Re: [PATCH] PCI: pciehp: Clear LBMS on hot-remove to prevent link speed reduction

[Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>]

On 4/23/24 8:33 PM, Smita Koralahalli wrote:
> Clear Link Bandwidth Management Status (LBMS) if set, on a hot-remove event.
>
> The hot-remove event could result in target link speed reduction if LBMS
> is set, due to a delay in Presence Detect State Change (PDSC) happening
> after a Data Link Layer State Change event (DLLSC).

What is the actual impact? Since this happens during hot-remove,
and there is no device, the link retrain will fail, right?

>
> In reality, PDSC and DLLSC events rarely come in simultaneously. Delay in
> PDSC can sometimes be too late and the slot could have already been
> powered down just by a DLLSC event. And the delayed PDSC could falsely be
> interpreted as an interrupt raised to turn the slot on. This false process
> of powering the slot on, without a link forces the kernel to retrain the
> link if LBMS is set, to a lower speed to restablish the link thereby
> bringing down the link speeds [2].
>
> According to PCIe r6.2 sec 7.5.3.8 [1], it is derived that, LBMS cannot
> be set for an unconnected link and if set, it serves the purpose of

I did not find this detail in the spec. Can you copy paste the lines
in the spec that mentions the case where it cannot set in an
unconnected link? All I see are the cases where it can be set.

> indicating that there is actually a device down an inactive link.
> However, hardware could have already set LBMS when the device was
> connected to the port i.e when the state was DL_Up or DL_Active. Some

Isn't LBMS only set during DL_Down transition ? Why would it be
set during DL_Up?

> hardwares would have even attempted retrain going into recovery mode,
> just before transitioning to DL_Down.
>
> Thus the set LBMS is never cleared and might force software to cause link
> speed drops when there is no link [2].
>
> Dmesg before:
> 	pcieport 0000:20:01.1: pciehp: Slot(59): Link Down
> 	pcieport 0000:20:01.1: pciehp: Slot(59): Card present
> 	pcieport 0000:20:01.1: broken device, retraining non-functional downstream link at 2.5GT/s
> 	pcieport 0000:20:01.1: retraining failed
> 	pcieport 0000:20:01.1: pciehp: Slot(59): No link
>
> Dmesg after:
> 	pcieport 0000:20:01.1: pciehp: Slot(59): Link Down
> 	pcieport 0000:20:01.1: pciehp: Slot(59): Card present
> 	pcieport 0000:20:01.1: pciehp: Slot(59): No link
>
> [1] PCI Express Base Specification Revision 6.2, Jan 25 2024.
>     https://members.pcisig.com/wg/PCI-SIG/document/20590
> [2] Commit a89c82249c37 ("PCI: Work around PCIe link training failures")
>
> Signed-off-by: Smita Koralahalli <Smita.KoralahalliChannabasappa@amd.com>
> ---
> 1. Should be based on top of fixes for link retrain status in
> pcie_wait_for_link_delay()
> https://patchwork.kernel.org/project/linux-pci/list/?series=824858
> https://lore.kernel.org/linux-pci/53b2239b-4a23-a948-a422-4005cbf76148@linux.intel.com/
>
> Without the fixes patch output would be:
> 	pcieport 0000:20:01.1: pciehp: Slot(59): Link Down
> 	pcieport 0000:20:01.1: pciehp: Slot(59): Card present
> 	pcieport 0000:20:01.1: broken device, retraining non-functional downstream link at 2.5GT/s
> 	pcieport 0000:20:01.1: retraining failed
> 	pcieport 0000:20:01.1: pciehp: Slot(59): No device found.
>
> 2. I initially attempted to wait for both events PDSC and DLLSC to happen
> and then turn on the slot.
> Similar to: https://lore.kernel.org/lkml/20190205210701.25387-1-mr.nuke.me@gmail.com/
> but before turning on the slot.
>
> Something like:
> -		ctrl->state = POWERON_STATE;
> -		mutex_unlock(&ctrl->state_lock);
> -		if (present)
> +		if (present && link_active) {
> +			ctrl->state = POWERON_STATE;
> +			mutex_unlock(&ctrl->state_lock);
> 			ctrl_info(ctrl, "Slot(%s): Card present\n",
> 				  slot_name(ctrl));
> -		if (link_active)
> 			ctrl_info(ctrl, "Slot(%s): Link Up\n",
> 				  slot_name(ctrl));
> -		ctrl->request_result = pciehp_enable_slot(ctrl);
> -		break;
> +			ctrl->request_result = pciehp_enable_slot(ctrl);
> +			break;
> +		}
> +		else {
> +			mutex_unlock(&ctrl->state_lock);
> +			break;
> +		}
>
> This would also avoid printing the lines below on a remove event.
> pcieport 0000:20:01.1: pciehp: Slot(59): Card present
> pcieport 0000:20:01.1: pciehp: Slot(59): No link
>
> I understand this would likely be not applicable in places where broken
> devices hardwire PDS to zero and PDSC would never happen. But I'm open to
> making changes if this is more applicable. Because, SW cannot directly
> track the interference of HW in attempting link retrain and setting LBMS.
>
> 3. I tried introducing delay similar to pcie_wait_for_presence() but I
> was not successful in picking the right numbers. Hence hit with the same
> link speed drop.
>
> 4. For some reason I was unable to clear LBMS with:
> 	pcie_capability_clear_word(ctrl->pcie->port, PCI_EXP_LNKSTA,
> 				   PCI_EXP_LNKSTA_LBMS);
> ---
>  drivers/pci/hotplug/pciehp_pci.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/pci/hotplug/pciehp_pci.c b/drivers/pci/hotplug/pciehp_pci.c
> index ad12515a4a12..9155fdfd1d37 100644
> --- a/drivers/pci/hotplug/pciehp_pci.c
> +++ b/drivers/pci/hotplug/pciehp_pci.c
> @@ -92,7 +92,7 @@ void pciehp_unconfigure_device(struct controller *ctrl, bool presence)
>  {
>  	struct pci_dev *dev, *temp;
>  	struct pci_bus *parent = ctrl->pcie->port->subordinate;
> -	u16 command;
> +	u16 command, lnksta;
>  
>  	ctrl_dbg(ctrl, "%s: domain:bus:dev = %04x:%02x:00\n",
>  		 __func__, pci_domain_nr(parent), parent->number);
> @@ -134,4 +134,10 @@ void pciehp_unconfigure_device(struct controller *ctrl, bool presence)
>  	}
>  
>  	pci_unlock_rescan_remove();
> +
> +	/* Clear LBMS on removal */
> +	pcie_capability_read_word(ctrl->pcie->port, PCI_EXP_LNKSTA, &lnksta);
> +	if (lnksta & PCI_EXP_LNKSTA_LBMS)
> +		pcie_capability_write_word(ctrl->pcie->port, PCI_EXP_LNKSTA,
> +					   PCI_EXP_LNKSTA_LBMS);
>  }

-- 
Sathyanarayanan Kuppuswamy
Linux Kernel Developer