[Intel-gfx] Why idle_freq is set to RPn and not RPe

[Szwichtenberg, Radoslaw]

Hello Chris,

Happy New Year!
Thanks for answers so far. I have some additional questions.

You wrote that this change was made to take a defensive position to ensure minimum power consumption - did we do any power measurements to confirm the benefit? How does this change affect user experience and overall performance? Did we do any performance impact measurements? Is same change done in Windows implementation? Was this change in the algorithm discussed with HW team?

Thanks!
Radek

> -----Original Message-----
> From: Kamble, Sagar A
> Sent: Thursday, December 31, 2015 6:38 PM
> To: Chris Wilson; Szwichtenberg, Radoslaw; S, Deepak; Intel Graphics
> Development; Goel, Akash
> Subject: Re: Why idle_freq is set to RPn and not RPe
> 
> 
> 
> On 12/30/2015 4:20 PM, Chris Wilson wrote:
> > On Wed, Dec 30, 2015 at 04:09:46PM +0530, Kamble, Sagar A wrote:
> >>     Turbo frequency range is Rpe to Rp0 when GPU is active as, on
> workload
> >>     submission frequency is taken to Rpe.
> >>
> >>     Does the HW require us to drop to RPn before entering RC6?
> >>     If we can enter RC6 even with other frequencies I think we can keep
> >>     running at Rpe on Idle.
> > Remember that we quite frequently prevent the hardware going into RC6,
> I assume this is threshold times in TO/EI mode for which GT is idle but not
> power gated.
> > and that it has been known for the hardware to fail to enter RC6
> > itself (through driver error or whatnot).
> And assume this is because of forcewake/rc6 setup errors in driver paths
> which should not happen in best case :) Agree that running at Rpn makes
> sense.
> >   Going to the extreme, why wouldn't
> > you set Rp0 on idle, since that will give the best restart latency?
> True. We can have different logic that starts from Rp0 and comes down if
> perf is met.
> > -Chris
> >
> Thanks for the inputs Chris.
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