[Intel-gfx] [PATCH] drm/i915: Park the GPU on module load

[Chris Wilson]

Quoting Tvrtko Ursulin (2018-09-14 10:40:43)
> 
> On 14/09/2018 09:34, Chris Wilson wrote:
> > Once we have flushed the first request through the system to both load a
> > context and record the default state; tell the GPU to park and idle
> > itself, putting itself immediately (hopefully at least) into a
> > powersaving state, and allowing ourselves to start from known state
> > after setting up all our bookkeeping.
> 
> Otherwise this would happen after a second or so, when the idle work 
> handler does it. Crucial thing to mention is I think the park/unpark 
> cycle which sets up the pinned default state.

Hmm, I didn't think it was worth highlight it here as one assumes that
all the bookkeeping is equally required :) I pulled out for special
mention in the comment, because I thought that was relevant to the
function where we are acquiring the default_state.
 
> Alternative to patch could be to extract 
> __intel_engine_pin_default_state to be called from here and from 
> intel_engines_unpark.
> 
> But I guess there is some value in going powersave as soon as possible 
> so okay.

Yeah. Later on (as in patches that have been on the ml for several
months with feature requests depending on them...) we acquire a
load_powersaving_context() function, which should make this a little more
interesting.

> > Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> > Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> > ---
> >   drivers/gpu/drm/i915/i915_gem.c | 8 ++++++++
> >   1 file changed, 8 insertions(+)
> > 
> > diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> > index 89834ce19acd..be9d012d851b 100644
> > --- a/drivers/gpu/drm/i915/i915_gem.c
> > +++ b/drivers/gpu/drm/i915/i915_gem.c
> > @@ -5414,6 +5414,14 @@ static int __intel_engines_record_defaults(struct drm_i915_private *i915)
> >   
> >       assert_kernel_context_is_current(i915);
> >   
> > +     /*
> > +      * Immediately park the GPU so that we enable powersaving and
> > +      * treat it as idle. The next time we issue a request, we will
> > +      * unpark and start using the engine->pinned_default_state, otherwise
> > +      * it is in limbo and an early reset may fail.
> > +      */
> > +     __i915_gem_park(i915);
> 
> Why not after we have grabbed the default state?

I was just wanting to tie it to the switch to kernel-context + idle.
I was thinking this was the closed point to the normal park sequence.
 
> I was worried this might access a sleeping device but apparently at 
> least vma unbind takes the rpm ref. Still, just this I think makes it 
> more logical to park after we have grabbed the default state, instead o 
> putting the device to sleep and then immediately waking up below.

Oh, we have wakerefs aplenty on module load, we are not assuming gt.awake
here.
-Chris