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

[Juri Lelli <juri.lelli@arm.com>]

Hi,

On 10/12/15 14:14, Dietmar Eggemann wrote:
> On 01/12/15 11:20, Juri Lelli wrote:
> > Hi Vincent,
> > 
> > On 30/11/15 10:59, Vincent Guittot wrote:
> >> Hi Juri,
> >>
> >> On 24 November 2015 at 11:54, Juri Lelli <juri.lelli@arm.com> wrote:
> 
> [...]
> 
> >>>>> +==========================================
> >>>>> +3 - capacity-scale
> >>>>> +==========================================
> >>>>> +
> >>>>> +CPUs capacities are defined with respect to capacity-scale property in the cpus
> >>>>> +node [1]. The property is optional; if not defined a 1024 capacity-scale is
> >>>>> +assumed. This property defines both the highest CPU capacity present in the
> >>>>> +system and granularity of CPU capacity values.
> >>>>
> >>>> I don't really see the point of this vs. having an absolute scale.
> >>>>
> >>>
> >>> IMHO, we need this for several reasons, one being to address one of your
> >>> concerns below: vendors are free to choose their scale without being
> >>> forced to publish absolute data. Another reason is that it might make
> >>> life easier in certain cases; for example, someone could implement a
> >>> system with a few clusters of, say, A57s, but some run at half the clock
> >>> of the others (e.g., you have a 1.2GHz cluster and a 600MHz cluster); in
> >>> this case I think it is just easier to define capacity-scale as 1200 and
> >>> capacities as 1200 and 600. Last reason that I can think of right now is
> >>> that we don't probably want to bound ourself to some particular range
> >>> from the beginning, as that range might be enough now, but it could
> >>> change in the future (as in, right now [1-1024] looks fine for
> >>> scheduling purposes, but that might change).
> >>
> >> Like Rob, i don't really see the benefit of this optional
> >> capacity-scale property. Parsing the capacity of all cpu nodes should
> >> give you a range as well.
> >> IMHO, this property looks like an optimization of the code that will
> >> parse the dt more than a HW description
> >>
> > 
> > I agree that we can come up with the same information just looking at
> > the biggest capacity value of all CPUs and treat that value as
> > capacity-scale. I just thought that having that explicit made things
> > clearer, as it could be not easy to immediately see from a DT with many
> > CPUs which is the biggest capacity value. But, yes, we could remove that
> > anyway.
> 
> +1! This capacity-scale complicates things unnecessarily. It was hard
> for me to understand the meaning of it. Your 2. example sets
> 'capacity-scale = <2>' but also 'capacity = <2>' for cpu[01] and
> 'capacity = <1>' for cpu[23]. This can be easily replaced by 'capacity =
> <1024>' for cpu[01] and 'capacity = <512>' for cpu[23]. Much more
> readable, as it was mentioned already in this thread.
> 
> I understand that we don't want to limit the range of capacity values in
> the dt file to [1..1024] nor enforce that the cpu w/ the highest
> capacity has to have the value of 1024 in the dt file so the scheduler
> has to scale accordingly if we want to limit capacity to its supported
> capacity range (like with EAS [1..1024]).
> 

OK, I guess I can easily remove capacity-value and simply normalize CPU
capacities w.r.t. the highest capacity in the DT.

Thanks,

- Juri