drm/i915: Fix possible overflow when recording semaphore states.

semaphore _sync_seqno, _seqno and _mbox are smaller than number of rings.
This optimization is to remove the ring itself from the list and the logic to do that
is at intel_ring_sync_index as below:

/*
 * rcs -> 0 = vcs, 1 = bcs, 2 = vecs, 3 = vcs2;
 * vcs -> 0 = bcs, 1 = vecs, 2 = vcs2, 3 = rcs;
 * bcs -> 0 = vecs, 1 = vcs2. 2 = rcs, 3 = vcs;
 * vecs -> 0 = vcs2, 1 = rcs, 2 = vcs, 3 = bcs;
 * vcs2 -> 0 = rcs, 1 = vcs, 2 = bcs, 3 = vecs;
*/

v2: Skip when from == to (Damien).
v3: avoid computing idx when from == to (Damien).
    use ring == to instead of ring->id == to->id (Damien).
    use continue instead of return (Rodrigo).
v4: avoid all unecessary computation (Damien).
    reduce idx to loop scope (Damien).

Cc: Damien Lespiau <damien.lespiau@intel.com>
Cc: Ben Widawsky <benjamin.widawsky@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_gpu_error.c

@@ -764,7 +764,7 @@ static void gen8_record_semaphore_state(struct drm_i915_private *dev_priv,
 					struct intel_engine_cs *ring,
 					struct drm_i915_error_ring *ering)
 {
-	struct intel_engine_cs *useless;
+	struct intel_engine_cs *to;
 	int i;
 
 	if (!i915_semaphore_is_enabled(dev_priv->dev))
@@ -776,13 +776,20 @@ static void gen8_record_semaphore_state(struct drm_i915_private *dev_priv,
 						 dev_priv->semaphore_obj,
 						 &dev_priv->gtt.base);
 
-	for_each_ring(useless, dev_priv, i) {
-		u16 signal_offset =
-			(GEN8_SIGNAL_OFFSET(ring, i) & PAGE_MASK) / 4;
-		u32 *tmp = error->semaphore_obj->pages[0];
+	for_each_ring(to, dev_priv, i) {
+		int idx;
+		u16 signal_offset;
+		u32 *tmp;
 
-		ering->semaphore_mboxes[i] = tmp[signal_offset];
-		ering->semaphore_seqno[i] = ring->semaphore.sync_seqno[i];
+		if (ring == to)
+			continue;
+
+		signal_offset = (GEN8_SIGNAL_OFFSET(ring, i) & PAGE_MASK) / 4;
+		tmp = error->semaphore_obj->pages[0];
+		idx = intel_ring_sync_index(ring, to);
+
+		ering->semaphore_mboxes[idx] = tmp[signal_offset];
+		ering->semaphore_seqno[idx] = ring->semaphore.sync_seqno[idx];
 	}
 }