Re: [PATCH v5 5/5] drm/xe: Suppress Surprise Link Down on non-hotplug device

["Tauro, Riana" <riana.tauro@intel.com>]

On 5/12/2026 6:56 PM, Mallesh Koujalagi wrote:
> If the slot is not hotplug capable, pcie_suppress_surprise_link_down()
> masks the Surprise Link Down bit (PCI_ERR_UNC_SURPDN) in the USP's AER
> Uncorrectable Error Mask register before punit_error_handler()
> triggers the cold reset.
Provide more details on why. Avoid function names and macros unless 
necessary.
Make it more human readable.

>
> Signed-off-by: Mallesh Koujalagi <mallesh.koujalagi@intel.com>
> ---
>   drivers/gpu/drm/xe/xe_ras.c | 51 +++++++++++++++++++++++++++++++++++++
>   1 file changed, 51 insertions(+)
>
> diff --git a/drivers/gpu/drm/xe/xe_ras.c b/drivers/gpu/drm/xe/xe_ras.c
> index 604470565bf3..67b4f25370c9 100644
> --- a/drivers/gpu/drm/xe/xe_ras.c
> +++ b/drivers/gpu/drm/xe/xe_ras.c
> @@ -224,8 +224,59 @@ static enum xe_ras_recovery_action handle_core_compute_errors(struct xe_device *
>   	return XE_RAS_RECOVERY_ACTION_RECOVERED;
>   }
>   
> +#ifdef CONFIG_PCIEAER
> +static bool pcie_slot_is_hotplug_capable(struct pci_dev *usp)
> +{
> +	struct pci_dev *root_port = pci_upstream_bridge(usp);
> +	u32 sltcap;
> +
> +	if (!root_port)
> +		return false;
 From the PCIe spec

"If this register is implemented but the Slot Implemented bit is Clear, 
the field behavior of this entire register is undefined."

Please check the slot implemented bit (8) in  PCI Express Capabilities 
Register before reading the slot capability register.

> +
> +	if (pcie_capability_read_dword(root_port, PCI_EXP_SLTCAP, &sltcap))
> +		return false;
> +
> +	return (sltcap & (PCI_EXP_SLTCAP_HPC | PCI_EXP_SLTCAP_PCP)) ==
> +		(PCI_EXP_SLTCAP_HPC | PCI_EXP_SLTCAP_PCP);

Do we need to check power controller here? Isn't hotpluggable denoted by
PCI_EXP_SLTCAP_HPC ?

> +}
> +
> +static void pcie_suppress_surprise_link_down(struct pci_dev *usp)
> +{
> +	u32 aer_uncorr_mask;
> +	u16 aer_cap;
> +
> +	aer_cap = usp->aer_cap;
> +	if (!aer_cap)
> +		return;
add debug log
> +
> +	pci_read_config_dword(usp, aer_cap + PCI_ERR_UNCOR_MASK, &aer_uncorr_mask);
> +	aer_uncorr_mask |= PCI_ERR_UNC_SURPDN;
> +	pci_write_config_dword(usp, aer_cap + PCI_ERR_UNCOR_MASK, aer_uncorr_mask);

save config space?

Thanks
Riana

> +	dev_dbg(&usp->dev, "Non-hotplug slot: Surprise Link Down masked for cold reset\n");
> +}
> +#endif /* CONFIG_PCIEAER */
> +
>   static void punit_error_handler(struct xe_device *xe)
>   {
> +#ifdef CONFIG_PCIEAER
> +	struct pci_dev *pdev = to_pci_dev(xe->drm.dev);
> +	struct pci_dev *vsp, *usp;
> +
> +	/*
> +	 * Device Hierarchy:
> +	 *
> +	 * Root Port --> Upstream Switch Port (USP) --> Virtual Switch Port (VSP) --> SGunit
> +	 *
> +	 * Cold reset power-cycles the slot, dropping the PCIe link. On a non-hotplug
> +	 * slot this triggers a spurious Surprise Link Down AER event on the USP.
> +	 * Suppress it if the slot is not hotplug capable.
> +	 */
> +	vsp = pci_upstream_bridge(pdev);
> +	usp = vsp ? pci_upstream_bridge(vsp) : NULL;
> +
> +	if (usp && !pcie_slot_is_hotplug_capable(usp))
> +		pcie_suppress_surprise_link_down(usp);
> +#endif
>   	xe_device_set_wedged_method(xe, DRM_WEDGE_RECOVERY_COLD_RESET);
>   	xe_device_declare_wedged(xe);
>   }