Re: [PATCH] arm64: perf_event: Fix time_offset for arch timer

[Will Deacon]

On Thu, Apr 30, 2020 at 06:04:36PM +0200, Peter Zijlstra wrote:
> On Thu, Apr 30, 2020 at 04:29:23PM +0100, Marc Zyngier wrote:
> 
> > I wonder if we could/should make __sched_clock_offset available even when
> > CONFIG_HAVE_UNSTABLE_SCHED_CLOCK isn't defined. It feels like it would
> > help with this particular can or worm...
> 
> Errrgh. __sched_clock_offset is only needed on x86 because we transition
> from one clock device to another on boot. It really shouldn't exist on
> anything sane.

I think we still transition from jiffies on arm64, because we don't register
with sched_clock until the timer driver probes. Marc, is that right?

> Let me try and understand your particular problem better.

I think the long and short of it is that userspace needs a way to convert
the raw counter cycles into a ns value that can be compared against values
coming out of sched_clock. To do this accurately, I think it needs the
cycles value at the point when sched_clock was initialised.

Will