Re: [rtc-linux] Re: [PATCH 4/9 RESEND] RFC: timekeeping: introduceflag systime_was_set

[Alexander Holler]

Am 14.06.2013 19:41, schrieb John Stultz:
> On 06/14/2013 09:52 AM, Alexander Holler wrote:
> > In order to let an RTC set the time at boot without the problem that a
> > second RTC overwrites it, the flag systime_was_set is introduced.
> >
> > systime_was_set will be true, if a persistent clock sets the time at
> > boot,
> > or if do_settimeofday() is called (e.g. by the RTC subsystem or
> > userspace).
> >
> > Signed-off-by: Alexander Holler <holler@xxxxxxxxxxxxx>
> > ---
> >   include/linux/time.h      |  6 ++++++
> >   kernel/time/timekeeping.c | 10 +++++++++-
> >   2 files changed, 15 insertions(+), 1 deletion(-)
> >
> > diff --git a/include/linux/time.h b/include/linux/time.h
> > index d5d229b..888280f 100644
> > --- a/include/linux/time.h
> > +++ b/include/linux/time.h
> > @@ -129,6 +129,12 @@ extern int update_persistent_clock(struct
> > timespec now);
> >   void timekeeping_init(void);
> >   extern int timekeeping_suspended;
> > +/*
> > + * Will be true if the system time was set at least once by
> > + * a persistent clock, RTC or userspace.
> > + */
> > +extern bool systime_was_set;
> > +
>
> Probably should make this static to timekeeping.c and create an accessor
> function so you don't have to export locking rules on this.
>
>
> >   unsigned long get_seconds(void);
> >   struct timespec current_kernel_time(void);
> >   struct timespec __current_kernel_time(void); /* does not take
> > xtime_lock */
> > diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
> > index baeeb5c..07d8531 100644
> > --- a/kernel/time/timekeeping.c
> > +++ b/kernel/time/timekeeping.c
> > @@ -37,6 +37,9 @@ int __read_mostly timekeeping_suspended;
> >   /* Flag for if there is a persistent clock on this platform */
> >   bool __read_mostly persistent_clock_exist = false;
> > +/* Flag for if the system time was set at least once */
> > +bool __read_mostly systime_was_set;
> > +
> Probably should also move this to be part of the timekeeper structure
> (since it will be protected by the timekeeper lock.

I wanted to avoid locks for this silly flag at all. It is only set once 
at boot (and resume) and set to 0 at suspend. And I don't see any 
possible race condition which could make a lock necessary. Therefor I've 
decided to not use a lock or atomic_* in order to skip any delay in 
setting the time.

Of course, I might be wrong and there might be a use case where multiple 
things do set the system time concurrently and nothing else did set 
system time before, but I found that extremly unlikely.


> >   static inline void tk_normalize_xtime(struct timekeeper *tk)
> >   {
> >       while (tk->xtime_nsec >= ((u64)NSEC_PER_SEC << tk->shift)) {
> > @@ -498,6 +501,9 @@ int do_settimeofday(const struct timespec *tv)
> >       raw_spin_lock_irqsave(&timekeeper_lock, flags);
> >       write_seqcount_begin(&timekeeper_seq);
> > +    systime_was_set = true;
> > +
> > +
> >       timekeeping_forward_now(tk);
> >       xt = tk_xtime(tk);
>
> Might also want to add the flag to inject_offset as well, since that
> could be used to set the time.

I wasn't sure about that because I had only a quick look at 
inject_offset() and had the impression it's only able to inject a 
relative small offset (so not usable at boot). And, as written sometimes 
before, I haven't had a deep look at suspend/resume, which might be the 
only place where it is really used to set the clock when
systime_was_set is false.

Regards,

Alexander
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