powerpc/pseries/eeh: Allow zero to be a valid PE configuration address

There's no real reason why zero can't be a valid PE configuration address.
Under qemu each sPAPR PHB (i.e. EEH supporting) has the passed-though
devices on bus zero, so the PE address of bus <dddd>:00 should be zero.

However, all previous versions of Linux will reject that, so Qemu at least
goes out of it's way to avoid it. The Qemu implementation of
ibm,get-config-addr-info2 RTAS has the following comment:

> /*
>  * We always have PE address of form "00BB0001". "BB"
>  * represents the bus number of PE's primary bus.
>  */

So qemu puts a one into the register portion of the PE's config_addr to
avoid it being zero. The whole is pretty silly considering that RTAS will
return a negative error code if it can't map the device's config_addr to a
PE.

This patch fixes Linux to treat zero as a valid PE address. This shouldn't
have any real effects due to the Qemu hack mentioned above. And the fact
that Linux EEH has worked historically on PowerVM means they never pass
through devices on bus zero so we would never see the problem there either.
Signed-off-by: Oliver O'Halloran <oohall@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200918093050.37344-8-oohall@gmail.com

arch/powerpc/platforms/pseries/eeh_pseries.c

@@ -87,21 +87,20 @@ void pseries_pcibios_bus_add_device(struct pci_dev *pdev)
  * pseries_eeh_get_pe_config_addr - Find the pe_config_addr for a device
  * @pdn: pci_dn of the input device
  *
- * Retrieve the assocated config address. Actually, there're 2 RTAS
- * function calls dedicated for the purpose. We need implement
- * it through the new function and then the old one. Besides,
- * you should make sure the config address is figured out from
- * FDT node before calling the function.
+ * The EEH RTAS calls use a tuple consisting of: (buid_hi, buid_lo,
+ * pe_config_addr) as a handle to a given PE. This function finds the
+ * pe_config_addr based on the device's config addr.
  *
- * It's notable that zero'ed return value means invalid PE config
- * address.
+ * Keep in mind that the pe_config_addr *might* be numerically identical to the
+ * device's config addr, but the two are conceptually distinct.
+ *
+ * Returns the pe_config_addr, or a negative error code.
  */
 static int pseries_eeh_get_pe_config_addr(struct pci_dn *pdn)
 {
 	int config_addr = rtas_config_addr(pdn->busno, pdn->devfn, 0);
 	struct pci_controller *phb = pdn->phb;
-	int ret = 0;
-	int rets[3];
+	int ret, rets[3];
 
 	if (ibm_get_config_addr_info2 != RTAS_UNKNOWN_SERVICE) {
 		/*
@@ -112,16 +111,16 @@ static int pseries_eeh_get_pe_config_addr(struct pci_dn *pdn)
 				config_addr, BUID_HI(phb->buid),
 				BUID_LO(phb->buid), 1);
 		if (ret || (rets[0] == 0))
-			return 0;
+			return -ENOENT;
 
-		/* Retrieve the associated PE config address */
+		/* Retrieve the associated PE config address with function 0 */
 		ret = rtas_call(ibm_get_config_addr_info2, 4, 2, rets,
 				config_addr, BUID_HI(phb->buid),
 				BUID_LO(phb->buid), 0);
 		if (ret) {
 			pr_warn("%s: Failed to get address for PHB#%x-PE#%x\n",
 				__func__, phb->global_number, config_addr);
-			return 0;
+			return -ENXIO;
 		}
 
 		return rets[0];
@@ -134,13 +133,20 @@ static int pseries_eeh_get_pe_config_addr(struct pci_dn *pdn)
 		if (ret) {
 			pr_warn("%s: Failed to get address for PHB#%x-PE#%x\n",
 				__func__, phb->global_number, config_addr);
-			return 0;
+			return -ENXIO;
 		}
 
 		return rets[0];
 	}
 
-	return ret;
+	/*
+	 * PAPR does describe a process for finding the pe_config_addr that was
+	 * used before the ibm,get-config-addr-info calls were added. However,
+	 * I haven't found *any* systems that don't have that RTAS call
+	 * implemented. If you happen to find one that needs the old DT based
+	 * process, patches are welcome!
+	 */
+	return -ENOENT;
 }
 
 /**
@@ -417,7 +423,7 @@ void pseries_eeh_init_edev(struct pci_dn *pdn)
 
 	/* first up, find the pe_config_addr for the PE containing the device */
 	addr = pseries_eeh_get_pe_config_addr(pdn);
-	if (addr == 0) {
+	if (addr < 0) {
 		eeh_edev_dbg(edev, "Unable to find pe_config_addr\n");
 		goto err;
 	}
@@ -897,7 +903,7 @@ static int __init eeh_pseries_init(void)
 			config_addr = pseries_eeh_get_pe_config_addr(pdn);
 
 			/* invalid PE config addr */
-			if (config_addr == 0)
+			if (config_addr < 0)
 				continue;
 
 			pseries_eeh_phb_reset(phb, config_addr, EEH_RESET_FUNDAMENTAL);