linux-kernel - Re: [PATCH] clocksource: arm_arch_timer: print timer value at init time

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Date:   Mon, 6 Feb 2017 17:41:07 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     Daniel Lezcano <daniel.lezcano@...aro.org>
Cc:     Olof Johansson <olof@...om.net>, marc.zyngier@....com,
        daniel.lezcano@...aro.or, tglx@...utronix.de,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] clocksource: arm_arch_timer: print timer value at init
 time

On Sat, Feb 04, 2017 at 09:41:18AM +0100, Daniel Lezcano wrote:
> On Mon, Dec 19, 2016 at 09:47:37AM -0800, Olof Johansson wrote:
> > This is useful to get an indication of how much time we spent in firmware.
> > 
> > It's not guaranteed that the timer started at 0 on reset, so it's just
> > an approximation, and might very well be invalid on some systems. But
> > it's still a useful metric to have access to.
> 
> Hi Olof,
> 
> [ ... ]
> 
> > --- a/drivers/clocksource/arm_arch_timer.c
> > +++ b/drivers/clocksource/arm_arch_timer.c
> > @@ -521,6 +521,8 @@ arch_timer_detect_rate(void __iomem *cntbase, struct device_node *np)
> >  
> >  static void arch_timer_banner(unsigned type)
> >  {
> > +	unsigned long cnt = arch_timer_read_counter();
> > +
> 
> arch_timer_banner() is called before arch_counter_register() where the
> arch_timer_read_counter() function pointer is set.
> 
> Perhaps the arch_timer_banner() and arch_counter_register() should be swapped in
> arch_timer_common_init().

That would make sense to me.

> >  	pr_info("Architected %s%s%s timer(s) running at %lu.%02luMHz (%s%s%s).\n",
> >  		     type & ARCH_CP15_TIMER ? "cp15" : "",
> >  		     type == (ARCH_CP15_TIMER | ARCH_MEM_TIMER) ?  " and " : "",
> > @@ -534,6 +536,8 @@ static void arch_timer_banner(unsigned type)
> >  		     type & ARCH_MEM_TIMER ?
> >  			arch_timer_mem_use_virtual ? "virt" : "phys" :
> >  			"");
> > +	pr_info("Initial timer value: 0x%lx: %ld.%02lds\n",
> > +		cnt, cnt/arch_timer_rate, (cnt/(arch_timer_rate/100)) % 100);

Our tiemrs should be precise enough to give us a few more digits here
(e.g. down to ns, like printk). Are there any helpers we can use to do
that?

It would also be nice to log which counter we're reading from.

Thanks,
Mark.