Why do we have coarse variants of ktime_get in Linux kernel?

Question

I was going through the following link:

https://www.kernel.org/doc/html/latest/core-api/timekeeping.html

We have both coarse (ktime_get_coarse()) and non-coarse (ktime_get()) versions.

Which one should we use and when?

It says coarse versions are faster than non-coarse versions.

Do the non-coarse versions read the hardware counter or not?

Answer 1

Q:

Which one should we use and when?

Seems it's well described on your link.
A:

These are quicker than the non-coarse versions, but less accurate.

So if you need to be quick and you don't care about accuracy - use coarse (there will be inaccuracy right up to 10ms with 100HZ and just up to 1ms with 1000HZ). If accuracy is a priority, then use non-coarse API.

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Q:

Whether the non-coarse versions reads the hardware counter or not?

A:
ktime_get() (unlike the ktime_get_coarse()) eventually invokes timekeeping_get_delta() which reads clocksource with tk_clock_read():

static inline u64 tk_clock_read(const struct tk_read_base *tkr)
{
    struct clocksource *clock = READ_ONCE(tkr->clock);

    return clock->read(clock);
}

Inside it invokes an appropriate callback for hardware timer to read.
You can search for such callbacks in your sources grepping through the clocksources: grep -rnI 'read =' drivers/clocksource.
E.g. here is setting of tpu_clocksource_read().