PCI: Clean up pci_scan_slot()

While determining the next PCI function is factored out of pci_scan_slot()
into next_fn(), the former still handles the first function as a special
case, which duplicates the code from the scan loop.

Furthermore the non-ARI branch of next_fn() is generally hard to understand
and especially the check for multifunction devices is hidden in the
handling of NULL devices for non-contiguous multifunction. It also signals
that no further functions need to be scanned by returning 0 via wraparound
and this is a valid function number.

Improve upon this by transforming the conditions in next_fn() to be easier
to understand.

By changing next_fn() to return -ENODEV instead of 0 when there is no next
function we can then handle the initial function inside the loop and
deduplicate the shared handling. This also makes it more explicit that only
function 0 must exist.

No functional change is intended.

Link: https://lore.kernel.org/r/20220628143100.3228092-2-schnelle@linux.ibm.comSigned-off-by: Niklas Schnelle <schnelle@linux.ibm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Cc: Jan Kiszka <jan.kiszka@siemens.com>

drivers/pci/probe.c

@@ -2579,8 +2579,7 @@ struct pci_dev *pci_scan_single_device(struct pci_bus *bus, int devfn)
 }
 EXPORT_SYMBOL(pci_scan_single_device);
 
-static unsigned int next_fn(struct pci_bus *bus, struct pci_dev *dev,
-			    unsigned int fn)
+static int next_fn(struct pci_bus *bus, struct pci_dev *dev, int fn)
 {
 	int pos;
 	u16 cap = 0;
@@ -2588,24 +2587,26 @@ static unsigned int next_fn(struct pci_bus *bus, struct pci_dev *dev,
 
 	if (pci_ari_enabled(bus)) {
 		if (!dev)
-			return 0;
+			return -ENODEV;
 		pos = pci_find_ext_capability(dev, PCI_EXT_CAP_ID_ARI);
 		if (!pos)
-			return 0;
+			return -ENODEV;
 
 		pci_read_config_word(dev, pos + PCI_ARI_CAP, &cap);
 		next_fn = PCI_ARI_CAP_NFN(cap);
 		if (next_fn <= fn)
-			return 0;	/* protect against malformed list */
+			return -ENODEV;	/* protect against malformed list */
 
 		return next_fn;
 	}
 
-	/* dev may be NULL for non-contiguous multifunction devices */
-	if (!dev || dev->multifunction)
-		return (fn + 1) % 8;
+	if (fn >= 7)
+		return -ENODEV;
+	/* only multifunction devices may have more functions */
+	if (dev && !dev->multifunction)
+		return -ENODEV;
 
-	return 0;
+	return fn + 1;
 }
 
 static int only_one_child(struct pci_bus *bus)
@@ -2643,26 +2644,25 @@ static int only_one_child(struct pci_bus *bus)
  */
 int pci_scan_slot(struct pci_bus *bus, int devfn)
 {
-	unsigned int fn, nr = 0;
 	struct pci_dev *dev;
+	int fn = 0, nr = 0;
 
 	if (only_one_child(bus) && (devfn > 0))
 		return 0; /* Already scanned the entire slot */
 
-	dev = pci_scan_single_device(bus, devfn);
-	if (!dev)
-		return 0;
-	if (!pci_dev_is_added(dev))
-		nr++;
-
-	for (fn = next_fn(bus, dev, 0); fn > 0; fn = next_fn(bus, dev, fn)) {
+	do {
 		dev = pci_scan_single_device(bus, devfn + fn);
 		if (dev) {
 			if (!pci_dev_is_added(dev))
 				nr++;
-			dev->multifunction = 1;
+			if (fn > 0)
+				dev->multifunction = 1;
+		} else if (fn == 0) {
+			/* function 0 is required */
+			break;
 		}
-	}
+		fn = next_fn(bus, dev, fn);
+	} while (fn >= 0);
 
 	/* Only one slot has PCIe device */
 	if (bus->self && nr)