[PATCH 1/6] PCI: of: Avoid config reads for disabled bridge nodes

[Xilin Wu <sophon@radxa.com>]

The TC9563 PCI power-control driver powers off external downstream ports
whose device tree nodes have status = "disabled".

On the Radxa Dragon Q8B, reading the Vendor ID from one of these
powered-off port functions during PCI enumeration raises an Arm SError
instead of returning an all-ones PCI error response. This prevents the
board from completing boot.

Commit 1a8c251cff20 ("PCI: move OF status = "disabled" detection to
dev->match_driver") made disabled PCI functions remain discoverable so
PCI fixups can still be applied, while suppressing driver binding later.
Preserve that behavior for endpoint functions.

For an unavailable node describing a PCI-to-PCI bridge, however, the
node also represents a subordinate bus that must not be enumerated.
Check the device tree node before reading the Vendor ID and return a PCI
error response without accessing config space when such a bridge is
disabled.

Functions not described by device tree and disabled endpoint functions
remain discoverable.

Signed-off-by: Xilin Wu <sophon@radxa.com>
---
 drivers/pci/of.c    | 27 +++++++++++++++++++++++++--
 drivers/pci/pci.h   |  5 +++++
 drivers/pci/probe.c |  5 +++++
 3 files changed, 35 insertions(+), 2 deletions(-)

diff --git a/drivers/pci/of.c b/drivers/pci/of.c
index a51dff91b196..37e9062065d7 100644
--- a/drivers/pci/of.c
+++ b/drivers/pci/of.c
@@ -24,8 +24,8 @@
  * pci_set_of_node - Find and set device's DT device_node
  * @dev: the PCI device structure to fill
  *
- * Returns 0 on success with of_node set or when no device is described in the
- * DT. Returns -ENODEV if the device is present, but disabled in the DT.
+ * Return: 0 on success with of_node set or when no device is described in the
+ * DT.
  */
 int pci_set_of_node(struct pci_dev *dev)
 {
@@ -46,6 +46,29 @@ int pci_set_of_node(struct pci_dev *dev)
 	return 0;
 }
 
+/**
+ * pci_of_device_skip_config_read - check whether to skip probing a PCI device
+ * @bus: PCI bus to scan
+ * @devfn: device/function number to check
+ *
+ * Return: true only when firmware explicitly describes an unavailable PCI
+ * bridge/port node. Disabled endpoints remain discoverable so fixups still run,
+ * while driver binding is suppressed later in pci_bus_add_device().
+ */
+bool pci_of_device_skip_config_read(struct pci_bus *bus, unsigned int devfn)
+{
+	if (!bus->dev.of_node)
+		return false;
+
+	struct device_node *node __free(device_node) =
+		of_pci_find_child_device(bus->dev.of_node, devfn);
+
+	if (!node || of_device_is_available(node))
+		return false;
+
+	return of_node_is_type(node, "pci");
+}
+
 void pci_release_of_node(struct pci_dev *dev)
 {
 	of_node_put(dev->dev.of_node);
diff --git a/drivers/pci/pci.h b/drivers/pci/pci.h
index ba3c3fddddc2..7cb1d04d947d 100644
--- a/drivers/pci/pci.h
+++ b/drivers/pci/pci.h
@@ -1244,6 +1244,7 @@ u32 of_pci_get_slot_power_limit(struct device_node *node,
 				u8 *slot_power_limit_scale);
 bool of_pci_preserve_config(struct device_node *node);
 int pci_set_of_node(struct pci_dev *dev);
+bool pci_of_device_skip_config_read(struct pci_bus *bus, unsigned int devfn);
 void pci_release_of_node(struct pci_dev *dev);
 void pci_set_bus_of_node(struct pci_bus *bus);
 void pci_release_bus_of_node(struct pci_bus *bus);
@@ -1284,6 +1285,10 @@ static inline bool of_pci_preserve_config(struct device_node *node)
 }
 
 static inline int pci_set_of_node(struct pci_dev *dev) { return 0; }
+static inline bool pci_of_device_skip_config_read(struct pci_bus *bus, unsigned int devfn)
+{
+	return false;
+}
 static inline void pci_release_of_node(struct pci_dev *dev) { }
 static inline void pci_set_bus_of_node(struct pci_bus *bus) { }
 static inline void pci_release_bus_of_node(struct pci_bus *bus) { }
diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
index 27008e2ea5af..ce29bf78a6f7 100644
--- a/drivers/pci/probe.c
+++ b/drivers/pci/probe.c
@@ -2586,6 +2586,11 @@ bool pci_bus_generic_read_dev_vendor_id(struct pci_bus *bus, int devfn, u32 *l,
 bool pci_bus_read_dev_vendor_id(struct pci_bus *bus, int devfn, u32 *l,
 				int timeout)
 {
+	if (pci_of_device_skip_config_read(bus, devfn)) {
+		PCI_SET_ERROR_RESPONSE(l);
+		return false;
+	}
+
 	return pci_bus_generic_read_dev_vendor_id(bus, devfn, l, timeout);
 }
 EXPORT_SYMBOL(pci_bus_read_dev_vendor_id);

-- 
2.55.0