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

[Vidya Sagar]

Hi Bjorn,
Just checking on this thread.
Is there anything else you want me to clarify on?

Thanks,
Vidya Sagar

On 14-03-2024 06:09, Vidya Sagar wrote:
> 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