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

[Mika Kuoppala]

Chris Wilson <chris at chris-wilson.co.uk> writes:

> Quoting Chris Wilson (2017-08-21 10:28:16)
>> 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.
>
> Hmm, or we could be more precise and just set the clocks high rather
> than queue a task. The complication isn't worth it for just a single
> callsite, but I am contemplating supplying boost/clocks information
> along with the request.

For the purposes of suspend, I think the approach is simple and
good enough.

Can David give a Tested-by?

Reviewed-by: Mika Kuoppala <mika.kuoppala at intel.com>

> -Chris