[Intel-gfx] [PATCH] drm/i915: Implement a better i945gm vblank irq vs. C-states workaround

[Ville Syrjälä]

On Thu, Oct 03, 2019 at 03:12:11PM +0100, Chris Wilson wrote:
> Quoting Ville Syrjala (2019-10-03 15:02:31)
> > From: Ville Syrjälä <ville.syrjala at linux.intel.com>
> > 
> > The current "disable C3+" workaround for the delayed vblank
> > irqs on i945gm no longer works. I'm not sure what changed, but
> > now I need to also disable C2. I also got my hands on a i915gm
> > machine that suffers from the same issue.
> > 
> > After some furious poking of registers I managed to find a
> > better workaround: The "Do not Turn off Core Render Clock in C
> > states" bit. With that I no longer have to disable any C-states,
> > and as a nice bonus the power cost is only ~1/4 of the
> > "disable C3+" method (which mind you doesn't even work anymore,
> > and so would have an even higher power cost if we made it work
> > by also disabling C2).
> > 
> > So let's throw out all the cpuidle/qos crap and just toggle
> > the magic bit as needed. And we extend the workaround to cover
> > i915gm as well.
> 
> Nice discovery. ScratchPad0 suggests that it may have been a late
> addition, there might be some chips out there that don't have the magic
> bit. Working on most is better than broken on all.

Yeah. And at least 100% of *my* broken machines now work ;)

> 
> > Cc: Chris Wilson <chris at chris-wilson.co.uk>
> > Signed-off-by: Ville Syrjälä <ville.syrjala at linux.intel.com>
> 
> Empirical results rule, and I for one am much happier without needing
> qos dma_latency,
> Acked-by: Chris Wilson <chris at chris-wilson.co.uk>
> -Chris

-- 
Ville Syrjälä
Intel