linux-kernel - Re: [RFC 2/2] perf/core: change errno for sampling event not supported in hardware

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Date:	Tue, 10 May 2016 23:33:13 -0400 (EDT)
From:	Vince Weaver <vincent.weaver@...ne.edu>
To:	Vineet Gupta <vgupta@...opsys.com>
cc:	Vince Weaver <vincent.weaver@...ne.edu>,
	linux-perf-users@...r.kernel.org, peterz@...radead.org,
	acme@...hat.com, linux-snps-arc@...ts.infradead.org,
	linux-kernel@...r.kernel.org, mingo@...nel.org
Subject: Re: [RFC 2/2] perf/core: change errno for sampling event not supported
 in hardware

On Mon, 9 May 2016, Vineet Gupta wrote:

> On Monday 09 May 2016 07:24 PM, Vince Weaver wrote:
> > On Mon, 9 May 2016, Vineet Gupta wrote:
> > 
> >> This allows userspace to identify this case specifically from the
> >> catch all error msg it prints currently.
> >>
> >> This is an ABI change
> > 
> > An ABI change which will probably break things.
> 
> 
> Right thats what I feared. But hold on, I don't think we need to change the ABI to
> achieve what we want. Gosh why did I even take that path.
> 
> Currently the errno switch case in perf_evsel__open_strerror() in doesn't handle
> ENOTSUPP. So how about we add that - augmented with the same sample_period !0
> check to barf for lack of sampling support.
> 
> Do you see anything wrong with that ?

no, but it would be nice if one of the actual maintainers would chime in 
with an opinion.

In any case if ENOTSUPP is being returned to userspace I should update the 
perf_event manpage to reflect that.

Vince