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

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Message-ID: <20181120145315.GA42987@e119886-lin.cambridge.arm.com>
Date:   Tue, 20 Nov 2018 14:53:16 +0000
From:   Andrew Murray <andrew.murray@....com>
To:     Pavel Tatashin <pasha.tatashin@...een.com>
Cc:     marc.zyngier@....com, 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>,
        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

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