Date:	Mon, 17 Aug 2015 14:40:36 -0700
From:	Drew Richardson <drew.richardson@....com>
To:	Will Deacon <will.deacon@....com>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Russell King <linux@....linux.org.uk>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Wade Cherry <Wade.Cherry@....com>
Subject: Re: [PATCH] arm: perf: Add event descriptions

On Wed, Aug 26, 2015 at 05:51:49PM +0100, Will Deacon wrote:
> On Mon, Aug 17, 2015 at 09:58:09PM +0100, Drew Richardson wrote:
> > On Tue, Aug 04, 2015 at 01:15:23AM +0100, Drew Richardson wrote:
> > > Add additional information about hardware events to make counters self
> > > describing. This makes the hardware PMUs easier to use as perf list
> > > contains the possible events instead of users having to refer to
> > > documentation like the ARM TRMs. This could also allow tools like
> > > oprofile to support PMUs without requiring an update.
> > > 
> > > Signed-off-by: Drew Richardson <drew.richardson@....com>
> > > ---
> > >  arch/arm/kernel/perf_event.c    |   1 +
> > >  arch/arm/kernel/perf_event_v7.c | 617 ++++++++++++++++++++++++++++++++++++++++
> > >  2 files changed, 618 insertions(+)
> > 
> > Any concerns with merging this?
> 
> We should revisit this after the merge window (since the ARM perf code is
> moving out to drivers/)

Not a problem

> but I do have reservations about putting all these
> strings into the kernel. Why can't we put this into something like libpfm
> instead?
> 
> Will
> 

My intent is to make it easier for people to use hardware counters on
ARM, particularly with new CPUs. Currently, the developer of the tool
needs to extract the event information from the ARM TRMs, add it to
their tool and make a new release. This work is done by multiple
tools, for example for the Cortex-A15:

http://sourceforge.net/p/perfmon2/libpfm4/ci/master/tree/lib/events/arm_cortex_a15_events.h
http://sourceforge.net/p/oprofile/oprofile/ci/master/tree/events/arm/armv7-ca15/events
https://github.com/ARM-software/gator/blob/master/daemon/events-Cortex-A15.xml

There are likely more tools, these are the ones I could easily
find. Note that the new ARM Cortex-A72 isn't supported by most of
them. And the perf command line tool doesn't have any of this.

Once the tool supports the new CPUs events the user needs to make sure
they're running a recent version of the tool that contains these
events for their new ARM CPU. Technically the user could add the event
information themselves and upstream the change, but not all users will
do this.

So my suggestion to solve the problem is that the kernel can have the
list of events as proposed in the patch. This is already done for some
PMUs like arm-ccn and arm-cci. If the tool doesn't support the
particular CPU, they can fall back to the event list provided by the
kernel. There will be some information missing like user-friendly
descriptions of the events, but it's hopefully enough that users can
do what they need to do and update their tool at a later time.

I'd be glad to wrap this all in a config option so users can disable
it to reduce bloat. Perhaps there is some other central location
outside the kernel that could store this information?

Thanks,

Drew
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