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SubjectRe: [PATCH 10/23] clocksource: remove update_callback

* Daniel Walker <dwalker@mvista.com> wrote:

> Uses the block notifier to replace the functionality of
> update_callback(). update_callback() was a special case specifically
> for the tsc, but including it in the clocksource structure duplicated
> it needlessly for other clocks.

Firstly, it think it should be mentioned that Thomas' queue already does
this, in clocksource-remove-the-update-callback.patch (hence he should
have been Cc:-ed). Your queue 'drops' Thomas' patch then redoes it here
without mentioning that this is another version of what is in Thomas's
queue. So we get this situation:

clocksource-remove-the-update-callback.patch
drop-clocksource-remove-the-update-callback.patch
clocksource_remove_update_callback.patch

that all flip-flops the same thing.

Secondly, your patch seems to do other changes as well:

> @@ -179,6 +172,7 @@ int recalibrate_cpu_khz(void)
> if (cpu_has_tsc) {
> cpu_khz = calculate_cpu_khz();
> tsc_khz = cpu_khz;
> + mark_tsc_unstable();
> cpu_data[0].loops_per_jiffy =
> cpufreq_scale(cpu_data[0].loops_per_jiffy,
> cpu_khz_old, cpu_khz);

this adds a new event to a place that didnt have it before. (If this is
fixing up an initialization artifact then that needs a comment at
least.)

plus:

> struct clocksource *clock = &clocksource_jiffies;
> +atomic_t clock_recalc_interval = ATOMIC_INIT(0);

is not mentioned in the changelog. It's also needlessly global.
Furthermore, it seems to be a rather unclean method of passing
information from clocksource_callback() into change_clocksource():

> @@ -176,8 +177,9 @@ static int change_clocksource(void)
> printk(KERN_INFO "Time: %s clocksource has been installed.\n",
> clock->name);
> return 1;
> - } else if (clock->update_callback) {
> - return clock->update_callback();
> + } else if (unlikely(atomic_read(&clock_recalc_interval))) {
> + atomic_set(&clock_recalc_interval, 0);
> + return 1;

that's quite bad: you lost an information passing facility by going to a
notifier, and you try to work it around via a global atomic variable.
Which also looks quite racy as well.

The clean solution is i think what Thomas did: he calls straight into
clocksource_change_rating(). And look at Thomas' patch:

3 files changed, 17 insertions(+), 37 deletions(-)

versus yours:

5 files changed, 45 insertions(+), 42 deletions(-)

Thomas' looks definitely simpler to me.

ingo
-
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