am using the cycle counter in AArch64, I am not getting cycles properly. I have enabled read of pmccntr_el0 in user space using a small kernel module. I have sample code like:

asm volatile("isb;mrs %0, pmccntr_el0" : "=r"(prev));


  asm volatile("isb;mrs %0, pmccntr_el0" : "=r"(curr));

    delta = curr-prev;

I expected delta to be in the range of 1400000000 as a57 in our design runs at 1400MHz

But I am getting around 32100000 which means the cycle counter frequency is ~3.21MHz

The value of Control register is pmcr=41013001 indicating divider is off.

With Generic timer counter registers, I am getting the values as expected. The below code gives

asm volatile ("isb; mrs %0, cntvct_el0" : "=r" (ts));

  sleep (2);

  asm volatile ("isb; mrs %0, cntvct_el0" : "=r" (te));

  asm volatile ("isb; mrs %0, cntfrq_el0" : "=r" (freq));

  printf ("Aarch64 %20ld cycles\n", (unsigned long long)(te - ts));

  printf (" Frequency = %u\n",freq);

I get count of 512021629 cycles for 2 sec as expected for 256MHz frequency which I got from cntfrq_el0.

Is there something basic I am missing for PMCCNTR_EL0?

thanks and regards,