k_uptime_get_32() does not behave as documented

[pabigot]

It was recently pointed out on slack that k_uptime_get_32() does not return The lower 32-bits of the elapsed time since the system booted, in milliseconds.

It returns instead the number of milliseconds corresponding to the low 32 bits of the tick counter.

u32_t z_impl_k_uptime_get_32(void)
{
        return __ticks_to_ms(z_tick_get_32());
}

The values are significantly different when you look at a 32-bit rollover of the tick clock:

At 10000 Hz ticks and 2^32 +/- 50:
t0 = 0x0000ffffffce = 4294967246 ticks => t0m = 429496724 ms
t1 = 0x000100000032 = 4294967346 ticks => t1m = 429496734 ms
t1-t0 = 100 ticks = 10 ms
and t1m-t0m = 10 ms
tt0 = 0x0000ffffffce = 4294967246 ticks => tt0m = 429496724 ms
tt1 = 0x000000000032 = 50 ticks => tt1m = 5 ms
tt1-tt0 = 100 ticks = 10 ms
but tt1m-tt1m = 3865470577 ms

This explains #17155 (comment) which had claimed that the current implementation followed an algorithm that I noted was incorrect. Apparently it does.

[pabigot]

I'm going to work up a patch that marks it deprecated, removes all uses from master, and documents the misbehavior.

[andyross]

Doesn't #17155 already fix this, though? The resolution to that discussion was that the 32 bit conversion results should be correct to the full precision of the output word, and that's what the new conversion utilities implement and test for.

[carlescufi]

Doesn't #17155 already fix this, though? The resolution to that discussion was that the 32 bit conversion results should be correct to the full precision of the output word, and that's what the new conversion utilities implement and test for.

@pabigot might want to weigh in, but I think the reason for this is the fact that we can get a fix in time for 2.0, whereas #17155 will only be merged in time for 2.1.

[andyross]

That's fair. Mostly I'm just whining about extra work when rebasing, I guess. I agree with the fix on the whole.

[pabigot]

My take-away from TSC is that nobody thought this was important enough to fix for 2.0 (otherwise TSC would have had to vote to approve it). So it's not going in.