nvml: recognize and keep NVSwitches

Set their subtype as NVSwitch so that the core doesn't remove
these bridges without any child.

Signed-off-by: Brice Goglin <Brice.Goglin@inria.fr>

hwloc/topology-nvml.c

@@ -80,7 +80,11 @@ hwloc__nvml_get_peer_obj_by_pci(struct hwloc_topology *topology, hwloc_obj_t gpu
     return obj;
   }
   case 0x10de: {
-    /* NVIDIA NVSwitch, return the PCI object, we don't have anything better. */
+    /* NVIDIA NVSwitch, return the PCI object, we don't have anything better.
+     * Mark it as subtype NVSwitch so that the core doesn't remove it.
+     */
+    if (!obj->subtype)
+      obj->subtype = strdup("NVSwitch");
     return obj;
   }
   default: {

hwloc/topology.c

@@ -2345,11 +2345,13 @@ hwloc__filter_bridges(hwloc_topology_t topology, hwloc_obj_t root, unsigned dept
 
     /* remove bridges that have no child,
      * and pci-to-non-pci bridges (pcidev) that no child either.
+     * keep NVSwitch since they may be used in NVLink matrices.
      */
     if (filter == HWLOC_TYPE_FILTER_KEEP_IMPORTANT
 	&& !child->io_first_child
         && (child->type == HWLOC_OBJ_BRIDGE
-            || (child->type == HWLOC_OBJ_PCI_DEVICE && (child->attr->pcidev.class_id >> 8) == 0x06))) {
+            || (child->type == HWLOC_OBJ_PCI_DEVICE && (child->attr->pcidev.class_id >> 8) == 0x06
+                && (!child->subtype || strcmp(child->subtype, "NVSwitch"))))) {
       unlink_and_free_single_object(pchild);
       topology->modified = 1;
     }