modpost GPL error using ktime_* - what to use instead?

Question

I'm building a non-GPL kernel module (non-negotiable) and in order to get timing information everywhere I look here and elsewhere tells me to use the new ktime_* API, but when I build I get this error:

FATAL: modpost: GPL-incompatible module xxx.ko uses GPL-only symbol 'ktime_get'

All the older APIs have been removed from the kernel, so how can I get something more accurate than jiffies?

@iot_builder Our legal department says so. I won't go into any more detail. — Ken Y-N
– Ken Y-N, Commented Oct 8 at 4:02

Ken Y-N · Accepted Answer · 2025-10-09 05:35:10Z

1

I'm on ARM, so I got an ARM-only solution.

This page shows how to interface to the timer, and this page has sample code that I could use (it's in a GPL library, so I'm OK to borrow for internal evaluation only!) - for the record, this is the code:

unsigned long notrace timer_read_counter(void)
{
    unsigned long cntpct;

    isb();
    asm volatile("mrs %0, cntpct_el0" : "=r" (cntpct));

    return cntpct;
}

Thre's also the companion:

unsigned long notrace get_tbclk(void)
{
    unsigned long cntfrq;
    asm volatile("mrs %0, cntfrq_el0" : "=r" (cntfrq));
    return cntfrq;
}

On my hardware this returns 19.2MHz, or about 52 nanoseconds per tick, which is good enough for me.

answered Oct 9 at 5:35

Ken Y-N

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1 Comment

Craig Estey Oct 9 at 18:39

You need an isb inst. Every example I have seen has one including the one you linked to.

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