Subject: Re: [PATCH V4] POWER: perf_event: Skip updating kernel counters if register value shrinks

From: Eric B Munson <[email-masked]>
Date: Wed, 27 Apr 2011 09:20:54 -0400

On Wed, 27 Apr 2011, David Laight wrote:

>  
> > prev and val are both 64 bit variables holding 32 bit numbers, we do
> not
> > accumulate in either, they are both replaced by values directly from
> the
> > registers.
> > So prev > val will not always be true.
> 
> The code seems to be:
>     prev = local64_read(&event->hw.prev_count);
>     val = read_pmc(event->hw.idx);
>     delta = check_and_compute_delta(prev, val);
>     local64_add(delta, &event->count);
> Which looks very much like 'prev' being a 64bit counter generated
> from the 32bit pmc register.
> 

Which implies that it will only ever be 32 bits wide, just stored in 64.