Arch Linux

pogeymanz

Member

Registered: 2008-03-11

Posts: 1,020

Are the i915 options enable_rc6, enable_fbc, etc, still needed?

I still have in my /etc/modprobe.d/i915.conf

 options i915 i915_enable_rc6=7 i915_enable_fbc=1 lvds_downclock=1 

Are these still helpful for energy saving?

Gusar

Member

Registered: 2009-08-25

Posts: 3,605

Re: Are the i915 options enable_rc6, enable_fbc, etc, still needed?

Only Sandy Bridge has multiple rc6 states (rc6, rc6p, rc6pp), Ivy Bridge and Haswell have just one. I don't know what the default for Sandy Bridge is, but Ivy and Haswell have rc6 on by default, so the option isn't needed on those. It's very easy to check what Sandy's default is, remove the option and then look at what powertop says.

fbc is set to "use per-chip default", however I think it's actually off everywhere because there's issues with it on certain chipsets/monitors, so you still need to explicitly enable it. lvds_downclock is off by default, so you need that one too (but note that only laptops use lvds/edp, this option does nothing on desktops).

Edit: There's also another option, enable_psr (panel self-refresh), only valid for eDP displays. If you have a modern laptop with an eDP display, you could also turn that one on and see if it has any effect.

Last edited by Gusar (2014-05-17 16:16:56)

WonderWoofy

Member

From: Los Gatos, CA

Registered: 2012-05-19

Posts: 8,414

Re: Are the i915 options enable_rc6, enable_fbc, etc, still needed?

Only Sandy Bridge has multiple rc6 states (rc6, rc6p, rc6pp), Ivy Bridge and Haswell have just one.

I'm not sure this is true.  I have an Ivy Bridge machine and by default it is able to use rc6 and rc6p.  Setting i915_enable_rc6=7 also makes powertop report it going into rc6pp as well.

Gusar

Member

Registered: 2009-08-25

Posts: 3,605

Re: Are the i915 options enable_rc6, enable_fbc, etc, still needed?

I checked, you're right, Ivy Bridge does have rc6p and rc6pp, Ironlake, Haswell and Broadwell don't. I had no idea Ironlake had any kind of rc6, you learn something new every day . Hmm, now I'm wondering about ValleyView/Baytrail. It's based on Ivy Bridge, so it could have them, but I'm not sure.

These are the per-chip defaults:

• pre-Ironlake: no rc6, rc6p, rc6pp

• Ironlake: rc6 disabled, no rc6p or rc6pp

• Sandy Bridge: rc6 enabled, rc6p and rc6pp disabled

• Ivy Bridge: rc6 and rc6p enabled, rc6pp disabled

• ValleyView/Baytrail: rc6 enabled, no idea if rc6p and rc6pp exist

• Haswell and Broadwell: rc6 enabled, no rc6p or rc6pp

Edit: On some further research I also found info on ValleyView/Baytrail: despite being based on Ivy Bridge, it does not have rc6p and rc6pp. I guess the minor power savings they provide just aren't worth the added complexity, so they got dropped after Ivy Bridge. So it's safe to say that Cherryview (Baytrail's successor) won't have them either.

Last edited by Gusar (2014-05-17 23:10:18)