linux-kernel - Re: [PATCH 1/2] perf: return ENOENT instead of ENOTSUPP

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Message-ID: <1434018301.1495.73.camel@twins>
Date:	Thu, 11 Jun 2015 12:25:01 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Hendrik Brueckner <brueckner@...ux.vnet.ibm.com>
Cc:	Ingo Molnar <mingo@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Vince Weaver <vincent.weaver@...ne.edu>,
	Paul Mackerras <paulus@...ba.org>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	linux-kernel@...r.kernel.org, linux-s390@...r.kernel.org
Subject: Re: [PATCH 1/2] perf: return ENOENT instead of ENOTSUPP

On Thu, 2015-06-11 at 11:59 +0200, Hendrik Brueckner wrote:
> The ENOTSUPP (which actually should be EOPNOTSUPP for user space) does not
> trigger a fallback event selection, for example, by perf record.
> If hardware support for the cycles perf event is available, but the hardware
> does not provide interrupts, returning ENOTSUPP causes perf to end.  Returning
> ENOENT causes the perf tool to fallback to a software-based cycle PMU that
> supports interrupts.
> 
> The commit 53b25335dd ("perf: Disable sampled events if no PMU interrupt")
> introduced that incompatible change.

That's 3.16

>  		if (event->pmu->capabilities & PERF_PMU_CAP_NO_INTERRUPT) {
> -			err = -ENOTSUPP;
> +			err = -ENOENT;
>  			goto err_alloc;
>  		}
>  	}

And now you would be changing an API that's been around for at least 4
releases.

Also, I really think -ENOENT is the wrong return here, you're asking for
things that's not supported, not for something that's not there.
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