[Intel-gfx] [PATCH 2/8] drm/i915: Make request's wait-for-space explicit

[Chris Wilson]

Quoting Mika Kuoppala (2017-11-15 13:51:13)
> Chris Wilson <chris at chris-wilson.co.uk> writes:
> 
> > At the start of building a request, we would wait for roughly enough
> > space to fit the average request (to reduce the likelihood of having to
> > wait and abort partway through request construction). To achieve we
> > would try to begin a 0-length command packet, this just adds extra
> > confusion so make the wait-for-space explicit, as in the next patch we
> > want to move it from the backend to the i915_gem_request_alloc() so it
> > can ensure that the wait-for-space is the first operation in building a
> > new request.
> >
> > Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> > ---
> >  drivers/gpu/drm/i915/intel_lrc.c        |  8 ++---
> >  drivers/gpu/drm/i915/intel_ringbuffer.c | 56 +++++++++++++++++++++------------
> >  drivers/gpu/drm/i915/intel_ringbuffer.h |  1 +
> >  3 files changed, 41 insertions(+), 24 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
> > index 58d050a9a866..ebd9596fe83b 100644
> > --- a/drivers/gpu/drm/i915/intel_lrc.c
> > +++ b/drivers/gpu/drm/i915/intel_lrc.c
> > @@ -1180,7 +1180,7 @@ static int execlists_request_alloc(struct drm_i915_gem_request *request)
> >  {
> >       struct intel_engine_cs *engine = request->engine;
> >       struct intel_context *ce = &request->ctx->engine[engine->id];
> > -     u32 *cs;
> > +     int ret;
> >  
> >       GEM_BUG_ON(!ce->pin_count);
> >  
> > @@ -1190,9 +1190,9 @@ static int execlists_request_alloc(struct drm_i915_gem_request *request)
> >        */
> >       request->reserved_space += EXECLISTS_REQUEST_SIZE;
> >  
> > -     cs = intel_ring_begin(request, 0);
> > -     if (IS_ERR(cs))
> > -             return PTR_ERR(cs);
> > +     ret = intel_ring_wait_for_space(request->ring, request->reserved_space);
> > +     if (ret)
> > +             return ret;
> >
> 
> We lose the wrap handling with this. What guarantees
> at this point, that we dont need to wrap?

We don't care. We wait for sufficient space to wrap, but we don't need
to advance/wrap the ring pointer here as we are not emitting a packet.
-Chris