cxl/region: Fix 'distance' calculation with passthrough ports

When programming port decode targets, the algorithm wants to ensure that
two devices are compatible to be programmed as peers beneath a given
port. A compatible peer is a target that shares the same dport, and
where that target's interleave position also routes it to the same
dport. Compatibility is determined by the device's interleave position
being >= to distance. For example, if a given dport can only map every
Nth position then positions less than N away from the last target
programmed are incompatible.

The @distance for the host-bridge's cxl_port in a simple dual-ported
host-bridge configuration with 2 direct-attached devices is 1, i.e. An
x2 region divided by 2 dports to reach 2 region targets.

An x4 region under an x2 host-bridge would need 2 intervening switches
where the @distance at the host bridge level is 2 (x4 region divided by
2 switches to reach 4 devices).

However, the distance between peers underneath a single ported
host-bridge is always zero because there is no limit to the number of
devices that can be mapped. In other words, there are no decoders to
program in a passthrough, all descendants are mapped and distance only
starts matters for the intervening descendant ports of the passthrough
port.

Add tracking for the number of dports mapped to a port, and use that to
detect the passthrough case for calculating @distance.

Cc: <[e-mail]>
Reported-by: Bobo WL <[e-mail]>
Reported-by: Jonathan Cameron <[e-mail]>
Link: http://lore.kernel.org/r/[e-mail]
Fixes: 27b3f8d13830 ("cxl/region: Program target lists")
Reviewed-by: Vishal Verma <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Dan Williams <[e-mail]>

drivers/cxl/core/port.c

···
 static int add_dport(struct cxl_port *port, struct cxl_dport *new)
 {
 	struct cxl_dport *dup;
+	int rc;
 
 	device_lock_assert(&port->dev);
 	dup = find_dport(port, new->port_id);
···
 			dev_name(dup->dport));
 		return -EBUSY;
 	}
-	return xa_insert(&port->dports, (unsigned long)new->dport, new,
-			 GFP_KERNEL);
+
+	rc = xa_insert(&port->dports, (unsigned long)new->dport, new,
+		       GFP_KERNEL);
+	if (rc)
+		return rc;
+
+	port->nr_dports++;
+	return 0;
 }
 
 /*

drivers/cxl/core/region.c

···
 	if (cxl_rr->nr_targets_set) {
 		int i, distance;
 
-		distance = p->nr_targets / cxl_rr->nr_targets;
+		/*
+		 * Passthrough ports impose no distance requirements between
+		 * peers
+		 */
+		if (port->nr_dports == 1)
+			distance = 0;
+		else
+			distance = p->nr_targets / cxl_rr->nr_targets;
 		for (i = 0; i < cxl_rr->nr_targets_set; i++)
 			if (ep->dport == cxlsd->target[i]) {
 				rc = check_last_peer(cxled, ep, cxl_rr,

drivers/cxl/cxl.h

···
  * @regions: cxl_region_ref instances, regions mapped by this port
  * @parent_dport: dport that points to this port in the parent
  * @decoder_ida: allocator for decoder ids
+ * @nr_dports: number of entries in @dports
  * @hdm_end: track last allocated HDM decoder instance for allocation ordering
  * @commit_end: cursor to track highest committed decoder for commit ordering
  * @component_reg_phys: component register capability base address (optional)
···
 	struct xarray regions;
 	struct cxl_dport *parent_dport;
 	struct ida decoder_ida;
+	int nr_dports;
 	int hdm_end;
 	int commit_end;
 	resource_size_t component_reg_phys;