Re: [PATCH v6 1/9] PCI: Check for PCIe downtraining conditions

[Bjorn Helgaas <helgaas@kernel.org>]

On Mon, Aug 06, 2018 at 06:25:35PM -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.

After this series, there are no callers of pcie_print_link_status(),
which means we *only* print something if a device is capable of more
bandwidth than the fabric can deliver.

ISTR some desire to have this information for NICs even if the device
isn't limited, so I'm just double-checking to make sure the driver
guys are OK with this change.

There are no callers of __pcie_print_link_status() outside the PCI
core, so I would move the declaration from include/linux/pci.h to
drivers/pci/pci.h.

If we agree that we *never* need to print anything unless a device is
constrained by the fabric, I would get rid of the "verbose" flag and
keep everything in pcie_print_link_status().

> Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
> ---
>  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 611adcd9c169..1c8c26dd2cb2 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 c133ccfa002e..d212de231259 100644
> --- a/include/linux/pci.h
> +++ b/include/linux/pci.h
> @@ -1087,6 +1087,7 @@ 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);
> -- 
> 2.17.1
>