Re: [PATCH V2] PCI: Clear errors logged in Secondary Status Register

[Vidya Sagar]

On 23-01-2024 04:30, Bjorn Helgaas wrote:
> External email: Use caution opening links or attachments
>
>
> On Tue, Jan 16, 2024 at 08:02:58PM +0530, Vidya Sagar wrote:
> > The enumeration process leaves the 'Received Master Abort' bit set in
> > the Secondary Status Register of the downstream port in the following
> > scenarios.
> >
> > (1) The device connected to the downstream port has ARI capability
> >      and that makes the kernel set the 'ARI Forwarding Enable' bit in
> >      the Device Control 2 Register of the downstream port. This
> >      effectively makes the downstream port forward the configuration
> >      requests targeting the devices downstream of it, even though they
> >      don't exist in reality. It causes the downstream devices return
> >      completions with UR set in the status in turn causing 'Received
> >      Master Abort' bit set.
> >
> >      In contrast, if the downstream device doesn't have ARI capability,
> >      the 'ARI Forwarding Enable' bit in the downstream port is not set
> >      and any configuration requests targeting the downstream devices
> >      that don't exist are terminated (section 6.13 of PCI Express Base
> >      6.0 spec) in the downstream port itself resulting in no change of
> >      the 'Received Master Abort' bit.
> >
> > (2) A PCIe switch is connected to the downstream port and when the
> >      enumeration flow tries to explore the presence of devices that
> >      don't really exist downstream of the switch, the downstream
> >      port receives the completions with UR set causing the 'Received
> >      Master Abort' bit set.
> Are these the only possible ways this error is logged?  I expected
> them to be logged when we enumerate below a Root Port that has nothing
> attached, for example.
In this case, there won't be any TLP sent downstream. I talked about 
this scenario in the
second paragraph of point (1) above.
> Does clearing them in pci_scan_bridge_extend() cover all ways this
> error might be logged during enumeration?  I can't remember whether
> all enumeration goes through this path.
So far in my testing, clearing it in pci_scan_bridge_extend() covers all 
the cases.

> > Clear 'Received Master Abort' bit to keep the bridge device in a clean
> > state post enumeration.
> >
> > Signed-off-by: Vidya Sagar <vidyas@xxxxxxxxxx>
> > ---
> > V2:
> > * Changed commit message based on Bjorn's feedback
> >
> >   drivers/pci/probe.c | 3 +++
> >   1 file changed, 3 insertions(+)
> >
> > diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
> > index 795534589b98..640d2871b061 100644
> > --- a/drivers/pci/probe.c
> > +++ b/drivers/pci/probe.c
> > @@ -1470,6 +1470,9 @@ static int pci_scan_bridge_extend(struct pci_bus *bus, struct pci_dev *dev,
> >        }
> >
> >   out:
> > +     /* Clear errors in the Secondary Status Register */
> > +     pci_write_config_word(dev, PCI_SEC_STATUS, 0xffff);
> > +
> >        pci_write_config_word(dev, PCI_BRIDGE_CONTROL, bctl);
> >
> >        pm_runtime_put(&dev->dev);
> > --
> > 2.25.1