linux-kernel - Re: Issues with "PCI/LINK: Report degraded links via link bandwidth notification"

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Date:   Thu, 28 Jan 2021 18:07:36 -0600
From:   "Alex G." <mr.nuke.me@...il.com>
To:     Sinan Kaya <okaya@...nel.org>, Bjorn Helgaas <helgaas@...nel.org>,
        Alexandru Gagniuc <alex_gagniuc@...lteam.com>,
        Keith Busch <keith.busch@...el.com>
Cc:     Jan Vesely <jano.vesely@...il.com>, Lukas Wunner <lukas@...ner.de>,
        Alex Williamson <alex.williamson@...hat.com>,
        Austin Bolen <austin_bolen@...l.com>,
        Shyam Iyer <Shyam_Iyer@...l.com>, linux-pci@...r.kernel.org,
        linux-kernel@...r.kernel.org, Christoph Hellwig <hch@....de>,
        Lucas Stach <l.stach@...gutronix.de>,
        Dave Airlie <airlied@...il.com>,
        Ben Skeggs <skeggsb@...il.com>,
        Alex Deucher <alexdeucher@...il.com>,
        Myron Stowe <myron.stowe@...hat.com>,
        "A. Vladimirov" <vladimirov.atanas@...il.com>
Subject: Re: Issues with "PCI/LINK: Report degraded links via link bandwidth
 notification"

On 1/28/21 5:51 PM, Sinan Kaya wrote:
> On 1/28/2021 6:39 PM, Bjorn Helgaas wrote:
>> AFAICT, this thread petered out with no resolution.
>>
>> If the bandwidth change notifications are important to somebody,
>> please speak up, preferably with a patch that makes the notifications
>> disabled by default and adds a parameter to enable them (or some other
>> strategy that makes sense).
>>
>> I think these are potentially useful, so I don't really want to just
>> revert them, but if nobody thinks these are important enough to fix,
>> that's a possibility.
> 
> Hide behind debug or expert option by default? or even mark it as BROKEN
> until someone fixes it?
> 
Instead of making it a config option, wouldn't it be better as a kernel 
parameter? People encountering this seem quite competent in passing 
kernel arguments, so having a "pcie_bw_notification=off" would solve 
their problems.

As far as marking this as broken, I've seen no conclusive evidence of to 
tell if its a sw bug or actual hardware problem. Could we have a sysfs 
to disable this on a per-downstream-port basis?

e.g.
     echo 0 > /sys/bus/pci/devices/0000:00:04.0/bw_notification_enabled

This probably won't be ideal if there are many devices downtraining 
their links ad-hoc. At worst we'd have a way to silence those messages 
if we do encounter such devices.

Alex