Re: [PATCH v5] PCI: Check for PCIe downtraining conditions

[Jakub Kicinski]

On Mon, 23 Jul 2018 15:03:38 -0500, Alexandru Gagniuc wrote:
> PCIe downtraining happens when both the device and PCIe port are
> capable of a larger bus width or higher speed than negotiated.
> Downtraining might be indicative of other problems in the system, and
> identifying this from userspace is neither intuitive, nor
> straightforward.
> 
> The easiest way to detect this is with pcie_print_link_status(),
> since the bottleneck is usually the link that is downtrained. It's not
> a perfect solution, but it works extremely well in most cases.
> 
> Signed-off-by: Alexandru Gagniuc <mr.nuke.me@xxxxxxxxx>
> ---
> 
> For the sake of review, I've created a __pcie_print_link_status() which
> takes a 'verbose' argument. If we agree want to go this route, and update
> the users of pcie_print_link_status(), I can split this up in two patches.
> I prefer just printing this information in the core functions, and letting
> drivers not have to worry about this. Though there seems to be strong for
> not going that route, so here it goes:

FWIW the networking drivers print PCIe BW because sometimes the network
bandwidth is simply over-provisioned on multi port cards, e.g. 80Gbps
card on a x8 link.

Sorry to bike shed, but currently the networking cards print the info
during probe.  Would it make sense to move your message closer to probe
time?  Rather than when device is added.  If driver structure is
available, we could also consider adding a boolean to struct pci_driver
to indicate if driver wants the verbose message?  This way we avoid
duplicated prints.

I have no objection to current patch, it LGTM.  Just a thought.

>  drivers/pci/pci.c   | 22 ++++++++++++++++++----
>  drivers/pci/probe.c | 21 +++++++++++++++++++++
>  include/linux/pci.h |  1 +
>  3 files changed, 40 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> index 316496e99da9..414ad7b3abdb 100644
> --- a/drivers/pci/pci.c
> +++ b/drivers/pci/pci.c
> @@ -5302,14 +5302,15 @@ u32 pcie_bandwidth_capable(struct pci_dev *dev, enum pci_bus_speed *speed,
>  }
>  
>  /**
> - * pcie_print_link_status - Report the PCI device's link speed and width
> + * __pcie_print_link_status - Report the PCI device's link speed and width
>   * @dev: PCI device to query
> + * @verbose: Be verbose -- print info even when enough bandwidth is available.
>   *
>   * Report the available bandwidth at the device.  If this is less than the
>   * device is capable of, report the device's maximum possible bandwidth and
>   * the upstream link that limits its performance to less than that.
>   */
> -void pcie_print_link_status(struct pci_dev *dev)
> +void __pcie_print_link_status(struct pci_dev *dev, bool verbose)
>  {
>  	enum pcie_link_width width, width_cap;
>  	enum pci_bus_speed speed, speed_cap;
> @@ -5319,11 +5320,11 @@ void pcie_print_link_status(struct pci_dev *dev)
>  	bw_cap = pcie_bandwidth_capable(dev, &speed_cap, &width_cap);
>  	bw_avail = pcie_bandwidth_available(dev, &limiting_dev, &speed, &width);
>  
> -	if (bw_avail >= bw_cap)
> +	if (bw_avail >= bw_cap && verbose)
>  		pci_info(dev, "%u.%03u Gb/s available PCIe bandwidth (%s x%d link)\n",
>  			 bw_cap / 1000, bw_cap % 1000,
>  			 PCIE_SPEED2STR(speed_cap), width_cap);
> -	else
> +	else if (bw_avail < bw_cap)
>  		pci_info(dev, "%u.%03u Gb/s available PCIe bandwidth, limited by %s x%d link at %s (capable of %u.%03u Gb/s with %s x%d link)\n",
>  			 bw_avail / 1000, bw_avail % 1000,
>  			 PCIE_SPEED2STR(speed), width,
> @@ -5331,6 +5332,19 @@ void pcie_print_link_status(struct pci_dev *dev)
>  			 bw_cap / 1000, bw_cap % 1000,
>  			 PCIE_SPEED2STR(speed_cap), width_cap);
>  }
> +
> +/**
> + * pcie_print_link_status - Report the PCI device's link speed and width
> + * @dev: PCI device to query
> + *
> + * Report the available bandwidth at the device.  If this is less than the
> + * device is capable of, report the device's maximum possible bandwidth and
> + * the upstream link that limits its performance to less than that.
> + */
> +void pcie_print_link_status(struct pci_dev *dev)
> +{
> +	__pcie_print_link_status(dev, true);
> +}
>  EXPORT_SYMBOL(pcie_print_link_status);
>  
>  /**
> diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
> index ac876e32de4b..1f7336377c3b 100644
> --- a/drivers/pci/probe.c
> +++ b/drivers/pci/probe.c
> @@ -2205,6 +2205,24 @@ static struct pci_dev *pci_scan_device(struct pci_bus *bus, int devfn)
>  	return dev;
>  }
>  
> +static void pcie_check_upstream_link(struct pci_dev *dev)
> +{
> +	if (!pci_is_pcie(dev))
> +		return;
> +
> +	/* Look from the device up to avoid downstream ports with no devices. */
> +	if ((pci_pcie_type(dev) != PCI_EXP_TYPE_ENDPOINT) &&
> +	    (pci_pcie_type(dev) != PCI_EXP_TYPE_LEG_END) &&
> +	    (pci_pcie_type(dev) != PCI_EXP_TYPE_UPSTREAM))
> +		return;
> +
> +	/* Multi-function PCIe share the same link/status. */
> +	if (PCI_FUNC(dev->devfn) != 0 || dev->is_virtfn)
> +		return;
> +
> +	__pcie_print_link_status(dev, false);
> +}
> +
>  static void pci_init_capabilities(struct pci_dev *dev)
>  {
>  	/* Enhanced Allocation */
> @@ -2240,6 +2258,9 @@ static void pci_init_capabilities(struct pci_dev *dev)
>  	/* Advanced Error Reporting */
>  	pci_aer_init(dev);
>  
> +	/* Check link and detect downtrain errors */
> +	pcie_check_upstream_link(dev);
> +
>  	if (pci_probe_reset_function(dev) == 0)
>  		dev->reset_fn = 1;
>  }
> diff --git a/include/linux/pci.h b/include/linux/pci.h
> index abd5d5e17aee..15bfab8f7a1b 100644
> --- a/include/linux/pci.h
> +++ b/include/linux/pci.h
> @@ -1088,6 +1088,7 @@ int pcie_set_mps(struct pci_dev *dev, int mps);
>  u32 pcie_bandwidth_available(struct pci_dev *dev, struct pci_dev **limiting_dev,
>  			     enum pci_bus_speed *speed,
>  			     enum pcie_link_width *width);
> +void __pcie_print_link_status(struct pci_dev *dev, bool verbose);
>  void pcie_print_link_status(struct pci_dev *dev);
>  int pcie_flr(struct pci_dev *dev);
>  int __pci_reset_function_locked(struct pci_dev *dev);