[PATCH V2] 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>
---
v2:
* Before deciding to hide 'bus' reset method, add an extra check to make sure
  that the link is indeed operating in the CXL mode and not in PCIe mode as
  the spec clearly says that a '0' in 'Unmask SBR' doesn't have any effect if
  the link is not operating in the CXL mode.

 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..a176566ba56f 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 && bridge->is_cxl && cxl_sbr_masked(bridge))
 		return -ENOTTY;
-	}
 
 	rc = pci_dev_reset_iommu_prepare(dev);
 	if (rc) {
-- 
2.25.1