linux-kernel - Re: [PATCH] PCI: pciehp: Report degraded links via link bandwidth notification

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Message-ID: <20181129173553.GD178809@google.com>
Date:   Thu, 29 Nov 2018 11:35:53 -0600
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Alexandru Gagniuc <mr.nuke.me@...il.com>
Cc:     austin_bolen@...l.com, alex_gagniuc@...lteam.com,
        keith.busch@...el.com, Shyam_Iyer@...l.com, lukas@...ner.de,
        Mika Westerberg <mika.westerberg@...ux.intel.com>,
        Sinan Kaya <okaya@...eaurora.org>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        Oza Pawandeep <poza@...eaurora.org>, linux-pci@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] PCI: pciehp: Report degraded links via link bandwidth
 notification

On Wed, Nov 28, 2018 at 06:08:24PM -0600, Alexandru Gagniuc wrote:
> A warning is generated when a PCIe device is probed with a degraded
> link, but there was no similar mechanism to warn when the link becomes
> degraded after probing. The Link Bandwidth Notification provides this
> mechanism.
> 
> Use the link bandwidth notification interrupt to detect bandwidth
> changes, and rescan the bandwidth, looking for the weakest point. This
> is the same logic used in probe().

I like the concept of this.  What I don't like is the fact that it's
tied to pciehp, since I don't think the concept of Link Bandwidth
Notification is related to hotplug.  So I think we'll only notice this
for ports that support hotplug.  Maybe it's worth doing it this way
anyway, even if it could be generalized in the future?

> Signed-off-by: Alexandru Gagniuc <mr.nuke.me@...il.com>
> ---
>  drivers/pci/hotplug/pciehp_hpc.c | 35 +++++++++++++++++++++++++++++++-
>  1 file changed, 34 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/pci/hotplug/pciehp_hpc.c b/drivers/pci/hotplug/pciehp_hpc.c
> index 7dd443aea5a5..834672000b59 100644
> --- a/drivers/pci/hotplug/pciehp_hpc.c
> +++ b/drivers/pci/hotplug/pciehp_hpc.c
> @@ -515,7 +515,8 @@ static irqreturn_t pciehp_isr(int irq, void *dev_id)
>  	struct controller *ctrl = (struct controller *)dev_id;
>  	struct pci_dev *pdev = ctrl_dev(ctrl);
>  	struct device *parent = pdev->dev.parent;
> -	u16 status, events;
> +	struct pci_dev *endpoint;
> +	u16 status, events, link_status;
>  
>  	/*
>  	 * Interrupts only occur in D3hot or shallower and only if enabled
> @@ -525,6 +526,17 @@ static irqreturn_t pciehp_isr(int irq, void *dev_id)
>  	    (!(ctrl->slot_ctrl & PCI_EXP_SLTCTL_HPIE) && !pciehp_poll_mode))
>  		return IRQ_NONE;
>  
> +	pcie_capability_read_word(pdev, PCI_EXP_LNKSTA, &link_status);
> +
> +	if (link_status & PCI_EXP_LNKSTA_LBMS) {
> +		if (pdev->subordinate && pdev->subordinate->self)
> +			endpoint = pdev->subordinate->self;
> +		else
> +			endpoint = pdev;
> +		__pcie_print_link_status(endpoint, false);
> +		pcie_capability_write_word(pdev, PCI_EXP_LNKSTA, link_status);
> +	}
> +
>  	/*
>  	 * Keep the port accessible by holding a runtime PM ref on its parent.
>  	 * Defer resume of the parent to the IRQ thread if it's suspended.
> @@ -677,6 +689,24 @@ static int pciehp_poll(void *data)
>  	return 0;
>  }
>  
> +static bool pcie_link_bandwidth_notification_supported(struct controller *ctrl)
> +{
> +	int ret;
> +	u32 cap;
> +
> +	ret = pcie_capability_read_dword(ctrl_dev(ctrl), PCI_EXP_LNKCAP, &cap);
> +	return (ret == PCIBIOS_SUCCESSFUL) && (cap & PCI_EXP_LNKCAP_LBNC);
> +}
> +
> +static void pcie_enable_link_bandwidth_notification(struct controller *ctrl)
> +{
> +	u16 lnk_ctl;
> +
> +	pcie_capability_read_word(ctrl_dev(ctrl), PCI_EXP_LNKCTL, &lnk_ctl);
> +	lnk_ctl |= PCI_EXP_LNKCTL_LBMIE;
> +	pcie_capability_write_word(ctrl_dev(ctrl), PCI_EXP_LNKCTL, lnk_ctl);
> +}
> +
>  static void pcie_enable_notification(struct controller *ctrl)
>  {
>  	u16 cmd, mask;
> @@ -713,6 +743,9 @@ static void pcie_enable_notification(struct controller *ctrl)
>  	pcie_write_cmd_nowait(ctrl, cmd, mask);
>  	ctrl_dbg(ctrl, "%s: SLOTCTRL %x write cmd %x\n", __func__,
>  		 pci_pcie_cap(ctrl->pcie->port) + PCI_EXP_SLTCTL, cmd);
> +
> +	if (pcie_link_bandwidth_notification_supported(ctrl))
> +		pcie_enable_link_bandwidth_notification(ctrl);
>  }
>  
>  static void pcie_disable_notification(struct controller *ctrl)
> -- 
> 2.17.1
>