[Intel-gfx] [PATCH 1/2] drm/i915: Re-add enable_rc6 modparam

[Rodrigo Vivi]

On Thu, May 16, 2019 at 03:49:19PM +0000, Summers, Stuart wrote:
> On Thu, 2019-05-16 at 18:42 +0300, Jani Nikula wrote:
> > On Thu, 16 May 2019, "Summers, Stuart" <stuart.summers at intel.com>
> > wrote:
> > > On Thu, 2019-05-16 at 12:59 +0300, Jani Nikula wrote:
> > > > On Tue, 14 May 2019, Rodrigo Vivi <rodrigo.vivi at intel.com> wrote:
> > > > > One possibility that just came to my mind now is, what if we
> > > > > make
> > > > > this only for platforms that are still protected by
> > > > > is_alpha_support=1
> > > > > (soon becoming require_force_probe=1)
> > > > 
> > > > Please don't conflate alpha_support or force_probe with
> > > > *anything*
> > > > else.
> > > > 
> > > > > But this is just one side of the coin... when product is out
> > > > > there
> > > > > and we want the user to debug the issue to see if it is a RC6
> > > > > bug
> > > > > we have no way to verify that. :/
> > > > 
> > > > The problem is, if it works with rc6 disabled, it doesn't prove
> > > > it's
> > > > an
> > > > rc6 bug either.

Well, RC6 is the main GT power gating. The issue could be on may other
individual power gating, but if by disabling RC6 the issue is gone
it is a very good indication that it is a GT-PM bug somewhere.

> > > 
> > > Good point. I'm not saying we should enforce a process of disabling
> > > RC6
> > > for the platform if enable_rc6=0 results in success. I'm just
> > > saying
> > > having the option is useful from a debug perspective. We will still
> > > need to do the appropriate full analysis, including the normal code
> > > review process on a pre-case basis when debug involves this
> > > parameter.
> > > But the parameter itself is still useful.
> > 
> > The trouble starts when users figure out that enable_rc6=0 works
> > around
> > a particular problem they have (likely by way of disabling runtime
> > pm,
> > not directly related to rc6). You could argue this is a good thing,
> > but
> > unfortunately we generally never hear from them again, and the root
> > cause remains unsolved, with degraded user experience wrt power
> > management.

This is indeed bad. We should probably be clear that by disabling that
they are draining their power and probably killing battery life.

> 
> So I understand the reasoning here, and agree that isn't an ideal
> situation. I'd also like a way to debug more efficiently. What did you
> think about the suggestion from Tvrtko to only apply on
> CONFIG_DRM_I915_DEBUG?

if debug is on and parameter is set, right? Might be a good thing to
avoid the abuse on the parameter.

> 
> Or we could even wrap this entirely with a CONFIG_I915_DEBUG_PM -
> although I'd like to suggest we still use the module parameter, we
> could just use the config option to hide the modparam under normal
> operation.

I think this looks more like you are enabling more logs and
not that you are disabling... unless the plan is to go with
this flag plus logs and traces around PM. Than I think it is
a good idea.

Same rules needs to apply to  RC6, RPM, DC states,
display power well management, psr, etc.

Thanks,
Rodrigo.

> 
> Thanks,
> Stuart
> 
> > 
> > BR,
> > Jani.
> > 
> > 



> _______________________________________________
> Intel-gfx mailing list
> Intel-gfx at lists.freedesktop.org
> https://lists.freedesktop.org/mailman/listinfo/intel-gfx