udev-builtin-net_id: use firmware_node/sun for ID_NET_NAME_SLOT

pci_get_hotplug_slot() has the following limitations:
- if slots are not hotpluggable, they are not in /sys/bus/pci/slots.
- the address at /sys/bus/pci/slots/X/addr doesn't contains the function part,
  so on some system, 2 different slots with different _SUN end up with the same
  hotplug_slot, leading to naming conflicts.
- it tries all parent devices until it finds a slot number, which is incorrect,
  and what led to NAMING_BRIDGE_MULTIFUNCTION_SLOT being disabled.

The use of PCI hotplug to find the slot (ACPI _SUN) was introduced in
https://github.com/systemd/systemd/commit/0035597a30d120f70df2dd7da3d6128fb8ba6051
"udev: net_id - export PCI hotplug slot names" on 2012/11/26.
At the same time on the kernel side we got
https://github.com/torvalds/linux/commit/bb74ac23b10820d8722c3e1f4add9ef59e703f63
"ACPI: create _SUN sysfs file" on 2012/11/16.

Using PCI hotplug was the only way at the time, but now 12 years later we can use
firmware_node/sun sysfs file.
Looking at a small selection of server HW, for HPE (Gen10 DL325), the _SUN is attached
to the NIC device, whereas for Dell (R640/R6515/R6615) and Cisco (UCSC-C220-M5SX),
the _SUN is on the first parent pcieport.

We still fallback to pci_get_hotplug_slot() to handle the s390 case and
maybe some other coner cases (_SUN on grand parent device that is not a
bridge ?).

src/udev/udev-builtin-net_id.c

@@ -566,6 +566,51 @@ static int pci_get_hotplug_slot(sd_device *dev, uint32_t *ret) {
         return -ENOENT;
 }
 
+static int get_device_firmware_node_sun(sd_device *dev, uint32_t *ret) {
+        const char *attr;
+        int r;
+
+        assert(dev);
+        assert(ret);
+
+        r = device_get_sysattr_value_filtered(dev, "firmware_node/sun", &attr);
+        if (r < 0)
+                return log_device_debug_errno(dev, r, "Failed to read firmware_node/sun, ignoring: %m");
+
+        r = safe_atou32(attr, ret);
+        if (r < 0)
+                return log_device_warning_errno(dev, r, "Failed to parse firmware_node/sun '%s', ignoring: %m", attr);
+
+        return 0;
+}
+
+static int pci_get_slot_from_firmware_node_sun(sd_device *dev, uint32_t *ret) {
+        int r;
+        sd_device *slot_dev;
+
+        assert(dev);
+        assert(ret);
+
+        /* Try getting the ACPI _SUN for the device */
+        if (get_device_firmware_node_sun(dev, ret) >= 0)
+                return 0;
+
+        r = sd_device_get_parent_with_subsystem_devtype(dev, "pci", NULL, &slot_dev);
+        if (r < 0)
+                return log_device_debug_errno(dev, r, "Failed to find pci parent, ignoring: %m");
+
+        if (is_pci_bridge(slot_dev) && is_pci_multifunction(dev) <= 0)
+                return log_device_debug_errno(dev, SYNTHETIC_ERRNO(ESTALE),
+                                              "Not using slot information because the parent pcieport "
+                                              "is a bridge and the PCI device is not multifunction.");
+
+        /* Try getting the ACPI _SUN from the parent pcieport */
+        if (get_device_firmware_node_sun(slot_dev, ret) >= 0)
+                return 0;
+
+        return -ENOENT;
+}
+
 static int get_pci_slot_specifiers(
                 sd_device *dev,
                 char **ret_domain,
@@ -616,7 +661,7 @@ static int get_pci_slot_specifiers(
 
 static int names_pci_slot(sd_device *dev, sd_device *pci_dev, const char *prefix, const char *suffix, EventMode mode) {
         _cleanup_free_ char *domain = NULL, *bus_and_slot = NULL, *func = NULL, *port = NULL;
-        uint32_t hotplug_slot = 0;  /* avoid false maybe-uninitialized warning */
+        uint32_t slot = 0;  /* avoid false maybe-uninitialized warning */
         char str[ALTIFNAMSIZ];
         int r;
 
@@ -641,20 +686,27 @@ static int names_pci_slot(sd_device *dev, sd_device *pci_dev, const char *prefix
                          strna(domain), bus_and_slot, strna(func), strna(port),
                          special_glyph(SPECIAL_GLYPH_ARROW_RIGHT), empty_to_na(str));
 
-        r = pci_get_hotplug_slot(pci_dev, &hotplug_slot);
-        if (r < 0)
-                return r;
-        if (r > 0)
-                /* If the hotplug slot is found through the function ID, then drop the domain from the name.
-                 * See comments in parse_hotplug_slot_from_function_id(). */
-                domain = mfree(domain);
+        if (naming_scheme_has(NAMING_FIRMWARE_NODE_SUN))
+                r = pci_get_slot_from_firmware_node_sun(pci_dev, &slot);
+        else
+                r = -1;
+        /* If we don't find a slot using firmware_node/sun, fallback to hotplug_slot */
+        if (r < 0) {
+                r = pci_get_hotplug_slot(pci_dev, &slot);
+                if (r < 0)
+                        return r;
+                if (r > 0)
+                        /* If the hotplug slot is found through the function ID, then drop the domain from the name.
+                        * See comments in parse_hotplug_slot_from_function_id(). */
+                        domain = mfree(domain);
+        }
 
         if (snprintf_ok(str, sizeof str, "%s%ss%"PRIu32"%s%s%s",
-                        prefix, strempty(domain), hotplug_slot, strempty(func), strempty(port), strempty(suffix)))
+                        prefix, strempty(domain), slot, strempty(func), strempty(port), strempty(suffix)))
                 udev_builtin_add_property(dev, mode, "ID_NET_NAME_SLOT", str);
 
         log_device_debug(dev, "Slot identifier: domain=%s slot=%"PRIu32" func=%s port=%s %s %s",
-                         strna(domain), hotplug_slot, strna(func), strna(port),
+                         strna(domain), slot, strna(func), strna(port),
                          special_glyph(SPECIAL_GLYPH_ARROW_RIGHT), empty_to_na(str));
 
         return 0;