[Intel-gfx] [PATCH 2/5] drm/i915: Allow asynchronous waits on the i915_active barriers

[Chris Wilson]

Quoting Chris Wilson (2020-04-06 14:09:44)
> Quoting Tvrtko Ursulin (2020-04-06 13:06:03)
> > 
> > On 06/04/2020 10:12, Chris Wilson wrote:
> > > Allow the caller to also wait upon the barriers stored in i915_active.
> > > 
> > > Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> > > ---
> > >   drivers/gpu/drm/i915/i915_active.c | 60 ++++++++++++++++++++++++++++++
> > >   drivers/gpu/drm/i915/i915_active.h |  1 +
> > >   2 files changed, 61 insertions(+)
> > > 
> > > diff --git a/drivers/gpu/drm/i915/i915_active.c b/drivers/gpu/drm/i915/i915_active.c
> > > index d5e24be759f7..048ab9edd2c2 100644
> > > --- a/drivers/gpu/drm/i915/i915_active.c
> > > +++ b/drivers/gpu/drm/i915/i915_active.c
> > > @@ -542,6 +542,55 @@ static int __await_active(struct i915_active_fence *active,
> > >       return 0;
> > >   }
> > >   
> > > +struct wait_barrier {
> > > +     struct wait_queue_entry base;
> > > +     struct i915_active *ref;
> > > +};
> > > +
> > > +static int
> > > +barrier_wake(wait_queue_entry_t *wq, unsigned int mode, int flags, void *key)
> > > +{
> > > +     struct wait_barrier *wb = container_of(wq, typeof(*wb), base);
> > > +
> > > +     if (i915_active_is_idle(wb->ref)) { /* shared waitqueue, must check! */
> > 
> > Who shares it?
> 
> __var_waitqueue(ref) => uses a one of a set of global workqueues based
> off hash(ref)
> 
> Or we add a wait_queue_head_t to active, but we would still need to
> recheck as it may be reused as we are signaled.
> 
> > > +     if (flags & I915_ACTIVE_AWAIT_BARRIER) {
> > > +             err = flush_lazy_signals(ref);
> > > +             if (err)
> > > +                     return err;
> > > +
> > > +             err = __await_barrier(ref, arg);
> > > +             if (err)
> > > +                     return err;
> > >
> > 
> > Could have a single set of active_acquire_if_busy/release over the 
> > previous and this new block. Not sure if that would help with any 
> > atomicity concerns, or if there are such.
> 
> It would not affect correctness, it will just depend on taste.

Actually, flush_lazy_signals needs to be inside the active-ref, so we
should rearrange the acquires.
-Chris