[Intel-gfx] [PATCH] drm/i915: "Race-to-idle" on switching to the kernel context

[Chris Wilson]

Quoting Mika Kuoppala (2017-08-21 10:17:52)
> Chris Wilson <chris at chris-wilson.co.uk> writes:
> 
> > During suspend we want to flush out all active contexts and their
> > rendering. To do so we queue a request from the kernel's context, once
> > we know that request is done, we know the GPU is completely idle. To
> > speed up that switch bump the GPU clocks.
> >
> > Switching to the kernel context prior to idling is also used to enforce
> > a barrier before changing OA properties, and when evicting active
> > rendering from the global GTT. All cases where we do want to
> > race-to-idle.
> >
> > Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> > Cc: David Weinehall <david.weinehall at linux.intel.com>
> > ---
> >  drivers/gpu/drm/i915/i915_gem_context.c | 11 ++++++++---
> >  1 file changed, 8 insertions(+), 3 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/i915/i915_gem_context.c b/drivers/gpu/drm/i915/i915_gem_context.c
> > index 58a2a44f88bd..ca1423ad2708 100644
> > --- a/drivers/gpu/drm/i915/i915_gem_context.c
> > +++ b/drivers/gpu/drm/i915/i915_gem_context.c
> > @@ -895,6 +895,7 @@ int i915_gem_switch_to_kernel_context(struct drm_i915_private *dev_priv)
> >  
> >       for_each_engine(engine, dev_priv, id) {
> >               struct drm_i915_gem_request *req;
> > +             bool active = false;
> >               int ret;
> >  
> >               if (engine_has_kernel_context(engine))
> > @@ -913,13 +914,17 @@ int i915_gem_switch_to_kernel_context(struct drm_i915_private *dev_priv)
> >                       prev = i915_gem_active_raw(&tl->last_request,
> >                                                  &dev_priv->drm.struct_mutex);
> >                       if (prev)
> > -                             i915_sw_fence_await_sw_fence_gfp(&req->submit,
> > -                                                              &prev->submit,
> > -                                                              GFP_KERNEL);
> > +                             active |= i915_sw_fence_await_sw_fence_gfp(&req->submit,
> > +                                                                        &prev->submit,
> > +                                                                        GFP_KERNEL) > 0;
> 
> There is no point of kicking the clocks if we are the only request left?
> 
> Well logical as the request is empty, just pondering if the actual ctx
> save/restore would finish quicker.

I was thinking if it was just the context save itself, it not would be
enough of a difference to justify itself. Just gut feeling and not
measured, I worry about the irony of boosting from idle just to idle.
-Chris