tests: kernel: timer: Fix failing tests for custom k_busy_wait()

[adamkondraciuk]

This commit provides an additional threshold value for comparison when a custom k_busy_wait() implementation is chosen.

[andyross]

Would prefer to make excuses based on specific platform clock behavior. A busy_wait being "custom" doesn't necessarily mean it's inaccurate. See the comment above about nRF, where the skew is an understood thing that can't be reasonably worked around (well, without changing the CPU clock to be a PLL based off the same crystal the timer driver uses vs. the internal oscillator, which AIUI has power impact that Nordic doesn't want to accept on the default config).

[adamkondraciuk]

Would prefer to make excuses based on specific platform clock behavior. A busy_wait being "custom" doesn't necessarily mean it's inaccurate. See the comment above about nRF, where the skew is an understood thing that can't be reasonably worked around (well, without changing the CPU clock to be a PLL based off the same crystal the timer driver uses vs. the internal oscillator, which AIUI has power impact that Nordic doesn't want to accept on the default config).

Okay, how about that?

[andyross]

Thanks

[andyross]

That's frustrating. Can you file a bug on this, we should probably look at splitting this into two tests then. timer_api is one of the core/most-important/best-validation-smokes tests and we really don't want to be disabling it for technicalities.

[andyross]

Also, and I swear I'm not pointing fingers here, failures in timer_api are often very subtle things that look like "the test is wrong" or "it's just random/unfixable". It's happened again and again that tiny edge case failures on obscure platforms or configurations turn out to be evidence of bugs in the driver or timer subsystem. We really want this test to be on for everyone.

[adamkondraciuk]

We discovered that it is enough to reduce assertions level like here, however for the timer_behavior test it won't work. Is it acceptable to skip it for this target?