Re: [PATCH v3 0/7] perf: Add ioctl for PMU driver configuration

[Suzuki K Poulose]

On 08/20/2018 03:22 PM, Kim Phillips wrote:
> On Mon, 20 Aug 2018 11:03:03 +0100
> Suzuki K Poulose <suzuki.poulose@xxxxxxx> wrote:
>
> > On 08/16/2018 08:28 PM, Mathieu Poirier wrote:
> > > On Wed, 15 Aug 2018 at 09:28, Kim Phillips <kim.phillips@xxxxxxx> wrote:
> > > > On Wed, 15 Aug 2018 10:39:13 +0100
> > > > Will Deacon <will.deacon@xxxxxxx> wrote:
> > > >
> > > > > On Tue, Aug 14, 2018 at 01:42:27PM -0600, Mathieu Poirier wrote:
> > > > > > On Tue, 14 Aug 2018 at 11:09, Kim Phillips <kim.phillips@xxxxxxx> wrote:
> > > > > > > The other thing that's going on here is that I'm becoming numb to the
> > > > > > > loathsome "failed to mmap with 12 (Cannot allocate memory)" being
> > > > > > > returned no matter what the error is/was. E.g., an error that would
> > > > > > > indicate a sense of non-implementation would be much better
> > > > > > > appreciated than presumably what the above is doing, i.e., returning
> > > > > > > -ENOMEM.  That, backed up with specific details in the form of human
> > > > > > > readable text in dmesg would be *most* welcome.
> > > > > >
> > > > > > As part of the refactoring of the code to support CPU-wide scenarios I
> > > > > > intend to emit better diagnostic messages from the driver.  Modifying
> > > > > > rb_alloc_aux() to propagate the error message generated by the
> > > > > > architecture specific PMUs doesn't look hard either and I _may_ get to
> > > > > > it as part of this work.
> > > > >
> > > > > For the record, I will continue to oppose PMU drivers that dump diagnostics
> > > > > about user-controlled input into dmesg, but the coresight drivers are yours
> > > > > so it's up to you and I won't get in the way!
> > > >
> > > > That sounds technically self-contradicting to me.  Why shouldn't
> > > > coresight share the same policies as those used for PMU drivers?  Or
> > > > why not allow the individual vendor PMU driver authors control the
> > > > level of user-friendliness of their own drivers?
> > > >
> > > > That being said, Matheiu, would you accept patches that make coresight
> > > > more verbose in dmesg?
> > >
> > > It depends on the issue you're hoping to address.  I'd rather see the
> > > root cause of the problem fixed than adding temporary code.  Suzuki
> > > added the ETR perf API and I'm currently working on CPU-wide
> > > scenarios.  From there and with regards to what can happen in
> > > setup_aux(), we should have things covered.
> >
> > I think the main issue is the lack of error code propagation from
> > setup_aux() back to the perf_aux_output_handle_begin(), which always
> > return -ENOMEM. If we fix that, we could get better idea of whats
> > wrong.
>
> Why get a better idea when we can get the exact details?

The different values for error numbers are there for a reason...

> > If someone is planning to add verbose messages, they may do so by adding
> > dev_dbg() / pr_debug(), which can be turned on as and when needed.
>
> I disagree:  that just adds another usage and kernel configuration
> obstacle.  Why not use pr_err straight up?

I personally don't agree to usage of pr_err() in paths which are easily
triggered from user input. Also, we are moving all the "debugging"
messages to the dynamic debug, to prevent lockdep splats.

Suzuki