drm/i915: i8xx also doesn't like multiple oustanding pageflips

My i855GM suffers from a 80k/s interrupt storm without this.
So add 2nd gen to the list of things that don't like more than
one outstanding pageflip request.

Furthermore I've changed the busy loop into a ringbuffer wait.
Busy-loops that don't check whether the chip died are simply evil.
And performance should actually improve, because there's usually
a decent amount of rendering queued on the gpu, hopefully rendering
that MI_WAIT into a noop by the time it's executed.

The current code holds dev->struct_mutex while executing this loop,
hence stalling all other gem activity anyway.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: stable@kernel.org
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
[anholt: resolved against conflict]
Signed-off-by: Eric Anholt <eric@anholt.net>

drivers/gpu/drm/i915/intel_display.c

@@ -5098,14 +5098,16 @@ static int intel_crtc_page_flip(struct drm_crtc *crtc,
 	work->pending_flip_obj = obj;
 
 	if (intel_crtc->plane)
-		flip_mask = I915_DISPLAY_PLANE_B_FLIP_PENDING_INTERRUPT;
+		flip_mask = MI_WAIT_FOR_PLANE_B_FLIP;
 	else
-		flip_mask = I915_DISPLAY_PLANE_A_FLIP_PENDING_INTERRUPT;
+		flip_mask = MI_WAIT_FOR_PLANE_A_FLIP;
 
-	/* Wait for any previous flip to finish */
-	if (IS_GEN3(dev))
-		while (I915_READ(ISR) & flip_mask)
-			;
+	if (IS_GEN3(dev) || IS_GEN2(dev)) {
+		BEGIN_LP_RING(2);
+		OUT_RING(MI_WAIT_FOR_EVENT | flip_mask);
+		OUT_RING(0);
+		ADVANCE_LP_RING();
+	}
 
 	/* Offset into the new buffer for cases of shared fbs between CRTCs */
 	offset = obj_priv->gtt_offset;