drm/i915/gem: Honour O_NONBLOCK before throttling execbuf submissions

Check the user's flags on the struct file before deciding whether or not
to stall before submitting a request. This allows us to reasonably
cheaply honour O_NONBLOCK without checking at more critical phases
during request submission.

Suggested-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Steve Carbonari <steven.carbonari@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200225192206.1107336-3-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c

@@ -2327,15 +2327,22 @@ static int __eb_pin_engine(struct i915_execbuffer *eb, struct intel_context *ce)
 	intel_context_timeline_unlock(tl);
 
 	if (rq) {
-		if (i915_request_wait(rq,
-				      I915_WAIT_INTERRUPTIBLE,
-				      MAX_SCHEDULE_TIMEOUT) < 0) {
-			i915_request_put(rq);
-			err = -EINTR;
-			goto err_exit;
-		}
+		bool nonblock = eb->file->filp->f_flags & O_NONBLOCK;
+		long timeout;
+
+		timeout = MAX_SCHEDULE_TIMEOUT;
+		if (nonblock)
+			timeout = 0;
 
+		timeout = i915_request_wait(rq,
+					    I915_WAIT_INTERRUPTIBLE,
+					    timeout);
 		i915_request_put(rq);
+
+		if (timeout < 0) {
+			err = nonblock ? -EWOULDBLOCK : timeout;
+			goto err_exit;
+		}
 	}
 
 	eb->engine = ce->engine;