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

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Date:   Tue, 20 Nov 2018 09:40:10 -0500
From:   Pavel Tatashin <pasha.tatashin@...een.com>
To:     marc.zyngier@....com
Cc:     catalin.marinas@....com, will.deacon@....com,
        Andrew Morton <akpm@...ux-foundation.org>,
        rppt@...ux.vnet.ibm.com, Michal Hocko <mhocko@...e.com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        andrew.murray@....com, james.morse@....com, sboyd@...nel.org,
        linux-arm-kernel@...ts.infradead.org,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v1 1/1] arm64: Early boot time stamps

> > +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.

I will send v2 soon.

Thank you,
Pasha