Am Dienstag, 21. Juni 2016, 09:22:31 schrieb John Stultz:
On Tue, Jun 21, 2016 at 1:32 AM, Arnd Bergmann email@example.com wrote:
On Tuesday, June 21, 2016 8:20:10 AM CEST Stephan Mueller wrote:
Am Freitag, 17. Juni 2016, 17:59:41 schrieb Arnd Bergmann:
Compared to the previous __getnstimeofday(), the difference is
using "monotonic" timebase instead of "real", so the zero time
is when the system booted rather than Jan 1 1970
I haven't looked at the details of the calling code, but I'd worry for crypto uses, especially if its being used for entropy collection, using the monotonic clock instead of the realtime clock might be problematic.
Funnily it does not seem like that. All tests that I have conducted show that monotonic clocks behave equally as realtime clocks, because the uncertainty lies in the execution time of a set of instructions. All we need to do is to measure it with a timer that has a resolution that allows detecting these variations.
"raw" means we don't honor updates for the rate based on ntp,
which is probably better as the ntp state might be observable over the net (it probably doesn't matter, but it can't hurt)
So... this feels like a very vague explanation, and the lack of frequency correction here probably need a really good comment. Keeping multiple time domains is usually asking for trouble, but we added the MONOTONIC_RAW clock to address a few cases where people really wanted an abstract hardware counter, which was unaffected by frequency corrections. I'd really make sure its clear why this is what you want vs the standard system time domain so we don't run into problems understanding it later.
Perfect, that is what I would be interested in.
"fast" means that in very rare cases, the time might appear
to go backwards (it probably can't happen here because you are not called in an NMI).
"fast" really means "safe-for-nmi wrt to locking". The tradeoff being that when frequency adjustments occur, and if your code is delayed, you might see time go backwards by a small amount. This allows
My code would not see that as an issue.
tracing/sched code (or other code called from NMI) to not have to duplicate the timekeeping infrastructure.
I think without a much better explanation, using the "fast" method isn't really warranted here.
Thanks a lot. With that, I would think that the proposed ktime_get_raw_fast_ns is good for use, which is supported with testing on my system.