Re: [RFC PATCH v4 03/21] x86/hpet: Calculate ticks-per-second in a separate function

Thomas Gleixner
2019-06-14T15:59:48Z

On Fri, 14 Jun 2019, Thomas Gleixner wrote:
> On Thu, 23 May 2019, Ricardo Neri wrote:
> > int hpet_alloc(struct hpet_data *hdp)
> > {
> > u64 cap, mcfg;
> > @@ -844,7 +867,6 @@ int hpet_alloc(struct hpet_data *hdp)
> > struct hpets *hpetp;
> > struct hpet __iomem *hpet;
> > static struct hpets *last;
> > - unsigned long period;
> > unsigned long long temp;
> > u32 remainder;
> >
> > @@ -894,12 +916,7 @@ int hpet_alloc(struct hpet_data *hdp)
> >
> > last = hpetp;
> >
> > - period = (cap & HPET_COUNTER_CLK_PERIOD_MASK) >>
> > - HPET_COUNTER_CLK_PERIOD_SHIFT; /* fs, 10^-15 */
> > - temp = 1000000000000000uLL; /* 10^15 femtoseconds per second */
> > - temp += period >> 1; /* round */
> > - do_div(temp, period);
> > - hpetp->hp_tick_freq = temp; /* ticks per second */
> > + hpetp->hp_tick_freq = hpet_get_ticks_per_sec(cap);
>
> Why are we actually computing this over and over?
>
> In hpet_enable() which is the first function invoked we have:
>
> /*
> * The period is a femto seconds value. Convert it to a
> * frequency.
> */
> freq = FSEC_PER_SEC;
> do_div(freq, hpet_period);
> hpet_freq = freq;
>
> So we already have ticks per second, aka frequency, right? So why do we
> need yet another function instead of using the value which is computed
> once? The frequency of the HPET channels has to be identical no matter
> what. If it's not HPET is broken beyond repair.

Aside of that this change breaks the IA64 support for /dev/hpet.

Thanks,

tglx