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    SubjectRe: [PATCH v3 0/7] perf: Add ioctl for PMU driver configuration
    On Mon, 20 Aug 2018 15:36:47 +0100
    Suzuki K Poulose <suzuki.poulose@arm.com> wrote:

    > On 08/20/2018 03:22 PM, Kim Phillips wrote:
    > > On Mon, 20 Aug 2018 11:03:03 +0100
    > > Suzuki K Poulose <suzuki.poulose@arm.com> wrote:
    > >
    > >> On 08/16/2018 08:28 PM, Mathieu Poirier wrote:
    > >>> On Wed, 15 Aug 2018 at 09:28, Kim Phillips <kim.phillips@arm.com> wrote:
    > >>>>
    > >>>> On Wed, 15 Aug 2018 10:39:13 +0100
    > >>>> Will Deacon <will.deacon@arm.com> 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@arm.com> 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...

    But the same error number, e.g., EINVAL, can be returned for different
    reasons.

    > >> 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.

    Why not? pr_* are ratelimited.

    > Also, we are moving all the "debugging"
    > messages to the dynamic debug, to prevent lockdep splats.

    Are you referring to this?:

    https://lkml.org/lkml/2018/5/1/73

    Re-reading the thread, AFAICT, it was never proven that the splats
    occurred due to the dev_infos, and there's no dev_info in this
    stacktrace:

    https://lkml.org/lkml/2018/5/10/269

    But even if it were, wouldn't the splats also occur with dev_dbg?

    Kim

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