Re: [PATCH 6/7] clocksource: Add Pistachio clocksource-only driver

[Thomas Gleixner]

On Thu, 21 May 2015, Ezequiel Garcia wrote:
> On 05/21/2015 07:00 PM, Thomas Gleixner wrote:
> > On Thu, 21 May 2015, Ezequiel Garcia wrote:
> >> +static cycle_t clocksource_read_cycles(struct clocksource *cs)
> >> +{
> >> +	u32 counter, overflw;
> >> +	unsigned long flags;
> >> +
> >> +	raw_spin_lock_irqsave(&lock, flags);
> > 
> > Hmm. Is that lock really necessary to read that counter? The
> > clocksource is global. And if its actually used for timekeeping, the
> > lock can get heavy contended.
> > 
> 
> Yup, it is really (and sadly) necessary. The kernel hangs up completely
> without it when the counter is accesed by more than one CPU.
> 
> Apparently, those two timer registers overflow and counter must be read
> atomically.

Welcome to the wonderful world of useless timer hardware.

	tglx
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