PCI/MSI: Allow arch code to return the number of MSI-X available

There is code in msix_capability_init() which, when the requested number
of MSI-X couldn't be allocated, calculates how many MSI-X /could/ be
allocated and returns that to the driver. That allows the driver to then
make a second request, with a number of MSIs that should succeed.

The current code requires the arch code to setup as many msi_descs as it
can, and then return to the generic code. On some platforms the arch
code may already know how many MSI-X it can allocate, before it sets up
any of the msi_descs.

So change the logic such that if the arch code returns a positive error
code, that is taken to be the number of MSI-X that could be allocated.
If the error code is negative we still calculate the number available
using the old method.

Because it's a little subtle, make sure the error return code from
arch_setup_msi_irq() is always negative. That way only implementations
of arch_setup_msi_irqs() need to be careful about returning a positive
error code.
Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>

drivers/pci/msi.c

@@ -42,8 +42,10 @@ int arch_setup_msi_irqs(struct pci_dev *dev, int nvec, int type)
 
 	list_for_each_entry(entry, &dev->msi_list, list) {
 		ret = arch_setup_msi_irq(dev, entry);
-		if (ret)
+		if (ret < 0)
 			return ret;
+		if (ret > 0)
+			return -ENOSPC;
 	}
 
 	return 0;
@@ -487,7 +489,9 @@ static int msix_capability_init(struct pci_dev *dev,
 	}
 
 	ret = arch_setup_msi_irqs(dev, nvec, PCI_CAP_ID_MSIX);
-	if (ret) {
+	if (ret < 0) {
+		/* If we had some success report the number of irqs
+		 * we succeeded in setting up. */
 		int avail = 0;
 		list_for_each_entry(entry, &dev->msi_list, list) {
 			if (entry->irq != 0) {
@@ -495,14 +499,13 @@ static int msix_capability_init(struct pci_dev *dev,
 			}
 		}
 
-		msi_free_irqs(dev);
+		if (avail != 0)
+			ret = avail;
+	}
 
-		/* If we had some success report the number of irqs
-		 * we succeeded in setting up.
-		 */
-		if (avail == 0)
-			avail = ret;
-		return avail;
+	if (ret) {
+		msi_free_irqs(dev);
+		return ret;
 	}
 
 	i = 0;