linux-kernel - Re: [PATCH v4 2/2] ThunderX2: Add Cavium ThunderX2 SoC UNCORE PMU driver

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Date:   Fri, 27 Apr 2018 10:46:29 -0500
From:   Kim Phillips <kim.phillips@....com>
To:     Will Deacon <will.deacon@....com>
Cc:     Mark Rutland <mark.rutland@....com>,
        Ganapatrao Kulkarni <ganapatrao.kulkarni@...ium.com>,
        <linux-doc@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>, <jnair@...iumnetworks.com>,
        <Robert.Richter@...ium.com>, <Vadim.Lomovtsev@...ium.com>,
        <Jan.Glauber@...ium.com>, <gklkml16@...il.com>
Subject: Re: [PATCH v4 2/2] ThunderX2: Add Cavium ThunderX2 SoC UNCORE PMU
 driver

On Fri, 27 Apr 2018 15:37:20 +0100
Will Deacon <will.deacon@....com> wrote:

> On Fri, Apr 27, 2018 at 08:15:25AM -0500, Kim Phillips wrote:
> > On Fri, 27 Apr 2018 10:30:27 +0100
> > Mark Rutland <mark.rutland@....com> wrote:
> > > On Thu, Apr 26, 2018 at 05:06:24PM -0500, Kim Phillips wrote:
> > > > On Wed, 25 Apr 2018 14:30:47 +0530
> > > > Ganapatrao Kulkarni <ganapatrao.kulkarni@...ium.com> wrote:
> > > > 
> > > > > +static int thunderx2_uncore_event_init(struct perf_event *event)
> > > 
> > > > This PMU driver can be made more user-friendly by not just silently
> > > > returning an error code such as -EINVAL, but by emitting a useful
> > > > message describing the specific error via dmesg.
> > > 
> > > As has previously been discussed on several occasions, patches which log
> > > to dmesg in a pmu::event_init() path at any level above pr_debug() are
> > > not acceptable -- dmesg is not intended as a mechanism to inform users
> > > of driver-specific constraints.
> > 
> > I disagree - drivers do it all the time, using dev_err(), dev_warn(), etc.
> > 
> > > I would appreciate if in future you could qualify your suggestion with
> > > the requirement that pr_debug() is used.
> > 
> > It shouldn't - the driver isn't being debugged, it's in regular use.
> 
> For anything under drivers/perf/, I'd prefer not to have these prints
> and instead see efforts to improve error reporting via the perf system
> call interface.

We'd all prefer that, and for all PMU drivers, why should ones under
drivers/perf be treated differently?

As you are already aware, I've personally tried to fix this problem -
that has existed since before the introduction of the perf tool (I
consider it a syscall-independent enhanced error interface), multiple
times, and failed.

So until someone comes up with a solution that works for everyone
up to and including Linus Torvalds (who hasn't put up a problem
pulling PMU drivers emitting things to dmesg so far, by the way), this
keep PMU drivers' errors silent preference of yours is unnecessarily
impeding people trying to measure system performance on Arm based
machines - all other archs' maintainers are fine with PMU drivers using
dmesg.

> Anyway, I think this driver has bigger problems that need addressing.

To me it represents yet another PMU driver submission - as the years go
by - that is lacking in the user messaging area.  Which reminds me, can
you take another look at applying this?:

https://patchwork.kernel.org/patch/10068535/

Thanks,

Kim