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

[Vidya Sagar]

On 21/02/26 02:51, Dave Jiang wrote:
> External email: Use caution opening links or attachments
>
>
> On 2/20/26 12:52 PM, 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.
>
> The "Unmask SBR" bit is a toggle bit. It does not give indicator whether the device is capable of SBR or not.

Not sure how is this point relevant here. Can you help me understand?

BTW, what do you mean by "whether the device is capable of SBR or not". My understanding is that each downstream port

must support SBR. The patch I made would ensure that 'bus' entry is not shown under 'reset_methods' if the downstream

port is a CXL capable port and 'Unmask SBR' is '0'.

> The original thought was that if the user is issuing CXL SBR, you know what you are doing and the kernel will set that bit and issue the SBR.
>
> DJ
>
>> 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>
>> ---
>>  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) {