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
systemd/systemd@0035597
"udev: net_id - export PCI hotplug slot names" on 2012/11/26.
At the same time on the kernel side we got
torvalds/linux@bb74ac2
"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 ?).

(cherry picked from commit 0a4ecc54cb9f2d3418b970c51bfadb69c34ae9eb)

Resolves: RHEL-50103

man/systemd.net-naming-scheme.xml

@@ -510,9 +510,10 @@
           to distinguish between devices. However, name conflict can occur if these devices are not
           children of the same PCI bridge, e.g. there are multiple PCI bridges in the same slot.
           </para>
+
+          <para>PCI slot number is now read from <constant>firmware_node/sun</constant> sysfs file.</para>
           </listitem>
         </varlistentry>
-
       </variablelist>
 
     <para>By default <constant>rhel-9.0</constant> is used.</para>
@@ -666,7 +667,7 @@ ID_NET_NAME_ONBOARD_LABEL=Ethernet Port 1
     </example>
 
     <example>
-      <title>PCI Ethernet card in hotplug slot with firmware index number</title>
+      <title>PCI Ethernet card in slot with firmware index number</title>
 
       <programlisting># /sys/devices/pci0000:00/0000:00:1c.3/0000:05:00.0/net/ens1
 ID_NET_NAME_MAC=enx000000000466

src/shared/netif-naming-scheme.h

@@ -42,6 +42,7 @@ typedef enum NamingSchemeFlags {
                                                      * This is disabled since rhel-9.5, as it seems not to work at least for some setups. See upstream issue #28929. */
         NAMING_DEVICETREE_ALIASES        = 1 << 15, /* Generate names from devicetree aliases */
         NAMING_SR_IOV_R                  = 1 << 17, /* Use "r" suffix for SR-IOV VF representors */
+        NAMING_FIRMWARE_NODE_SUN         = 1 << 18, /* Use firmware_node/sun to get PCI slot number */
 
         /* And now the masks that combine the features above */
         NAMING_V238 = 0,
@@ -73,7 +74,7 @@ typedef enum NamingSchemeFlags {
         NAMING_RHEL_9_2 = NAMING_RHEL_9_0,
         NAMING_RHEL_9_3 = NAMING_RHEL_9_0 | NAMING_SR_IOV_R,
         NAMING_RHEL_9_4 = NAMING_RHEL_9_3,
-        NAMING_RHEL_9_5 = NAMING_RHEL_9_4 & ~NAMING_BRIDGE_MULTIFUNCTION_SLOT,
+        NAMING_RHEL_9_5 = (NAMING_RHEL_9_4 & ~NAMING_BRIDGE_MULTIFUNCTION_SLOT) | NAMING_FIRMWARE_NODE_SUN,
 
         EXTRA_NET_NAMING_SCHEMES
 

src/udev/udev-builtin-net_id.c

@@ -442,6 +442,51 @@ 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 dev_pci_slot(sd_device *dev, const LinkInfo *info, NetNames *names) {
         const char *sysname, *attr;
         unsigned domain, bus, slot, func;
@@ -517,13 +562,20 @@ static int dev_pci_slot(sd_device *dev, const LinkInfo *info, NetNames *names) {
                          domain, bus, slot, func, strempty(info->phys_port_name), dev_port,
                          special_glyph(SPECIAL_GLYPH_ARROW_RIGHT), empty_to_na(names->pci_path));
 
-        r = pci_get_hotplug_slot(names->pcidev, &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 = 0;
+        if (naming_scheme_has(NAMING_FIRMWARE_NODE_SUN))
+                r = pci_get_slot_from_firmware_node_sun(names->pcidev, &hotplug_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(names->pcidev, &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 = 0;
+        }
 
         s = names->pci_slot;
         l = sizeof(names->pci_slot);