clock_gettime function doesn't scale to real time.. and changing CNTFRQ_EL0 doesn't make any change..(arm64)

Chan Kim ckim at
Fri Nov 4 05:04:15 EDT 2022

Hello linux experts and newbies,

I have ported linux on our arm64 fpga board. Both 5.10.0 and 5.15.xx works
ok with minimal config.
I have run a simple application and timed the processing time using
clock_gettime function.
It felt like it took almost 2.3 seconds but the program say it took only
0.36 seconds.
Here is how I did  it in the application.

	Int main() {

	struct timespec start, stop;
	    float exec_time_sec, exec_time_nsec;
	//check start time
	    if (clock_gettime(CLOCK_REALTIME, &start) == -1 )  {
	        perror ("clock_gettime");

	Do something... (calculate fibonacci value for 1 ~ 30)

	//check end time

	    if (clock_gettime(CLOCK_REALTIME, &stop) == -1 )  {
	        perror ("clock_gettime");

	    //Normalize to mili second
	    exec_time_sec = (float)(stop.tv_sec - start.tv_sec);
	    exec_time_nsec = (float)((double)(stop.tv_nsec -

	    printf("Execution time : %f sec\n", exec_time_sec +
	return 0;

I used u-boot program for loading linux kernel and the u-boot program sets
the CNTFRQ_EL0 register with 5000000. 
(which is 5MHz, I heard the system clock runs at 5MHz in the board).
The description of the register in armv8 arch manual says :

This register is provided so that software can discover the frequency of the
system counter. It must
be programmed with this value as part of system initialization. The value of
the register is not
interpreted by hardware.

I tried setting the CNTFRQ_EL0 with 20Mhz, expecting the execution to be
displayed 4 times shorter but it is the same!
I couldn't find how linux uses clock_gettime.
How can I solve this problem?
Any advice will be deeply appreciated.

Thank you!

Chan Kim

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