Re: [PATCH] x86/apic: Handle missing global clockevent gracefully

[Aubrey Li]

On Mon, Aug 12, 2019 at 8:25 PM Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
>
> On Mon, 12 Aug 2019, Li, Aubrey wrote:
> > On 2019/8/9 20:54, Thomas Gleixner wrote:
> > > +   local_irq_disable();
> > >     /*
> > >      * Setup the APIC counter to maximum. There is no way the lapic
> > >      * can underflow in the 100ms detection time frame
> > >      */
> > >     __setup_APIC_LVTT(0xffffffff, 0, 0);
> > >
> > > -   /* Let the interrupts run */
> > > -   local_irq_enable();
> > > +   /*
> > > +    * Methods to terminate the calibration loop:
> > > +    *  1) Global clockevent if available (jiffies)
> > > +    *  2) TSC if available and frequency is known
> > > +    */
> > > +   jif_start = READ_ONCE(jiffies);
> > > +
> > > +   if (tsc_khz) {
> > > +           tsc_start = rdtsc();
> > > +           tsc_perj = div_u64((u64)tsc_khz * 1000, HZ);
> > > +   }
> > > +
> > > +   while (lapic_cal_loops <= LAPIC_CAL_LOOPS) {
> >
> > Is this loop still meaningful, can we just invoke the handler twice
> > before and after the tick?
>
> And that solves what?
>

I meant, can we do this one time?
- lapic_cal_t1 = read APIC counter
- /* Wait for a tick to elapse */
- lapic_cal_t2 = read APIC counter

I'm not clear why we still need this loop, to use the
existing lapic_cal_handler()?

Thanks,
-Aubrey