Re: [PATCH v2] PCI : Fix pcie_flag_reg in set_pcie_port_type

[Bjorn Helgaas]

[+cc Mika, Andy]

On Fri, Dec 13, 2024 at 12:32:41PM +0530, Gowthami Thiagarajan wrote:
> When an invalid PCIe topology is detected, the set_pcie_port_type function 
> does not set the port type correctly. This issue can occur in 
> configurations such as:
> 
> 	Root Port ---> Downstream Port ---> Root Port
> 
> In such cases, the topology is identified as invalid and due to the
> incorrect port type setting, the extended configuration space of the
> child device becomes inaccessible.

>From reading the code, it looks like the underlying problem is
components that advertise the wrong PCIe Device/Port Type.

set_pcie_port_type() already detects that incorrect Port Type and
tries to correct it, but it puts the corrected type in bits [3:0]
instead of [7:4] where it belongs.

This looks like a bug from ca78410403dd ("PCI: Get rid of
dev->has_secondary_link flag").  If so, we should add a Fixes: tag for
that and possibly a stable tag.

This looks like a clear bug.  What system tripped over this?  It's
useful to have bread crumbs like that in case we see other issues
caused by hardware/firmware defects like this.

> Signed-off-by: Gowthami Thiagarajan <gthiagarajan@xxxxxxxxxxx>
> ---
> v1->v2:
> 	Updated commit description
> 
>  drivers/pci/probe.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
> index 4f68414c3086..263ec21451d9 100644
> --- a/drivers/pci/probe.c
> +++ b/drivers/pci/probe.c
> @@ -1596,7 +1596,7 @@ void set_pcie_port_type(struct pci_dev *pdev)
>  		if (pcie_downstream_port(parent)) {
>  			pci_info(pdev, "claims to be downstream port but is acting as upstream port, correcting type\n");
>  			pdev->pcie_flags_reg &= ~PCI_EXP_FLAGS_TYPE;
> -			pdev->pcie_flags_reg |= PCI_EXP_TYPE_UPSTREAM;
> +			pdev->pcie_flags_reg |= PCI_EXP_TYPE_UPSTREAM << 4;
>  		}
>  	} else if (type == PCI_EXP_TYPE_UPSTREAM) {
>  		/*
> @@ -1607,7 +1607,7 @@ void set_pcie_port_type(struct pci_dev *pdev)
>  		if (pci_pcie_type(parent) == PCI_EXP_TYPE_UPSTREAM) {
>  			pci_info(pdev, "claims to be upstream port but is acting as downstream port, correcting type\n");
>  			pdev->pcie_flags_reg &= ~PCI_EXP_FLAGS_TYPE;
> -			pdev->pcie_flags_reg |= PCI_EXP_TYPE_DOWNSTREAM;
> +			pdev->pcie_flags_reg |= PCI_EXP_TYPE_DOWNSTREAM << 4;
>  		}
>  	}
>  }
> -- 
> 2.25.1
>