[Intel-gfx] [PATCH 1/2] drm/i915: Increase render/media power gating hysteresis for gen9+

[Chris Wilson]

Quoting Sagar Arun Kamble (2018-01-21 11:01:23)
> 
> 
> On 1/20/2018 3:01 PM, Chris Wilson wrote:
> > On gen9+, after an idle period the HW will disable the entire power well
> > to conserve power (by preventing current leakage). It takes around a 100
> > microseconds to bring the power well back online afterwards. With the
> > current hysteresis value of 25us, we do not have sufficient time to
> > respond to an interrupt and schedule the next execution before the HW
> > powers itself down. (At present, we prevent this by grabbing the
> > forcewake for prolonged periods of time, but that is overkill fixed in
> > the next patch.) The minimum we want to set the power gating hysteresis
> > to is the length of time it takes us to service the GPU, which across a
> > broad spectrum of machines is about 250us.
> >
> > (Note this also brings guc latency into the same ballpark as execlists.)
> >
> > Testcase: igt/gem_exec_nop/sequential
> > Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> > Cc: Joonas Lahtinen <joonas.lahtinen at linux.intel.com>
> > Cc: Michal Wajdeczko <michal.wajdeczko at intel.com>
> > Cc: Sagar Arun Kamble <sagar.a.kamble at intel.com>
> > Cc: Michel Thierry <michel.thierry at intel.com>
> > Cc: Michal Winiarski <michal.winiarski at intel.com>
> > ---
> >   drivers/gpu/drm/i915/intel_pm.c | 4 ++--
> >   1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/i915/intel_pm.c b/drivers/gpu/drm/i915/intel_pm.c
> > index 1db79a860b96..6748d3efb537 100644
> > --- a/drivers/gpu/drm/i915/intel_pm.c
> > +++ b/drivers/gpu/drm/i915/intel_pm.c
> > @@ -6627,8 +6627,8 @@ static void gen9_enable_rc6(struct drm_i915_private *dev_priv)
> >       I915_WRITE(GEN6_RC_SLEEP, 0);
> >   
> >       /* 2c: Program Coarse Power Gating Policies. */
> > -     I915_WRITE(GEN9_MEDIA_PG_IDLE_HYSTERESIS, 25);
> > -     I915_WRITE(GEN9_RENDER_PG_IDLE_HYSTERESIS, 25);
> > +     I915_WRITE(GEN9_MEDIA_PG_IDLE_HYSTERESIS, 250);
> > +     I915_WRITE(GEN9_RENDER_PG_IDLE_HYSTERESIS, 250);
> As per bspec this is in 1.28usec units so it should be 195.

Both numbers pulled out of the air, just read us above as approx.us.
-Chris