[PATCH V1] PCI: Hide SBR from reset_methods if masked by CXL

[Vidya Sagar <vidyas@nvidia.com>]

The CXL specification (e.g., CXL r3.1 v1.0, sec 8.1.5.2) defines
the "Unmask SBR" bit in the Port Control Extensions Register.
When this bit is 0 (default), asserting the Secondary Bus Reset (SBR) bit
in the Bridge Control register has no effect on the downstream bus.

Currently, the Linux PCI core checks this condition in
pci_reset_bus_function(). If SBR is masked, it returns -ENOTTY during the
execution of the reset. However, during the probe phase (when probe=true),
the function currently returns 0. This 0 return value incorrectly signals
to the PCI subsystem that SBR is a viable reset method for the device.

As a result, 'bus' is listed in the device's
/sys/bus/pci/devices/.../reset_methods attribute, even though the hardware
is incapable of performing it. If a user attempts to write bus to reset
method or triggers a reset that falls back to SBR, the operation fails
with: "bash: echo: write error: Inappropriate ioctl for device" error.

This patch modifies pci_reset_bus_function() to return -ENOTTY immediately
if cxl_sbr_masked() is true, regardless of the probe argument. This
ensures that 'bus' is not advertised in reset_methods when the hardware
prevents it, improving clarity for users and aligning the sysfs capability
report with actual hardware behavior.

Signed-off-by: Vidya Sagar <vidyas@nvidia.com>
---
 drivers/pci/pci.c | 6 +-----
 1 file changed, 1 insertion(+), 5 deletions(-)

diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
index f3244630bfd0..57e24300d1c7 100644
--- a/drivers/pci/pci.c
+++ b/drivers/pci/pci.c
@@ -4915,12 +4915,8 @@ static int pci_reset_bus_function(struct pci_dev *dev, bool probe)
 	 * If "dev" is below a CXL port that has SBR control masked, SBR
 	 * won't do anything, so return error.
 	 */
-	if (bridge && cxl_sbr_masked(bridge)) {
-		if (probe)
-			return 0;
-
+	if (bridge && cxl_sbr_masked(bridge))
 		return -ENOTTY;
-	}
 
 	rc = pci_dev_reset_iommu_prepare(dev);
 	if (rc) {
-- 
2.25.1