Re: [RFC V4 0/3] arm_pmu: acpi: variant support and QCOM Falkor extensions

[J. AgustÃn Vega-FrÃas]

Hi Will,

On Fri, Jul 13, 2018 at 10:33 AM, Will Deacon <will.deacon@xxxxxxx> wrote:

[snip]

> I'm mostly ok with this approach, but I have a concern with the way in which
> the sysfs interface for carving up the config fields is implemented. If this
> is intended to be a strict extension to the armv8 pmu architecture, then I
> don't think you should be overriding the attr_groups entirely. Rather, you
> should be taking the attr_groups from pmuv3 and then extending them in a way
> which avoids overlapping field allocations by construction.
>
> As it stands, you already have an overlap between the pcc bit and the
> chained counter bit which Suzuki has implemented and it will be very easy to
> introduce API breakage if we don't enforce this as part of the design.
>
> Will

FYI, I left Qualcomm on July 6, one of my former colleagues will submit
new iterations of this series. I will continue to comment on this and future
patchsets as a courtesy to my former colleagues and the community.

Thanks for pointing out the sysfs issue. My suggestion on how to address it is:

1. Reserve config and config1 for architectural format attributes and
   config2 for extension format attributes.
2. Add a struct attribute ** parameter to the extension init function so
   extensions can return the new attributes.
3. The extension framework code in arm_pmu_acpi.c can then allocate a new
   attribute array to contain the base and extension attributes and ensure
   all the new attributes are on config2.

Though a more elaborate approach can be implemented to find conflicts in
bit usage within config fields, it would require much more code for a
relatively simple problem. Thoughts?

Thanks,
AgustÃn