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Date: Tue, 11 May 2010 11:31:51 +0200
From: Ingo Molnar <mingo@elte.hu>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Lin Ming <ming.m.lin@intel.com>,
       Corey Ashford <cjashfor@linux.vnet.ibm.com>,
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       Will Deacon <will.deacon@arm.com>, Maynard Johnson <mpjohn@us.ibm.com>,
       Carl Love <carll@us.ibm.com>
Subject: Re: [RFC][PATCH 3/9] perf: export registerred pmus via sysfs
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* Peter Zijlstra <peterz@infradead.org> wrote:

> On Tue, 2010-05-11 at 11:18 +0200, Ingo Molnar wrote:
> > * Peter Zijlstra <peterz@infradead.org> wrote:
> > 
> > > The new PMUs will use a dynamic range that starts at PERF_TYPE_MAX.
> > 
> > I dont think we should use a dynamic range of event sources - it's a 
> > completely useless indirection that has no meaning to humans.
> > 
> > As far as machine interfaces go a much cleaner approach would be to allow an 
> > open fd to a sysfs file to be passed to sys_perf_event_open() - this would 
> > identify the event source. This needs a small extension of the ABI but we 
> > could thus get rid of the 'type' enumeration altogether and express _all_ 
> > event sources via fds to sysfs files.
> 
> Whatever, that's almost identical. [...]

It's not identical: as we dont expose our mapping structure externally and 
dont have to have some dynamic type ID allocation layer/mechanism. Also, using 
fds is an elegant, Linuxish way of expressing some object's identity and 
passing it along.

( It also removes the possibility to intentionally or accidentally have type 
  IDs that are not reachable via the sysfs and vica verse. )

> [...] What we can do is reserve perf_event_attr::type with bit 31 set for 
> fd's and use the fd->file lookup instead of the type->pmu lookup.

Yeah, that sounds good.

	Ingo