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SubjectRe: [PATCH] optimized ktime_get[_ts] for GENERIC_TIME=y
On Mon, 6 Jul 2009, Martin Schwidefsky wrote:
> +ktime_t ktime_get(void)
> +{
> + cycle_t cycle_now, cycle_delta;
> + struct timespec time;
> + unsigned long seq;
> + s64 nsecs;
> +
> + do {
> + seq = read_seqbegin(&xtime_lock);
> + time.tv_sec = xtime.tv_sec + wall_to_monotonic.tv_sec;
> + time.tv_nsec = xtime.tv_nsec + wall_to_monotonic.tv_nsec;

That's actually a violation of the timespec semantics. tv_nsec can end
up greater than (10^9 - 1). Please use separate sec and nsec variables.

secs = xtime.tv_sec + wall_to_monotonic.tv_sec;
nsecs = xtime.tv_nsec + wall_to_monotonic.tv_nsec;

> + /* read clocksource: */
> + cycle_now = clocksource_read(clock);
> +
> + /* calculate the delta since the last update_wall_time: */
> + cycle_delta = (cycle_now - clock->cycle_last) & clock->mask;
> +
> + /* convert to nanoseconds: */
> + nsecs = cyc2ns(clock, cycle_delta);

So that needs to be changed to:

nsecs += cyc2ns(clock, cycle_delta);

> +
> + } while (read_seqretry(&xtime_lock, seq));
> + nsecs += time.tv_sec * 1000000000ULL + time.tv_nsec;
> + return (ktime_t) { .tv64 = nsecs };

This will break all 32bit architectures which do not have
CONFIG_KTIME_SCALAR set.

With the above changes applied:

return ktime_add_ns(ktime_set(secs, 0), nsecs);

will do the trick. Might need some comments though :)

Thanks,

tglx


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