[Skiboot] [PATCH V7 2/3] occ: Fix Pstate ordering for P9

[Vaidyanathan Srinivasan]

* Vaidyanathan Srinivasan <svaidy at linux.vnet.ibm.com> [2017-05-26 12:38:52]:

> * Michael Neuling <mikey at neuling.org> [2017-05-22 11:13:38]:
> 
> > On Tue, 2017-02-14 at 02:01 +0530, Shilpasri G Bhat wrote:
> > > In P9 the pstate values are positive. They are continuous set of
> > > unsigned integers [0 to +N] where Pmax is 0 and Pmin is N. The
> > > linear ordering of pstates for P9 has changed compared to P8.
> > > P8 has neagtive pstate values advertised as [0 to -N] where Pmax
> > > is 0 and Pmin is -N. This patch adds helper routines to abstract
> > > pstate comparison with pmax and adds sanity pstate limit checks.
> > > This patch also fixes pstate arithmetic by using labs().
> > 
> > Can you put this comment inline in the code?  There is no mention of Pmax Pmin
> > where cmp_postive/negative_pstates() is making it difficult to understand what
> > it's actually doing.
> 
> Yes, we can explain this difference in specification inline in code.
> Will update in a repost.
> 
> > 
> > Alternatively, why don't we convert what we get on P8 to positive numbers, so
> > that all the pstates numbers are positive.  Then we don't need this clumsy
> > negative/positive code.
> 
> The firmware interface specification allows POWER8 IDs to be a signed
> number and practically we can have negative PState IDs that are higher
                                                                ^^ lower
> frequency and positive PState IDs that are lower frequencies.
                                             ^^ higher
                                              
> Treating them as unsigned make validation of the range and boundaries
> provided by OCC difficult between POWER8 and POWER9.
> 
> The good part is that the current firmware interface for POWER9 and
> future platforms treat the PState IDs as positive and hence future
> platforms and implementation are clean.
> 
> --Vaidy
> 
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