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

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Date:   Tue, 24 Jul 2018 16:39:29 +0300
From:   Tal Gilboa <talgi@...lanox.com>
To:     "Alex G." <mr.nuke.me@...il.com>,
        Jakub Kicinski <jakub.kicinski@...ronome.com>
Cc:     "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "bhelgaas@...gle.com" <bhelgaas@...gle.com>,
        "keith.busch@...el.com" <keith.busch@...el.com>,
        "alex_gagniuc@...lteam.com" <alex_gagniuc@...lteam.com>,
        "austin_bolen@...l.com" <austin_bolen@...l.com>,
        "shyam_iyer@...l.com" <shyam_iyer@...l.com>,
        "jeffrey.t.kirsher@...el.com" <jeffrey.t.kirsher@...el.com>,
        "ariel.elior@...ium.com" <ariel.elior@...ium.com>,
        "michael.chan@...adcom.com" <michael.chan@...adcom.com>,
        "ganeshgr@...lsio.com" <ganeshgr@...lsio.com>,
        Tariq Toukan <tariqt@...lanox.com>,
        "airlied@...il.com" <airlied@...il.com>,
        "alexander.deucher@....com" <alexander.deucher@....com>,
        "mike.marciniszyn@...el.com" <mike.marciniszyn@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [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 <mr.nuke.me@...il.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?
>>
>> 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