linux-kernel - Re: PCI: Work around PCIe link training failures

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Message-ID: <alpine.DEB.2.21.2408160312180.59022@angie.orcam.me.uk>
Date: Fri, 16 Aug 2024 14:57:09 +0100 (BST)
From: "Maciej W. Rozycki" <macro@...am.me.uk>
To: Matthew W Carlis <mattc@...estorage.com>
cc: alex.williamson@...hat.com, Bjorn Helgaas <bhelgaas@...gle.com>, 
    "David S. Miller" <davem@...emloft.net>, david.abdurachmanov@...il.com, 
    edumazet@...gle.com, helgaas@...nel.org, kuba@...nel.org, leon@...nel.org, 
    linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org, 
    linux-rdma@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org, lukas@...ner.de, 
    mahesh@...ux.ibm.com, mika.westerberg@...ux.intel.com, 
    netdev@...r.kernel.org, npiggin@...il.com, oohall@...il.com, 
    pabeni@...hat.com, pali@...nel.org, saeedm@...dia.com, sr@...x.de, 
    Jim Wilson <wilson@...iptree.org>
Subject: Re: PCI: Work around PCIe link training failures

On Thu, 15 Aug 2024, Matthew W Carlis wrote:

> > Well, in principle in a setup with reliable links the LBMS bit may never 
> > be set, e.g. this system of mine has been in 24/7 operation since the last 
> > reboot 410 days ago and for the devices that support Link Active reporting 
> > it shows:
> > ...
> > so out of 11 devices 6 have the LBMS bit clear.  But then 5 have it set, 
> > perhaps worryingly, so of course you're right, that it will get set in the 
> > field, though it's not enough by itself for your problem to trigger.
> 
> The way I look at it is that its essentially a probability distribution with time,
> but I try to avoid learning too much about the physical layer because I would find
> myself debugging more hardware issues lol. I also don't think LBMS/LABS being set
> by itself is very interesting without knowing the rate at which it is being set.

 Agreed.  Ilpo's upcoming bandwidth controller will hopefully give us such 
data.

> FWIW I have seen some devices in the past going into recovery state many times a
> second & still never downtrain, but at the same time they were setting the
> LBMS/LABS bits which maybe not quite spec compliant.
> 
> I would like to help test these changes, but I would like to avoid having to test
> each mentioned change individually. Does anyone have any preferences in how I batch
> the patches for testing? Would it be ok if I just pulled them all together on one go?

 Certainly fine with me, especially as 3/4 and 4/4 aren't really related 
to your failure scenario, and then you need 1/4 and 2/4 both at a time to 
address both aspects of the issue you have reported.

  Maciej