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SubjectRe: [PATCH v5] PCI: Check for PCIe downtraining conditions
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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?

>> +
>> 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);
>

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