[POWERPC] Simplify rtas_change_msi() error semantics

Currently rtas_change_msi() returns either the error code from RTAS, or if
the RTAS call succeeded the number of irqs that were configured by RTAS.
This makes checking the return value more complicated than it needs to be.

Instead, have rtas_change_msi() check that the number of irqs configured by
RTAS is equal to what we requested - and return an error otherwise. This makes
the return semantics match the usual 0 for success, something else for error.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>

arch/powerpc/platforms/pseries/msi.c

@@ -70,11 +70,15 @@ static int rtas_change_msi(struct pci_dn *pdn, u32 func, u32 num_irqs)
 		seq_num = rtas_ret[1];
 	} while (rtas_busy_delay(rc));
 
-	if (rc == 0) /* Success */
-		rc = rtas_ret[0];
+	/*
+	 * If the RTAS call succeeded, check the number of irqs is actually
+	 * what we asked for. If not, return an error.
+	 */
+	if (rc == 0 && rtas_ret[0] != num_irqs)
+		rc = -ENOSPC;
 
-	pr_debug("rtas_msi: ibm,change_msi(func=%d,num=%d) = (%d)\n",
-		 func, num_irqs, rc);
+	pr_debug("rtas_msi: ibm,change_msi(func=%d,num=%d), got %d rc = %d\n",
+		 func, num_irqs, rtas_ret[0], rc);
 
 	return rc;
 }
@@ -87,7 +91,7 @@ static void rtas_disable_msi(struct pci_dev *pdev)
 	if (!pdn)
 		return;
 
-	if (rtas_change_msi(pdn, RTAS_CHANGE_FN, 0) != 0)
+	if (rtas_change_msi(pdn, RTAS_CHANGE_FN, 0))
 		pr_debug("rtas_msi: Setting MSIs to 0 failed!\n");
 }
 
@@ -180,14 +184,14 @@ static int rtas_setup_msi_irqs(struct pci_dev *pdev, int nvec, int type)
 	if (type == PCI_CAP_ID_MSI) {
 		rc = rtas_change_msi(pdn, RTAS_CHANGE_MSI_FN, nvec);
 
-		if (rc != nvec) {
+		if (rc) {
 			pr_debug("rtas_msi: trying the old firmware call.\n");
 			rc = rtas_change_msi(pdn, RTAS_CHANGE_FN, nvec);
 		}
 	} else
 		rc = rtas_change_msi(pdn, RTAS_CHANGE_MSIX_FN, nvec);
 
-	if (rc != nvec) {
+	if (rc) {
 		pr_debug("rtas_msi: rtas_change_msi() failed\n");
 		return rc;
 	}