FAILED: Patch "s390/pci: Handle futile config accesses of disabled devices directly" failed to apply to 6.12-stable tree

The patch below does not apply to the 6.12-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

Thanks,
Sasha

------------------ original commit in Linus's tree ------------------

From 84d875e69818bed600edccb09be4a64b84a34a54 Mon Sep 17 00:00:00 2001
From: Niklas Schnelle <[email protected]>
Date: Thu, 8 Jan 2026 16:45:53 +0100
Subject: [PATCH] s390/pci: Handle futile config accesses of disabled devices
directly

On s390 PCI busses and slots with multiple functions may have holes because PCI functions are passed-through by the hypervisor on a per function basis and some functions may be in standby or reserved. This fact is indicated by returning true from the hypervisor_isolated_pci_functions() helper and triggers common code to scan all possible devfn values. Via pci_scan_single_device() this in turn causes config reads for the device and vendor IDs, even for PCI functions which are in standby and thereofore disabled.

So far these futile config reads, as well as potentially writes, which can never succeed were handled by the PCI load/store instructions themselves. This works as the platform just returns an error for a disabled and thus not usable function handle. It does cause spamming of error logs and additional overhead though.

Instead check if the used function handle is enabled in zpci_cfg_load() and zpci_cfg_write() and if not enable directly return -ENODEV. Also refactor zpci_cfg_load() and zpci_cfg_store() slightly to accommodate the new logic while meeting modern kernel style guidelines.

---
 arch/s390/pci/pci.c | 25 +++++++++++++++++--------
 1 file changed, 17 insertions(+), 8 deletions(-)

index 57f3980b98a92..7f44b0644a20e 100644
@@ -231,24 +231,33 @@ int zpci_fmb_disable_device(struct zpci_dev *zdev)
 static int zpci_cfg_load(struct zpci_dev *zdev, int offset, u32 *val, u8 len)
 {
 	u64 req = ZPCI_CREATE_REQ(zdev->fh, ZPCI_PCIAS_CFGSPC, len);
+	int rc = -ENODEV;
 	u64 data;
-	int rc;
+
+	if (!zdev_enabled(zdev))
+		goto out_err;
 
 	rc = __zpci_load(&data, req, offset);
-	if (!rc) {
-		data = le64_to_cpu((__force __le64) data);
-		data >>= (8 - len) * 8;
-		*val = (u32) data;
-	} else
-		*val = 0xffffffff;
+	if (rc)
+		goto out_err;
+	data = le64_to_cpu((__force __le64)data);
+	data >>= (8 - len) * 8;
+	*val = (u32)data;
+	return 0;
+
+out_err:
+	PCI_SET_ERROR_RESPONSE(val);
 	return rc;
 }
 
 static int zpci_cfg_store(struct zpci_dev *zdev, int offset, u32 val, u8 len)
 {
 	u64 req = ZPCI_CREATE_REQ(zdev->fh, ZPCI_PCIAS_CFGSPC, len);
+	int rc = -ENODEV;
 	u64 data = val;
-	int rc;
+
+	if (!zdev_enabled(zdev))
+		return rc;
 
 	data <<= (8 - len) * 8;
 	data = (__force u64) cpu_to_le64(data);

2.51.0