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    SubjectRe: [RFC][PATCH] new timeofday core subsystem (v.A0)
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    On Thu, 2004-09-02 at 18:39, George Anzinger wrote:
    > john stultz wrote:
    > > +static cycle_t jiffies_read(void)
    > > +{
    > > + cycle_t ret = get_jiffies_64();
    > > + return ret;
    > > +}
    > > +
    > > +static cycle_t jiffies_delta(cycle_t now, cycle_t then)
    > > +{
    > > + /* simple subtraction, no need to mask */
    > > + return now - then;
    > > +}
    >
    > This should be inline...

    Unfortunately they cannot be, as they are used as function pointers
    through the struct timesource interface.

    > > +
    > > +static nsec_t jiffies_cyc2ns(cycle_t cyc, cycle_t* remainder)
    > > +{
    > > +
    > > + cyc *= NSEC_PER_SEC/HZ;
    >
    > Hm... This assumes that 1/HZ is what is needed here. Today this value is
    > 999898. Not exactly reachable by NSEC_PER_SEC/HZ. Or did I miss something,
    > like the relationship of jiffie to 1/HZ and to real time.

    You're right. You'd think with the recent discussions I would have
    already fixed that, but it slipped by. I'll fix it to use ACTHZ in the
    next release. Good catch.


    > > +int ntp_leapsecond(struct timespec now)
    > > +{
    > > + /*
    > > + * Leap second processing. If in leap-insert state at
    > > + * the end of the day, the system clock is set back one
    > > + * second; if in leap-delete state, the system clock is
    > > + * set ahead one second. The microtime() routine or
    > > + * external clock driver will insure that reported time
    > > + * is always monotonic. The ugly divides should be
    > > + * replaced.
    >
    > ?? Is this really to be hidden? I rather though this was just the same as a
    > user driven clock setting. In any case, it is a slew of the wall clock and
    > should be presented to abs timer code so that the abs timers can be corrected.
    > This means a call to "clock_was_set()" AND an update of the mono_to_wall value.

    Hmm. Looking at that comment, it seems a bit outdated. I'll see if I can
    update it to be more clear.

    And yes, clock_was_set() is on my list. I realized I dropped it, but I
    wanted to make sure I was using it properly before I just threw it in.


    > > +void do_gettimeofday(struct timeval *tv)
    > > +{
    > > + nsec_t wall, sys;
    > > + unsigned long seq;
    > > +
    > > + /* atomically read wall and sys time */
    > > + do {
    > > + seq = read_seqbegin(&system_time_lock);
    > > +
    > > + wall = wall_time_offset;
    > > + sys = __monotonic_clock();
    > > +
    > > + } while (read_seqretry(&system_time_lock, seq));
    > > +
    > > + /* add them and convert to timeval */
    > > + *tv = ns2timeval(wall+sys);
    > > +}
    > I am not sure you don't want to seperate the locking from the clock read. This
    > so one lock can be put around larger bits of code. For example, in
    > posix-times.c we need to get all three clocks under the same lock (that being
    > monotonic, wall_time_offset, and jiffies (and possibly as sub jiffie value)).

    Well, all the values in timeofday.c are protected by the
    system_time_lock, so I'm trying to minimize the locks. I do want to kill
    off xtime and thus the xtime lock, so jiffies will be by itself and will
    need a jiffies_lock of some sort. However part of this effort is to ween
    folks off of using jiffies as a time value, thus in the future it should
    only be used inside the timer subsystem as a interrupt counter.

    Thinking about it more, the NTP code could probably drop the ntp_lock
    and just use the system_time_lock, but I think I'll push that
    optimization off for a bit.

    > Something like:
    > void get_tod_parts(nsec_t *wall, nsec_t *mon)
    > {
    > *wall = wall_time_offset;
    > *mon = __monotonic_clock();
    > }
    >
    > This could then be used in a larger function with out the double locking.

    I understood your point above, but I'm not sure I see double locking.
    __monotonic_clock() is lock free for exactly this reason.


    > > +int do_settimeofday(struct timespec *tv)
    > > +{
    > > + /* convert timespec to ns */
    > > + nsec_t newtime = timespec2ns(tv);
    > > +
    > > + /* atomically adjust wall_time_offset to the desired value */
    > > + write_seqlock_irq(&system_time_lock);
    > > +
    > > + wall_time_offset = newtime - __monotonic_clock();
    > > +
    > > + /* clear NTP settings */
    > > + ntp_clear();
    > > +
    > > + write_sequnlock_irq(&system_time_lock);
    >
    > Also need a clock_was_set() call here.

    Yep!

    Thanks for the feedback, George!
    -john

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