linux-kernel - Re: [PATCH v2 1/1] arm64: Early boot time stamps

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Date:   Wed, 21 Nov 2018 17:58:41 +0000
From:   Pavel Tatashin <pasha.tatashin@...een.com>
To:     Will Deacon <will.deacon@....com>
Cc:     catalin.marinas@....com, akpm@...ux-foundation.org,
        rppt@...ux.vnet.ibm.com, mhocko@...e.com,
        ard.biesheuvel@...aro.org, andrew.murray@....com,
        james.morse@....com, marc.zyngier@....com, sboyd@...nel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/1] arm64: Early boot time stamps

On 18-11-21 17:47:07, Will Deacon wrote:
> > +	/*
> > +	 * 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.
> > +	 */
> > +	if (!freq)
> > +		return;
> > +
> > +	arch_timer_read_counter = arch_counter_get_cntvct;
> 
> Why do you need to assign this here?
> 
> > +	sched_clock_register(arch_timer_read_counter, ARCH_TIMER_NBITS, freq);
> 
> arch_timer_read_counter can be reassigned once the arm_arch_timer driver
> has probed; what stops this from being unused as the sched_clock after that
> has happened? I worry that toggling the function pointer could lead to
> sched_clock() going backwards.

No reason, I will revert it back to use a local variable. I agree, time
can go backward for a period of time while we switch to permanent clock later,
if that clock is different. 

> 
> > +}
> > +
> >  void __init setup_arch(char **cmdline_p)
> >  {
> > +	sched_clock_early_init();
> > +
> >  	init_mm.start_code = (unsigned long) _text;
> >  	init_mm.end_code   = (unsigned long) _etext;
> >  	init_mm.end_data   = (unsigned long) _edata;
> 
> The patch from this point onwards just looks like a refactoring to me which
> should be independent of adding early printk timestamps. Also, it doesn't
> update the vdso logic, which hardwires a 56-bit mask for the counter values.

OK, I will split the patch, and will also address the hard coded value
here:

https://soleen.com/source/xref/linux/arch/arm64/kernel/vdso/gettimeofday.S?r=c80ed088#73

Are there more that you know of?

Thank you,
Pasha