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

[Tal Gilboa <talgi@mellanox.com>]

On 7/24/2018 12:52 AM, Tal Gilboa wrote:
> On 7/24/2018 12:01 AM, Jakub Kicinski wrote:
>> 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@gmail.com>
>>> ---
>>>
>>> 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.
> 
> I don't see the reason for having two functions. What's the problem with 
> adding the verbose argument to the original function?
> 
>>
>>>   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);
> 
> This is called for every PCIe device right? Won't there be a duplicated 
> print in case a device loads with lower PCIe bandwidth than needed?

Alex, can you comment on this please?

> 
>>> +
>>>       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);
>>