Date: Tue, 15 Dec 2015 18:10:03 +0000
From: Mark Rutland <mark.rutland@arm.com>
To: Mark Brown <broonie@kernel.org>
Cc: Juri Lelli <juri.lelli@arm.com>, Rob Herring <robh@kernel.org>,
        linux-kernel@vger.kernel.org, linux-pm@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org, devicetree@vger.kernel.org,
        peterz@infradead.org, vincent.guittot@linaro.org,
        linux@arm.linux.org.uk, sudeep.holla@arm.com,
        lorenzo.pieralisi@arm.com, catalin.marinas@arm.com,
        will.deacon@arm.com, morten.rasmussen@arm.com,
        dietmar.eggemann@arm.com, Pawel Moll <pawel.moll@arm.com>,
        Ian Campbell <ijc+devicetree@hellion.org.uk>,
        Kumar Gala <galak@codeaurora.org>,
        Maxime Ripard <maxime.ripard@free-electrons.com>,
        Olof Johansson <olof@lixom.net>,
        Gregory CLEMENT <gregory.clement@free-electrons.com>,
        Paul Walmsley <paul@pwsan.com>,
        Linus Walleij <linus.walleij@linaro.org>, Chen-Yu Tsai <wens@csie.org>,
        Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Subject: Re: [RFC PATCH 2/8] Documentation: arm: define DT cpu capacity
 bindings
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On Tue, Dec 15, 2015 at 05:45:16PM +0000, Mark Brown wrote:
> On Tue, Dec 15, 2015 at 05:28:37PM +0000, Mark Rutland wrote:
> > On Tue, Dec 15, 2015 at 05:17:13PM +0000, Mark Brown wrote:
> 
> > > Obviously people are going to get upset if we introduce performance
> > > regressions - but that's true always, we can also introduce problems
> > > with numbers people have put in DT.  It seems like it'd be harder to
> > > manage regressions due to externally provided magic numbers since
> > > there's inherently less information there.
> 
> > It's certainly still possible to have regressions in that case. Those
> > regressions would be due to code changes in the kernel, given the DT
> > didn't change.
> 
> > I'm not sure I follow w.r.t. "inherently less information", unless you
> > mean trying to debug without access to that DTB?
> 
> If what the kernel knows about the system is that it's got a bunch of
> cores with numbers assigned to them then all it's really got is those
> numbers.  If something changes that causes problems for some systems
> (eg, because the numbers have been picked poorly but in a way that
> happened to work well with the old code) that's not a lot to go on, the
> more we know about the system the more likely it is that we'll be able
> to adjust the assumptions in whatever new thing we do that causes
> problems for any particular systems where we run into trouble.

Regardless of where the numbers live (DT or kernel), all we have are
numbers. I can see that changing the in-kernel numbers would be possible
when modifyign the DT is not, but I don't see how that gives you more
information.

Mark.
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