Re: [PATCH V2] PCI: Hide SBR from reset_methods if masked by CXL

[Dave Jiang]

On 2/25/26 6:38 AM, Vidya Sagar wrote:
> 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@xxxxxxxxxx>

Reviewed-by: Dave Jiang <dave.jiang@xxxxxxxxx>

> ---
> 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) {