Re: [RFC PATCH 2/8] Documentation: arm: define DT cpu capacity bindings

Mark Rutland
2015-12-15T18:10:03Z

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.