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

[Thomas Gleixner]

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