[Intel-gfx] [PATCH 02/13] drm/i915: Only queue requests during execlists submission

[Chris Wilson]

On Fri, Aug 26, 2016 at 01:43:42PM +0300, Mika Kuoppala wrote:
> Chris Wilson <chris at chris-wilson.co.uk> writes:
> 
> > On Fri, Aug 26, 2016 at 12:41:16PM +0300, Mika Kuoppala wrote:
> >> Chris Wilson <chris at chris-wilson.co.uk> writes:
> >> 
> >> > Leave the more complicated request dequeueing to the tasklet and instead
> >> > just kick start the tasklet if we detect we are adding the first
> >> > request.
> >> >
> >> > Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> >> > ---
> >> >  drivers/gpu/drm/i915/intel_lrc.c | 28 ++++------------------------
> >> >  1 file changed, 4 insertions(+), 24 deletions(-)
> >> >
> >> > diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
> >> > index 6b49df4316f4..ca52b8e63305 100644
> >> > --- a/drivers/gpu/drm/i915/intel_lrc.c
> >> > +++ b/drivers/gpu/drm/i915/intel_lrc.c
> >> > @@ -609,35 +609,15 @@ static void intel_lrc_irq_handler(unsigned long data)
> >> >  static void execlists_submit_request(struct drm_i915_gem_request *request)
> >> >  {
> >> >  	struct intel_engine_cs *engine = request->engine;
> >> > -	struct drm_i915_gem_request *cursor;
> >> > -	int num_elements = 0;
> >> >  
> >> >  	spin_lock_bh(&engine->execlist_lock);
> >> >  
> >> > -	list_for_each_entry(cursor, &engine->execlist_queue, execlist_link)
> >> > -		if (++num_elements > 2)
> >> > -			break;
> >> > -
> >> > -	if (num_elements > 2) {
> >> > -		struct drm_i915_gem_request *tail_req;
> >> > -
> >> > -		tail_req = list_last_entry(&engine->execlist_queue,
> >> > -					   struct drm_i915_gem_request,
> >> > -					   execlist_link);
> >> > -
> >> > -		if (request->ctx == tail_req->ctx) {
> >> > -			WARN(tail_req->elsp_submitted != 0,
> >> > -				"More than 2 already-submitted reqs queued\n");
> >> > -			list_del(&tail_req->execlist_link);
> >> > -			i915_gem_request_put(tail_req);
> >> > -		}
> >> > -	}
> >> > -
> >> >  	i915_gem_request_get(request);
> >> > -	list_add_tail(&request->execlist_link, &engine->execlist_queue);
> >> >  	request->ctx_hw_id = request->ctx->hw_id;
> >> > -	if (num_elements == 0)
> >> > -		execlists_unqueue(engine);
> >> > +
> >> > +	list_add_tail(&request->execlist_link, &engine->execlist_queue);
> >> > +	if (request->execlist_link.prev == &engine->execlist_queue)
> >> 
> >> Why not list_first_entry()?
> >
> > Ah, because it's not as magic as I thought :)
> >
> > if (list_first_entry(&engine->execlist_queue,
> > 		     struct drm_i915_gem_request,
> > 		     execlist_link) == request)
> >
> > I think is reason enough.
> 
> Agreed. Looked at
> 
> if (list_is_singular(&engine->execlist_queue))
> 
> but that it will add needless empty check.
> 
> If that is abomination, just add comment that we need to resubmit
> if the list was empty.
> 
> Without digging the list internals, the next reader also
> could be more happy with
> 
> head->next == head->prev check.

As a temporary, we can use list_empty() before the add since we are
holding the spinlock.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre