[Intel-gfx] [PATCH 47/51] drm/i915: Update ironlake_enable_rc6() to do explicit request management

[Daniel Vetter]

On Fri, Feb 27, 2015 at 03:03:22PM +0200, Ville Syrjälä wrote:
> On Wed, Feb 25, 2015 at 10:31:13PM +0100, Daniel Vetter wrote:
> > On Wed, Feb 18, 2015 at 02:28:16PM +0000, John Harrison wrote:
> > > On 13/02/2015 17:03, Chris Wilson wrote:
> > > >On Fri, Feb 13, 2015 at 04:58:24PM +0000, John Harrison wrote:
> > > >>On 13/02/2015 12:19, Chris Wilson wrote:
> > > >>>On Fri, Feb 13, 2015 at 11:48:56AM +0000, John.C.Harrison at Intel.com wrote:
> > > >>>>From: John Harrison <John.C.Harrison at Intel.com>
> > > >>>>
> > > >>>>Updated ironlake_enable_rc6() to do explicit request creation and submission.
> > > >>>If you merged the context here with the common context switching code,
> > > >>>we don't even need to touch the ring here.
> > > >>>-Chris
> > > >>>
> > > >>It would certainly be preferable to not have any ring commands
> > > >>written from deep within the power management code. However, I
> > > >>didn't want to change anything I didn't really need to, especially
> > > >>in code that I'm not at all sure about. Plus I don't have an
> > > >>ironlake to test any significant change on.
> > > >I did and tested extensively.
> > > >-Chris
> > > 
> > > Do you have a patch that just does the move of this from PM code to context
> > > switch code? Something that I can drop into this series would be great. If
> > > not, exactly where about in the context switch code should it go? Should it
> > > be in the start of day initialisation, in the per context intialisation,
> > > every context switch, only the first switch after a resume, ...?
> > > 
> > > Tracing back to where/when this code is currently executed seems to be quite
> > > complicated. The _enable_rc6() function is called during ring reset but only
> > > for Gen6+ because Ironlake is broken according to the comment. It is also
> > > called by a system power management callback but it is unclear when that
> > > would occur. Finally, it is also called from the display code in
> > > intel_modeset_init_hw().
> > 
> > ilk rc6 is disabled by default because it crashes machines hard and
> > doesn't seem to be all that useful really. Compared to gen6+ rc6 which has
> > a massive impact on idle power consumption, more with each generation.
> > 
> > I've also never heard of anyone managing to make this work reliably. We
> > could also just rip the entire code out I think, at least I wouldn't be
> > surprised if it has bitrot completely by now.
> 
> I was running with it when Ben had those ILK context+rc6 patches flying
> around. But I admit that was some time ago.

Iirc we've enabled it for a bit with those, it fell to pieces too. And it
seems to be rare enough that testing on a developers machine doesn't cut
it (tried myself to enable it a few times and had to revert again).
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch