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

Chan Kim ckim at
Tue Nov 8 00:18:44 EST 2022

There is a typo : Setting cntfrq_el0 is done by " msr cntfrq_el0, x0 " in
the code.

>-----Original Message-----
>From: Chan Kim <ckim at>
>Sent: Tuesday, November 8, 2022 2:16 PM
>To: 'kernelnewbies at' <kernelnewbies at>
>Cc: 'Siddh Raman Pant' <code at>; 'Linus Probert'
><linus.probert at>
>Subject: RE: clock_gettime function doesn't scale to real time.. and
>changing CNTFRQ_EL0 doesn't make any change..(arm64)
>Hello all,
>I fixed this problem and now the time measurement and commands like sleep
>works just fine.
>Two points I fixed :
>- I had 'clock-frequency' property set with wrong frequency in my timer
>node in the device tree so I removed it.
>   The document says when the boot loader sets CNTFRQ register correctly,
>we don't have to provide 'clock-frequency' property value.
>- The correct frequency of the system counter (arm464) was 10MHz in our
>board. Previously I set CNTFRQ register with 5MHz but I fixed it to 10MHz.
>One more thing to note. The system counter has both system register view
>and memory mapped register view.
>Previously I said even if I set cntfrq_el0 register with some values (using
>system register, "msr cntfrq_el0, COUNTER_FREQUENCY") it did not change
>It was because I set the same register with old value (5MHz) using the
>memory mapped access later (like with "writel(COUNTER_FREQUENCY,
>Hope this helps someone later.
>Thank you!
>Chan Kim
>>-----Original Message-----
>>From: Chan Kim <ckim at>
>>Sent: Saturday, November 5, 2022 8:46 PM
>>To: 'Siddh Raman Pant' <code at>
>>Cc: 'Kernel Newbies' <kernelnewbies at>
>>Subject: RE: clock_gettime function doesn't scale to real time.. and
>>changing CNTFRQ_EL0 doesn't make any change..(arm64)
>>Hi,Siddh and Linus,
>>I tried using 'time' command to measure the time and my program output
>>is the same in commercial intel machine (ubuntu 20.04).
>>fib(041) = 165580141
>>fib(042) = 267914296
>>fibonacci finishing...
>>Execution time : 3.801840 sec
>>real	0m3.806s
>>user	0m3.802s
>>sys	0m0.005s
>>So it looks like the application program doesn't have problem.
>>In the program exec_time_nsec is actually in unit of second. Because
>>tv_nsec is in unit of nsec, I'm converting it to second by dividing it
>>with BILLION.
>>And yes, the CNTFRQ_EL0 is for software use. Hardware just runs with
>>clock and it know only the number of clocks and doesn't know 1000000
>>clock period physically represents what seconds. CNTFRQ_EL0 is letting
>>the software able to convert the number of clocks to physical time.
>>I'm at home now and can't do experiment with the board now. I'll try
>>following the pc_clock_gettime function as you showed to see how it works.
>>Thanks for the advices.
>>Chan Kim
>>>-----Original Message-----
>>>From: Siddh Raman Pant <code at>
>>>Sent: Saturday, November 5, 2022 4:51 AM
>>>To: Chan Kim <ckim at>
>>>Cc: Kernel Newbies <kernelnewbies at>
>>>Subject: Re: clock_gettime function doesn't scale to real time.. and
>>>changing CNTFRQ_EL0 doesn't make any change..(arm64)
>>>On Fri, 04 Nov 2022 14:34:15 +0530, Chan Kim wrote:
>>>> 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.
>>>Try a simple command line loop to see if the problem is with system or
>>>program: while $(sleep 1); do echo "Hi"; done;
>>>Also, while of no concern here, note that kernel also has a real-time
>>>config (CONFIG_PREEMPT_RT).
>>>> 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");
>>>> 	     exit(EXIT_FAILURE);
>>>> 	}
>>>>       Do something... (calculate fibonacci value for 1 ~ 30)
>>>>       //check end time
>>>> 	if (clock_gettime(CLOCK_REALTIME, &stop) == -1 ) {
>>>> 		perror ("clock_gettime");
>>>> 		exit(EXIT_FAILURE);
>>>> 	}
>>>> 	//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 +
>>>> exec_time_nsec);
>>>>       return 0;
>>>> }
>>>Adding to what Linus said in the other reply, CLOCK_REALTIME is not
>>>guaranteed to be monotonic. For calculation of intervals, you should
>>>Also, try just having Fibonacci calculation in main, and then use the
>>>`time` command to achieve what you want.
>>>> 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!
>>>Because as the description says, this register is not interpreted by
>>>the hardware. That means this won't affect hardware in any way, and
>>>thus won't increase frequency. That's why your execution time remains
>>>The value stored here is for use by software to know the clock
>>>frequency of the machine, so it can do its time related calculations
>>>What you did is equivalent of trying to hoodwink the system!
>>>> I couldn't find how linux uses clock_gettime.
>>>If you mean how clock_gettime function is defined, see it here:
>>>Otherwise: man clock_gettime
>>>> How can I solve this problem?
>>>> Any advice will be deeply appreciated.
>>>Try making the changes and let us know. This is new for me too!
>>>> Thank you!
>>>> Chan Kim
>>>Please use plain text email instead of HTML, that's the kernel mailing
>>>list etiquette. It is also superior once you get the hang of it! :)
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