On 31.03.2010 18:58, Maciej Pijanka wrote:
On Mon, 29 Mar 2010, Carl-Daniel Hailfinger wrote:
The current delay loop calculation is still from revision 1 of flashrom, and since then it had a logic bug which caused all delays to be twice as long as intended. Fix the delay duration.
Protect against delay loop overflows.
Detect a non-working delay loop.
Change the delay loop itself to ensure clever compiler optimizers won't eliminate it (as happens with clang/llvm in the current code). Some people suggested machine-specific asm, but the empty asm statement with the loop counter as register/memory input has the benefit of being perfectly cross-platform and working in gcc and clang.
If time goes backwards (catastrophical NTP time difference, manual time change), timing measurements were shot because the new-old time subtraction yielded negative numbers which weren't handled correctly because the variable is unsigned. Simply return 1 microsecond timing in that case.
If time goes forward too fast, pick the biggest possible timing measurement with a guaranteed overflow avoidance for all timing calculations.
Maybe in case of leap seconds or time changing simply detect, and either fail and restart whole timing calculation, from scratch (possibly noting attempt number). If after some fixed number of retries you can't get reliable result. simply fail?
The code does exactly what you suggested (unless I'm mistaken). The changelog needs improvement to actually tell people about this.
Check four times if the calculated timing is at most 10% too fast. This addresses OS scheduler interactions, e.g. being scheduled out during measurement which inflates measurements.
this makes sense but if we want that good results, maybe odd number of repeats, leave off minimum and maximum ones, then calculate over remaining (and minimum sane number of remaining atempts is imo 5, thus seven is needed to make sufficient number of iterations).
Hmmm... could be done, but I'd rather tell the user that timing is garbage than to add an elaborate algorithm to convert garbage to gold. ;-)
Anyway, imo code is good and readable and gives more acurate results on my machines.
Acked-by: Maciej Pijanka maciej.pijanka@gmail.com
Thanks, committed in r990.
Regards, Carl-Daniel