[PATCH v6 02/17] PCI/CXL: Probe the underlying bus reset in cxl_reset_bus_function()

[Nicolin Chen <nicolinc@nvidia.com>]

cxl_reset_bus_function() reports "supported" to a probe after checking only
that the upstream bridge carries a CXL port DVSEC. The underlying bus reset
can still be unavailable, e.g. on a bus shared with other devices, so both
the reset_methods[] array and the reset_method sysfs node end up listing a
"cxl_bus" that is guaranteed to fail with -ENOTTY when it is attempted.

Probe the underlying pci_dev_reset_slot_function() and then, if it is not
applicable, pci_parent_bus_reset(). These are the same two checks that the
actual reset runs, so a shared-bus CXL device no longer advertises a method
that can never succeed.

Probing via pci_reset_bus_function() would not work: its cxl_sbr_masked()
check rejects every CXL port with a masked SBR, while the do-reset path in
this function unmasks the SBR before resetting. Such a port would wrongly
probe as unsupported.

Move the port control register read above the probe branch as well, so that
probe runs the same checks. That read asserts no reset, and a failure means
the device cannot be reset this way, which is a probe-time answer.

Also pass an explicit PCI_RESET_DO_RESET at the do-reset call site, since
probe is always false at that point.

Fixes: 53c49b6e6dd2e ("PCI/CXL: Add 'cxl_bus' reset method for devices below CXL Ports")
Assisted-by: LLM
Signed-off-by: Nicolin Chen <nicolinc@nvidia.com>
---
 drivers/pci/pci.c | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
index 1eba412b7e0de..a6e0ecfd207a7 100644
--- a/drivers/pci/pci.c
+++ b/drivers/pci/pci.c
@@ -5029,13 +5029,21 @@ static int cxl_reset_bus_function(struct pci_dev *dev, bool probe)
 	if (!dvsec)
 		return -ENOTTY;
 
-	if (probe)
-		return 0;
-
 	rc = pci_read_config_word(bridge, dvsec + PCI_DVSEC_CXL_PORT_CTL, &reg);
 	if (rc)
 		return -ENOTTY;
 
+	/*
+	 * Do not probe via pci_reset_bus_function(), which would reject a
+	 * masked SBR that the do-reset path below unmasks before resetting.
+	 */
+	if (probe) {
+		rc = pci_dev_reset_slot_function(dev, PCI_RESET_PROBE);
+		if (rc != -ENOTTY)
+			return rc;
+		return pci_parent_bus_reset(dev, PCI_RESET_PROBE);
+	}
+
 	rc = pci_dev_reset_iommu_prepare(dev);
 	if (rc) {
 		pci_err(dev, "failed to stop IOMMU for a PCI reset: %d\n", rc);
@@ -5050,7 +5058,7 @@ static int cxl_reset_bus_function(struct pci_dev *dev, bool probe)
 				      val);
 	}
 
-	rc = pci_reset_bus_function(dev, probe);
+	rc = pci_reset_bus_function(dev, PCI_RESET_DO_RESET);
 
 	if (reg != val)
 		pci_write_config_word(bridge, dvsec + PCI_DVSEC_CXL_PORT_CTL,
-- 
2.43.0