[Skiboot] [PATCH] occ: Skip setting cores to nominal frequency in P9

[Vaidyanathan Srinivasan]

* ppaidipe <ppaidipe at linux.vnet.ibm.com> [2017-04-03 11:32:55]:

> Hi Shilpa
> 
> On 2017-04-01 14:59, Shilpasri G Bhat wrote:
> > In P9, once OCC is up, it is supposed to setup the cores to nominal
> > frequency. So skip this step in OPAL.
> > 
> > Signed-off-by: Shilpasri G Bhat <shilpa.bhat at linux.vnet.ibm.com>
> > ---
> >  hw/occ.c | 13 ++++++++-----
> >  1 file changed, 8 insertions(+), 5 deletions(-)
> > 
> > diff --git a/hw/occ.c b/hw/occ.c
> > index 2ffb677..b222521 100644
> > --- a/hw/occ.c
> > +++ b/hw/occ.c
> > @@ -897,11 +897,14 @@ void occ_pstates_init(void)
> >  		return;
> >  	}
> > 
> > -	/* Setup host based pstates and set nominal frequency */
> > -	for_each_chip(chip) {
> > -		for_each_available_core_in_chip(c, chip->id) {
> > -			cpu_pstates_prepare_core(chip, c, pstate_nom);
> > -		}
> > +	/*
> > +	 * Setup host based pstates and set nominal frequency only in
> > +	 * P8.
> > +	 */
> 
> Instead of skipping, is it good to add validating whether core comes with
> nominal frequency
> or psafe frequency? So that we can catch in OPAL log about any failure if
> OCC doesn't set
> up the core nominal frequency properly during boot time.
> What will be the impact, if core comes with psafe frequency?

Good idea to validate, but we should not hold bootup to do this
verification.  On P8 we had to wait for OCC boot and manually
transition to nominal, but on P9 and newer platforms we can proceed
the moment we get the PState table in HOMER and no further xscoms and
waits needed.  This will save us a bit of bootup time on OCC preloaded
systems and a ton of bootup time where skiboot has to initiate OCC
load.

These transitions can be verified even at petit boot time and since
OCC issues are not fatal to bootup, we should not delay bootup but
rather move the checks to later stages in boot process.

Actually if we are in psafe, the Linux kernel cpufreq driver will log
a message.  Also we will never load the driver if OCC never started
and provided us a PState table.  We log that in skiboot as well.

Verification of whether we entered Linux at nominal frequency is not
checked, but that is something we could do at later stage in bootup.
We would like to verify whether PState transition really work or not
and log a message from cpufreq platform driver.

--Vaidy