linux-kernel - Re: [PATCH 2/4] Documentation: Add documentation for APM X-Gene SoC PMU DTS binding

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Message-ID: <CACgAJHwZ=CwyvxLmuXdOAw2oFhfoC2jkdOY1HPenCsEaYLjrPw@mail.gmail.com>
Date:	Tue, 5 Apr 2016 11:51:02 -0700
From:	Tai Tri Nguyen <ttnguyen@....com>
To:	Mark Rutland <mark.rutland@....com>
Cc:	will.deacon@....com, catalin.marinas@....com,
	linux-kernel@...r.kernel.org, devicetree@...r.kernel.org,
	linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
	patches <patches@....com>
Subject: Re: [PATCH 2/4] Documentation: Add documentation for APM X-Gene SoC
 PMU DTS binding

Hi Mark,

On Mon, Apr 4, 2016 at 4:38 PM, Mark Rutland <mark.rutland@....com> wrote:
> On Mon, Apr 04, 2016 at 04:40:33PM -0700, Tai Tri Nguyen wrote:
>> On Fri, Apr 1, 2016 at 5:30 AM, Mark Rutland <mark.rutland@....com> wrote:
>> > On Thu, Mar 31, 2016 at 04:37:50PM -0700, Tai Nguyen wrote:
>> >> +This is APM X-Gene SoC PMU (Performance Monitoring Unit) module.
>> >> +The following PMU devices are supported:
>> >> +
>> >> +  L3C                        - L3 cache controller
>> >> +  IOB                        - IO bridge
>> >> +  MCB                        - Memory controller bridge
>> >> +  MC                 - Memory controller
>> >
>> > These sound like separate units. How do these relate?
>> >
>> > Is there an SOC-wide PMU that aggregates counters, or are these actually
>> > independent?
>> >
>>
>> Yes, they are independent, but sharing the same top level status interrupt.
>> There's no SOC-wide PMU which aggregates these counters.
>
> If they're just sharing the interrupt, why are they not separate nodes (and
> drivers) which simply happen to share an interrupt?
>
> Is there anything else shared?

Yes, a long with the interrupt they also share top level PMU status CSR region.

>
>> >> +The following section describes the SoC PMU DT node binding.
>> >> +
>> >> +Required properties:
>> >> +- compatible         : Shall be "apm,xgene-pmu" for revision 1 or
>> >> +                          "apm,xgene-pmu-v2" for revision 2.
>> >
>> > That name is very general. Is there not a more specific name for the SOC
>> > PMU?
>> >
>>
>> Beside the ARMv8 core PMU which has the compatible name "arm,armv8-pmuv3",
>> these are all the PMUs in X-Gene SoCs. Also, we are using the same PMU
>> driver across our platforms. I think a general name is what it should be.
>
> Depending on my prior question, I'm not sure that's necessarily true. If
> there's no actual shared SoC PMU logic as such, the names below for each
> individual PMU seem fine.
>

Given that they are sharing the same top level PMU status CSRs, a
combined driver
seems to be the best approach in my opinion.

>> >> +Required properties for L3C subnode:
>> >> +- compatible         : Shall be "apm,xgene-pmu-l3c".
>> >> +- reg                        : First resource shall be the L3C PMU resource.
>> >> +- index                      : Instance number of the L3C PMU.
>> >> +
>> >> +Required properties for IOB subnode:
>> >> +- compatible         : Shall be "apm,xgene-pmu-iob".
>> >> +- reg                        : First resource shall be the IOB PMU resource.
>> >> +- index                      : Instance number of the IOB PMU.
>> >> +
>> >> +Required properties for MCB subnode:
>> >> +- compatible         : Shall be "apm,xgene-pmu-mcb".
>> >> +- reg                        : First resource shall be the MCB PMU resource.
>> >> +- index                      : Instance number of the MCB PMU.
>> >> +
>> >> +Required properties for MC subnode:
>> >> +- compatible         : Shall be "apm,xgene-pmu-mc".
>> >> +- reg                        : First resource shall be the MC PMU resource.
>> >> +- index                      : Instance number of the MC PMU.
>> >
>> > What's the index property useful for?
>> >
>>
>> The index property is used for indicating the physical hardware PMU id.
>> For example, on X-Gene1 there are 4 memory controllers (MC), each of them has
>> its own PMU. The index property tells us which MC a PMU belongs to.
>> The same for MCB/L3C and IOB.
>
> Sure, but is this simply informative for the user, or does this have an impact
> on the programming model?
>

Yes, it does impact. For example, there are 4 PMUs associated with 4 MCs.
They all certainly have the same sort of events. The index will help
to determine
the right event users want to monitor. Below is an example of perf list output:
...
mc0/mcu-rd-request/
...
mc1/mcu-rd-request/
...

Regards,
-- 
Tai