linux-kernel - Re: [RFC/PATCH] perf: Expand definition of sysfs format attribute

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Date:	Mon, 8 Apr 2013 21:57:50 +1000
From:	Michael Ellerman <michael@...erman.id.au>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	acme@...radead.org, jolsa@...hat.com, linux-kernel@...r.kernel.org,
	linux-doc@...r.kernel.org, rob@...dley.net
Subject: Re: [RFC/PATCH] perf: Expand definition of sysfs format attribute

On Mon, Apr 08, 2013 at 11:05:15AM +0200, Peter Zijlstra wrote:
> On Mon, 2013-03-04 at 15:21 +1100, Michael Ellerman wrote:
> > Make it explicit that the format attributes may define overlapping bit
> > ranges. Unfortunately this was left unspecified originally, and all the
> > examples show non-overlapping ranges. I don't believe this is an ABI
> > change, as we are defining something that was previously undefined, but
> > others may disagree.
> > 
> > The POWER8 PMU would like to define overlapping ranges, as bit ranges in
> > the event code have different meanings for certain events. It will also
> > allow us to define an overarching "event" field, that encompasses all
> > others.
> > 
> > As far as I can see perf is comfortable with this change, however I am
> > not sure if there are any other users of the interface.
> > 
> > Signed-off-by: Michael Ellerman <michael@...erman.id.au>
> > ---
> >  Documentation/ABI/testing/sysfs-bus-event_source-devices-format |    6 ++++++
> >  1 file changed, 6 insertions(+)
> > 
> > diff --git a/Documentation/ABI/testing/sysfs-bus-event_source-devices-format b/Documentation/ABI/testing/sysfs-bus-event_source-devices-format
> > index 079afc7..77f47ff 100644
> > --- a/Documentation/ABI/testing/sysfs-bus-event_source-devices-format
> > +++ b/Documentation/ABI/testing/sysfs-bus-event_source-devices-format
> > @@ -9,6 +9,12 @@ Description:
> >  		we want to export, so that userspace can deal with sane
> >  		name/value pairs.
> >  
> > +		Userspace must be prepared for the possibility that attributes
> > +		define overlapping bit ranges. For example:
> > +			attr1 = 'config:0-23'
> > +			attr2 = 'config:0-7'
> > +			attr3 = 'config:12-35'
> > +
> >  		Example: 'config1:1,6-10,44'
> >  		Defines contents of attribute that occupies bits 1,6-10,44 of
> >  		perf_event_attr::config1.
> 
> ISTR discussing this with Jiri at some point.. I think we ended up with
> being fine with overlapping ranges but having perf issue a warning (not
> an error) when attributes of a single event have overlap.

OK great.

> I'm not sure the latter was ever implemented in the userspace side.

Yeah not that I can see with a quick look. It's fairly advanced usage
IMHO so a warning would be nice, but not essential.

cheers
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