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Subject: Re: [PATCH v2 1/6] perf: Add interface to add general events to
 sysfs
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: Lin Ming <ming.m.lin@intel.com>
Cc: Ingo Molnar <mingo@elte.hu>, Andi Kleen <andi@firstfloor.org>,
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Date: Mon, 25 Jul 2011 09:57:21 +0200
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On Tue, 2011-07-19 at 15:52 +0800, Lin Ming wrote:

> > I'd rather not have a function pointer here, and all the kobj thingies
> > can be hooking into the existing struct device, right?
> 
> I forgot to mention one important reason why I added a function pointer.
> 
> The events can only be added to sysfs after PMU sysfs is initialized in
> perf_event_sysfs_init ->
>     pmu_dev_alloc

Right.

> > @@ -5571,6 +5571,8 @@ static int pmu_dev_alloc(struct pmu *pmu)
> >        if (ret)
> >                goto free_dev;
> >
> > +       if (pmu->add_events)
> > +               pmu->add_events();
> 
> So we need a pmu callback which is called in pmu_dev_alloc to add events
> to sysfs.
> 
> You suggested a new interface,
> int perf_pmu_add_event(struct pmu *pmu, const char *name, u64 config)
> 
> But where should it be called?
> I guess you mean to call it in pmu init function, for example,
> uncore_pmu_init.
> 
> But pmu init function maybe called before perf_event_sysfs_init, which
> means the pmu sysfs has not been initialized yet.

Right, so there is no reason to call perf_pmu_register() really early,
except for the normal pmu and the software pmu (since they're used by
the watchdog muck).

The uncore for example can use late_initcall() just fine and use both
perf_pmu_register() and the proposed perf_pmu_add_event() from the same 
init call.

Only for the primary pmu and software thingies do we need to add an
extra init call.