Re: [PATCH v4 2/8] Documentation: arm: define DT cpu capacity bindings

[Vincent Guittot]

On 21 March 2016 at 11:53, Juri Lelli <juri.lelli@xxxxxxx> wrote:
> Hi Sai,
>
> On 18/03/16 10:49, Sai Gurrappadi wrote:
>> Hi Juri,
>>
>> On 03/18/2016 07:24 AM, Juri Lelli wrote:
>>
>> <snip>
>>
>> > +
>> > +==========================================
>> > +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.
>>
>> Any reason why this capacity number is not dynamically generated based on the
>> max frequency for each CPU? The DT property would then instead specify just
>> the micro-architectural differences between the CPU types.
>>
>
> I'm not sure I clearly understand your question, so I'll try to
> reiterate it.
>
> Are you asking why we don't dynamically profile the system, at boot for
> example, to get this number? Or do you ask why this number couldn't be
> only describing micro-arch differences (so, if I get it right, we should
> then multiply it by max freq to get the capacity of a CPU)?
>
> We already played with the first option (please refer to v2 and v3), but
> we ended up agreeing that dynamic profiling adds overhead to the boot
> process (while a DT approach can provide information to speed up boot)
> and it is in general not repeatable/reliable (as numbers can vary from
> boot to boot for different reasons).
>
> The second option I think can be feasible, but I'm not sure what we gain
> in practice. We will still need to specify a per-platform number, right?

So could we use dt binding like dhrystone = <xyz> (with a unit like
DMIPS/Mhz)  which can then be combined with OPP table to gives a
1st-order approximation of each CPU capacity ?

>
> Best,
>
> - Juri