linux-kernel - Re: [PATCH v2 2/4] Documentation: Add documentation for APM X-Gene SoC PMU DTS binding

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Message-ID: <20160531165630.GE4254@leverpostej>
Date:	Tue, 31 May 2016 17:56:30 +0100
From:	Mark Rutland <mark.rutland@....com>
To:	Tai Tri Nguyen <ttnguyen@....com>
Cc:	Rob Herring <robh@...nel.org>, Will Deacon <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 v2 2/4] Documentation: Add documentation for APM X-Gene
 SoC PMU DTS binding

On Mon, May 02, 2016 at 02:46:05PM -0700, Tai Tri Nguyen wrote:
> Hi Rob,
> 
> On Mon, May 2, 2016 at 1:56 PM, Rob Herring <robh@...nel.org> wrote:
> > On Wed, Apr 20, 2016 at 12:31:22PM +0100, Will Deacon wrote:
> >> On Mon, Apr 18, 2016 at 01:04:53PM -0700, Tai Tri Nguyen wrote:
> >> > >> +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.
> >> > >
> >> > > Don't use indexes. You probably need phandles to the nodes these are
> >> > > related to.
> >> > >
> >> > > How many variations of child nodes do you expect to have? 2, 10, 50? You
> >> > > might want to just collapse all this down to a single node and put this
> >> > > information in the driver if it is fixed for each SoC and there's only a
> >> > > handful.
> >> > >
> >> >
> >> > For each kind of PMU, for example memory controller PMU, I expect to
> >> > have the number of instances up to 8.
> >> > They are actually all independent PMU nodes and have their own CSR memory bases.
> >> > The indexes are used for exposing the devices to perf user only. It
> >> > doesn't have an impact on the programming model.
> >> > Mark also had the same concern.
> >>
> >> Regardless, I'll need an ack from Rob or Mark before I can merge this.
> >
> > I still have a concern with this. Needing an index to expose to the user
> > is generally not a valid reason. That's OS specific and therefore
> > doesn't belong in DT.
> >
> > Rob
> 
> I can use device name here. However, the perf event names will be
> different between DT and ACPI which I want to avoid.
> And the names don't look good at all.
> Also, specifically for MC and MCB PMUs, the indexes are compared
> against the active MC/MCB mask to find out whether they are populated
> or not.
> Without using the index property, I will also need a mapping function
> of physical device addresses and their physical ids.

What's wrong with using ${device}.{physical_address} as the PMU name?
That would be unique and consistent regardless of the firmware, no
mapping nor index property necessary.

That's sufficient for any user already familiar with the topology, a
familiarity you seem to be assuming regardless by not explicitly
describing the topology in the DT.

Thanks,
Mark.