On 09/10/2013 12:47 PM, Stephan Mueller wrote: > Am Dienstag, 10. September 2013, 12:38:56 schrieb John Stultz: > > Hi John, > >> On Tue, Sep 10, 2013 at 4:31 AM, Stephan Mueller <smuel...@chronox.de> > wrote: >>> Hi, >>> >>> /dev/random uses the get_cycles() function to obtain entropy in >>> addition to jiffies and the event value of hardware events. >>> >>> Typically the high-resolution timer of get_cycles delivers the >>> majority of entropy, because the event value is quite deterministic >>> and jiffies are very coarse. >> [snip] >> >>> The following patch uses the clocksource clock for a time value in >>> case get_cycles returns 0. As clocksource may not be available >>> during boot time, a flag is introduced which allows random.c to >>> check the availability of clocksource. >> [snip] >> >>> diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c >>> index 48b9fff..75b1613 100644 >>> --- a/kernel/time/timekeeping.c >>> +++ b/kernel/time/timekeeping.c >>> @@ -35,6 +35,7 @@ static struct timekeeper timekeeper; >>> >>> static DEFINE_RAW_SPINLOCK(timekeeper_lock); >>> static seqcount_t timekeeper_seq; >>> static struct timekeeper shadow_timekeeper; >>> >>> +static bool timekeeper_enabled = 0; >>> >>> /* flag for if timekeeping is suspended */ >>> int __read_mostly timekeeping_suspended; >>> >>> @@ -833,8 +834,15 @@ void __init timekeeping_init(void) >>> >>> write_seqcount_end(&timekeeper_seq); >>> raw_spin_unlock_irqrestore(&timekeeper_lock, flags); >>> >>> + timekeeper_enabled = 1; >>> >>> } >> So the end of timekeeping_init() may not be what you want here. This >> only means we've started up the timekeping core with only the default >> clocksource (with only few exceptions, this is almost always jiffies). >> Then as clocksource drivers are initialized, they are registered and >> the timekeeping core will switch over to the best available >> clocksource. Also, to avoid the churn at boot of switching to every >> clocksource registered, we queue them up and wait until fs_init time >> to switch to whatever is the best available then. >> >> So its likely with this patch that the systems all still end up using >> jiffies for their clocksource at least until fs_init time. > Thank you for the explanation. Is there any trigger that is fired at > fs_init time that one can read? Check out clocksource_done_booting(), as that's probably where you'd want to add something similar. Now this doesn't ensure non-jiffies clocksources are installed, just that we're far enough into boot that we assume most of the system's clocksources have been registered.
timekeeping_valid_for_hres() might be a better check for what you're looking for, since that means some free-running clocksource has been installed. thanks -john -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
