Re: [PATCH v1 1/1] arm64: Early boot time stamps

[Andrew Murray]

On Tue, Nov 20, 2018 at 09:40:10AM -0500, Pavel Tatashin wrote:
> > > +static __init void sched_clock_early_init(void)
> > > +{
> > > +     u64 freq = arch_timer_get_cntfrq();
> > > +     u64 (*read_time)(void) = arch_counter_get_cntvct;
> >
> > We already have arch_timer_read_counter which is exposed from
> > arm_arch_timer.h.
> 
> OK
> 
> >
> > > +
> > > +     /* Early clock is available only on platforms with known freqs */
> >
> > This comment is misleading. It should read something like:
> >
> >         /*
> >          * The arm64 boot protocol mandates that CNTFRQ_EL0 reflects
> >          * the timer frequency. To avoid breakage on misconfigured
> >          * systems, do not register the early sched_clock if the
> >          * programmed value if zero. Other random values will just
> >          * result in random output.
> >          */
> >
> 
> OK
> 
> > > +     if (!freq)
> > > +             return;
> > > +
> > > +     sched_clock_register(read_time, BITS_PER_LONG, freq);
> >
> > This doesn't seem right. The counter has an architected minimum of 56
> > bits, and you can't assume that it is going to be more than that.
> 
> Yeah, I saw 56 is used in arm_arch_timer.c, but I could not find where
> this minimum is defined in aarch64 specs. I will change it to 56.

See section G5.1.2 of the ARM ARM for details.

Thanks,

Andrew Murray

> 
> I will send v2 soon.
> 
> Thank you,
> Pasha