Quoting Mika Kuoppala (2017-11-15 13:51:13)
> Chris Wilson <chris@xxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxx>
> > ---
> > 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
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