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SubjectRe: [PATCH v5] PCI: Check for PCIe downtraining conditions
On 7/24/2018 2:59 AM, Alex G. wrote:
> On 07/23/2018 05:14 PM, Jakub Kicinski wrote:
>> On Tue, 24 Jul 2018 00:52:22 +0300, 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 <>
>>>>> ---
>>>>> 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?
>> IMHO it's reasonable to keep the default parameter to what 90% of users
>> want by a means on a wrapper.  The non-verbose output is provided by
>> the core already for all devices.
>> What do you think of my proposal above Tal?  That would make the extra
>> wrapper unnecessary since the verbose parameter would be part of the
>> driver structure, and it would avoid the duplicated output.
> I see how it might make sense to add another member to the driver
> struct, but is it worth the extra learning curve? It seems to be
> something with the potential to confuse new driver developers, and
> having a very marginal benefit.
> Although, if that's what people want...

I prefer the wrapper function. Looking at struct pci_driver it would
seem strange for it to hold a field for controlling verbosity (IMO).
This is a very (very) specific field in a very general struct.

> Alex

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