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

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Message-ID: <20180427160914.GA5286@arm.com>
Date:   Fri, 27 Apr 2018 17:09:14 +0100
From:   Will Deacon <will.deacon@....com>
To:     Kim Phillips <kim.phillips@....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

Kim,

[Ganapat: please don't let this discussion disrupt your PMU driver
 development. You can safely ignore it for now :)]

On Fri, Apr 27, 2018 at 10:46:29AM -0500, Kim Phillips wrote:
> 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?

Because they're the ones I maintain...

> 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.

Why is that my problem? Try harder?

> 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.

Good for them, although I'm pretty sure that at least the x86 folks are
against this crap too.

> > 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?:

As I said before, I'm not going to take anything that logs above pr_debug
for things that are directly triggerable from userspace. Spin a version
using pr_debug and I'll queue it.

Have a good weekend,

Will