linux-kernel - Re: [PATCH v5 0/5] pci: Work around ASMedia ASM2824 PCIe link training failures

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Message-ID: <20221101230709.fqlj3fxguprltyga@pali>
Date:   Wed, 2 Nov 2022 00:07:09 +0100
From:   Pali Rohár <pali@...nel.org>
To:     Bjorn Helgaas <bhelgaas@...gle.com>,
        Krzysztof Wilczyński <kw@...ux.com>
Cc:     "Maciej W. Rozycki" <macro@...am.me.uk>, Stefan Roese <sr@...x.de>,
        Jim Wilson <wilson@...iptree.org>,
        David Abdurachmanov <david.abdurachmanov@...il.com>,
        linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 0/5] pci: Work around ASMedia ASM2824 PCIe link
 training failures

Bjorn, Krzysztof: Gentle reminder.

On Sunday 09 October 2022 16:14:34 Pali Rohár wrote:
> Bjorn, Krzysztof: could you please look at this patch series and say
> what do you think about it? It is quite strange issue for which is
> defined PCI_ANY_ID quirk... And is needs to be somehow workarounded.
> 
> On Saturday 17 September 2022 13:03:05 Maciej W. Rozycki wrote:
> > Hi,
> > 
> >  This is v5 of the change to work around a PCIe link training phenomenon 
> > where a pair of devices both capable of operating at a link speed above 
> > 2.5GT/s seems unable to negotiate the link speed and continues training 
> > indefinitely with the Link Training bit switching on and off repeatedly 
> > and the data link layer never reaching the active state.
> > 
> >  This was originally observed in a configuration featuring a downstream 
> > port of the ASMedia ASM2824 Gen 3 switch wired to the upstream port of the 
> > Pericom PI7C9X2G304 Gen 2 switch.  However in the course of review I have 
> > come to the conclusion that similarly to the earlier similar change to 
> > U-Boot it is indeed expected to be safe to apply this workaround to any 
> > downstream port that has failed link negotiation provided that:
> > 
> > 1. the port is capable of reporting the data link layer link active 
> >    status (because unlike U-Boot we cannot busy-loop continuously polling 
> >    the link training bit),
> > 
> > and:
> > 
> > 2. we don't attempt to lift the 2.5GT/s speed restriction, imposed as the
> >    basis of the workaround, for devices not explicitly known to continue 
> >    working in that case.
> > 
> > It is expected to be safe because the workaround is applied to a failed 
> > link, that is one that does not (at the time this code is executed) work 
> > anyway, so trying to bring it up cannot make the situation worse.  So this 
> > version of the workaround is attempted for all PCIe devices discovered, 
> > and only the lifting of the 2.5GT/s speed restriction is qualified by the 
> > vendor:device ID, currently one of the ASMedia ASM2824 device only.
> > 
> >  Broadening the scope of the quirk has in turn made it necessary to make 
> > some adjustments to code elsewhere and consequently what was originally a 
> > single patch has now become a small series instead.
> > 
> >  This has been verified with a SiFive HiFive unmatched board, booting with 
> > or without the workaround activated in U-Boot, which covered both the link 
> > retraining part of the quirk and the lifting of speed restriction already 
> > imposed by U-Boot.
> > 
> >  Please see individual change descriptions for further details.
> > 
> >  Questions or comments?  Otherwise please apply.
> > 
> >   Maciej