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

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Date:   Sun, 15 Jul 2018 15:35:45 -0500
From:   J. Agustín Vega-Frías 
        <agustin.vega.frias@...il.com>
To:     Will Deacon <will.deacon@....com>
Cc:     Agustin Vega-Frias <agustinv@...eaurora.org>,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-acpi@...r.kernel.org, Mark Rutland <mark.rutland@....com>,
        Jeremy Linton <jeremy.linton@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Marc Zyngier <marc.zyngier@....com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Phani Pabba <pabba@...eaurora.org>,
        Richard Ruigrok <rruigrok@...eaurora.org>,
        Vijaya Kilari <vkilari@...eaurora.org>,
        Jeff Hugo <jhugo@...eaurora.org>,
        Rahul Ramasubramanian <rahulr@...eaurora.org>
Subject: Re: [RFC V4 0/3] arm_pmu: acpi: variant support and QCOM Falkor extensions

Hi Will,

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