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This provides an interface for arch code to find out how many nanoseconds are going to be added on to xtime by the next call to do_timer. The value returned is a fixed-point number in 52.12 format in nanoseconds. The reason for this format is that it gives the full precision that the timekeeping code is using internally. The motivation for this is to fix a problem that has arisen on 32-bit powerpc in that the value returned by do_gettimeofday drifts apart from xtime if NTP is being used. PowerPC is now using a lockless do_gettimeofday based on reading the timebase register and performing some simple arithmetic. (This method of getting the time is also exported to userspace via the VDSO.) However, the factor and offset it uses were calculated based on the nominal tick length and weren't being adjusted when NTP varied the tick length. Note that 64-bit powerpc has had the lockless do_gettimeofday for a long time now. It also had an extremely hairy routine that got called from the 32-bit compat routine for adjtimex, which adjusted the factor and offset according to what it thought the timekeeping code was going to do. Not only was this only called if a 32-bit task did adjtimex (i.e. not if a 64-bit task did adjtimex), it was also duplicating computations from kernel/timer.c and it wasn't clear that it was (still) correct. The simple solution is to ask the timekeeping code how long the current jiffy will be on each timer interrupt, after calling do_timer. If this jiffy will be a different length from the last one, we then need to compute new values for the factor and offset used in the lockless do_gettimeofday. In this way we can keep xtime and do_gettimeofday in sync, even when NTP is varying the tick length. Note that when adjtimex varies the tick length, it almost always introduces the variation from the next tick on. The only case I could see where adjtimex would vary the length of the current tick is when an old-style adjtime adjustment is being cancelled. (It's not clear to me why the adjustment has to be cancelled immediately rather than from the next tick on.) Thus I don't see any real need for a hook in adjtimex; the rare case of an old-style adjustment being cancelled can be fixed up at the next tick. Signed-off-by: Paul Mackerras <paulus@samba.org> Acked-by: john stultz <johnstul@us.ibm.com> Signed-off-by: Linus Torvalds <torvalds@osdl.org> |
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| .. | ||
| irq | ||
| power | ||
| .gitignore | ||
| acct.c | ||
| audit.c | ||
| auditsc.c | ||
| capability.c | ||
| compat.c | ||
| configs.c | ||
| cpu.c | ||
| cpuset.c | ||
| dma.c | ||
| exec_domain.c | ||
| exit.c | ||
| extable.c | ||
| fork.c | ||
| futex.c | ||
| hrtimer.c | ||
| intermodule.c | ||
| itimer.c | ||
| kallsyms.c | ||
| Kconfig.hz | ||
| Kconfig.preempt | ||
| kexec.c | ||
| kfifo.c | ||
| kmod.c | ||
| kprobes.c | ||
| ksysfs.c | ||
| kthread.c | ||
| Makefile | ||
| module.c | ||
| mutex-debug.c | ||
| mutex-debug.h | ||
| mutex.c | ||
| mutex.h | ||
| panic.c | ||
| params.c | ||
| pid.c | ||
| posix-cpu-timers.c | ||
| posix-timers.c | ||
| printk.c | ||
| profile.c | ||
| ptrace.c | ||
| rcupdate.c | ||
| rcutorture.c | ||
| resource.c | ||
| sched.c | ||
| seccomp.c | ||
| signal.c | ||
| softirq.c | ||
| softlockup.c | ||
| spinlock.c | ||
| stop_machine.c | ||
| sys_ni.c | ||
| sys.c | ||
| sysctl.c | ||
| time.c | ||
| timer.c | ||
| uid16.c | ||
| user.c | ||
| wait.c | ||
| workqueue.c | ||