drm/nouveau/fifo/gk104-: better detection of chid when parsing engine status

The previous commit simply changes the interface, but should result in
the same behaviour as previously.  This commit has been split out from
it as it can result in a different channel being selected.
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/nvkm/engine/fifo/gk104.c

@@ -48,6 +48,7 @@ static void
 gk104_fifo_engine_status(struct gk104_fifo *fifo, int engn,
 			 struct gk104_fifo_engine_status *status)
 {
+	struct nvkm_engine *engine = fifo->engine[engn].engine;
 	struct nvkm_subdev *subdev = &fifo->base.engine.subdev;
 	struct nvkm_device *device = subdev->device;
 	u32 stat = nvkm_rd32(device, 0x002640 + (engn * 0x08));
@@ -61,7 +62,24 @@ gk104_fifo_engine_status(struct gk104_fifo *fifo, int engn,
 	status->load     = !!(stat & 0x00002000);
 	status->prev.tsg = !!(stat & 0x00001000);
 	status->prev.id  =   (stat & 0x00000fff);
-	status->chan     = status->load ? &status->next : &status->prev;
+	status->chan     = NULL;
+
+	if (status->busy && status->chsw) {
+		if (status->load && status->save) {
+			if (engine && nvkm_engine_chsw_load(engine))
+				status->chan = &status->next;
+			else
+				status->chan = &status->prev;
+		} else
+		if (status->load) {
+			status->chan = &status->next;
+		} else {
+			status->chan = &status->prev;
+		}
+	} else
+	if (status->load) {
+		status->chan = &status->prev;
+	}
 
 	nvkm_debug(subdev, "engine %02d: busy %d faulted %d chsw %d "
 			   "save %d load %d %sid %d%s-> %sid %d%s\n",