Question

from here:

The value of HZ varies across kernel versions and
hardware platforms. On i386
the situation is as follows: on kernels up to and including 2.4.x, HZ was 100,
giving a jiffy value of 0.01 seconds; starting with 2.6.0, HZ was raised to
1000, giving a jiffy of 0.001 seconds. Since kernel 2.6.13, the HZ value is a
kernel configuration parameter and can be 100, 250 (the default) or 1000,
yielding a jiffies value of, respectively, 0.01, 0.004, or 0.001 seconds.
Since kernel 2.6.20, a further frequency is available: 300, a number that
divides evenly for the common video frame rates (PAL, 25 HZ; NTSC, 30 HZ).

So how can I transform 5usec to jiffies?

extern unsigned long usecs_to_jiffies(const unsigned int u);

it seems to be useless since jiffies resolution is not high enough to measure useconds.

Answer 1

When in doubt, read the code!

Here it is (a version of it can be found here):

unsigned long usecs_to_jiffies(const unsigned int u)
{
    if (u > jiffies_to_usecs(MAX_JIFFY_OFFSET))
        return MAX_JIFFY_OFFSET;
#if HZ <= USEC_PER_SEC && !(USEC_PER_SEC % HZ)
    return (u + (USEC_PER_SEC / HZ) - 1) / (USEC_PER_SEC / HZ);
#elif HZ > USEC_PER_SEC && !(HZ % USEC_PER_SEC)
    return u * (HZ / USEC_PER_SEC);
#else
    return (USEC_TO_HZ_MUL32 * u + USEC_TO_HZ_ADJ32)
        >> USEC_TO_HZ_SHR32;
#endif
}

So, it does some stuff to check if there is a shortcut, and if nothing else works, figures it out with some 64-bit math.

But 5usec will be one jiffies, no matter which bit of code it runs.