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

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Message-Id: <20240815194059.28798-1-mattc@purestorage.com>
Date: Thu, 15 Aug 2024 13:40:59 -0600
From: Matthew W Carlis <mattc@...estorage.com>
To: macro@...am.me.uk
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Subject: PCI: Work around PCIe link training failures

Sorry for the delay in my responses here I had some things get in my way.

On Fri, 9 Aug 2024 09:13:52 Oliver O'Halloran <oohall@...il.com> wrote:

> Ok? If we have to check for DPC being enabled in addition to checking
> the surprise bit in the slot capabilities then that's fine, we can do
> that. The question to be answered here is: how should this feature
> work on ports where it's normal for a device to be removed without any
> notice?

I'm not sure if its the correct thing to check however. I assumed that ports
using the pciehp driver would usually consider it "normal" for a device to
be removed actually, but maybe I have the idea of hp reversed.

On Fri, 9 Aug 2024 14:34:04 Maciej W. Rozycki <macro@...am.me.uk> 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.
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?

- Matt