Hi Rob, On 16/06/16 09:20, Juri Lelli wrote: > Hi, > > On 15/06/16 17:11, Rob Herring wrote: > > On Wed, Jun 15, 2016 at 5:17 AM, Juri Lelli <juri.lelli@xxxxxxx> wrote: > > [...] > > > > +========================================== > > > +2 - CPU capacity definition > > > +========================================== > > > + > > > +CPU capacity is a number that provides the scheduler information about CPUs > > > +heterogeneity. Such heterogeneity can come from micro-architectural differences > > > +(e.g., ARM big.LITTLE systems) or maximum frequency at which CPUs can run > > > +(e.g., SMP systems with multiple frequency domains). Heterogeneity in this > > > +context is about differing performance characteristics; this binding tries to > > > +capture a first-order approximation of the relative performance of CPUs. > > > + > > > +CPU capacities are obtained by running a suitable benchmark. This binding makes > > > +no aspersions on the validity or suitability of any particular benchmark, the > > > +final capacity should, however, be: > > > + > > > +* A "single-threaded" or CPU affine benchmark > > > +* Divided by the running frequency of the CPU executing the benchmark > > > +* Not subject to dynamic frequency scaling of the CPU > > > + > > > +For the time being we however advise usage of the Dhrystone benchmark. What > > > +above thus becomes: > > > + > > > +CPU capacities are obtained by running the Dhrystone benchmark on each CPU at > > > +max frequency. The obtained DMIPS score is then divided by the frequency (in > > > +MHz) at which the benchmark has been run, so that DMIPS/MHz are obtained. > > > +Such values are then normalized w.r.t. the highest score obtained in the > > > +system. > > > > So the property says it represents DMIPS/MHz, but we take that and > > "normalize" them back to a made up numbers? > > The normalization step is required if one wants to prevent > cross-platform comparisons (I think that's what vendors generally want). > They are not made up, they still come from measured DMIPS/MHz values. > > > Perhaps that step should > > be optional. Then paranoid Si vendors can put their fake numbers in > > and end users can update the dts files with real numbers. > > > > But, you can also decide to skip that step and put non normalized > numbers in. This documentation is advising people for what seems to be > be the most common way of coming up with values that, once in a DT, > won't be used to compare perf of different platforms. > > Maybe we want to add a paragraph clearly stating this point? > > > Is there any point in allowing people to pick their own scale? Why not > > just 100 (as in percent)? > > > > I think we agreed that not picking any particular scale is more flexible > and easier to use. For example, if there is a 2x factor between you cpus > you can simply put 1 and 2 there. But, if you need higher "resolution" > you can put whatever suits you better and we'll use the max as scale. > > Thanks a lot for the review. > Do you have any further comments on this point? Best, - Juri -- To unsubscribe from this list: send the line "unsubscribe devicetree" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
