linux-kernel - Re: [PATCH v3] PCI: Disallow retraining link for Atheros chips on non-Gen1 PCIe bridges

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Message-ID: <20210602155559.GA2028037@bjorn-Precision-5520>
Date:   Wed, 2 Jun 2021 10:55:59 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Pali Rohár <pali@...nel.org>
Cc:     Bjorn Helgaas <bhelgaas@...gle.com>,
        Kalle Valo <kvalo@...eaurora.org>,
        Toke Høiland-Jørgensen <toke@...hat.com>,
        Marek Behún <kabel@...nel.org>,
        Krzysztof Wilczyński <kw@...ux.com>,
        vtolkm@...il.com, Rob Herring <robh@...nel.org>,
        Ilias Apalodimas <ilias.apalodimas@...aro.org>,
        Thomas Petazzoni <thomas.petazzoni@...tlin.com>,
        linux-pci@...r.kernel.org, ath10k@...ts.infradead.org,
        linux-wireless@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] PCI: Disallow retraining link for Atheros chips on
 non-Gen1 PCIe bridges

On Wed, Jun 02, 2021 at 02:08:16PM +0200, Pali Rohár wrote:
> On Tuesday 01 June 2021 19:00:36 Bjorn Helgaas wrote:

> > I wonder if this could be restructured as a generic quirk in quirks.c
> > that simply set the bridge's TLS to 2.5 GT/s during enumeration.  Or
> > would the retrain fail even in that case?
> 
> If I understand it correctly then PCIe link is already up when kernel
> starts enumeration. So setting Bridge TLS to 2.5 GT/s does not change
> anything here.
> 
> Moreover it would have side effect that cards which are already set to
> 5+ GT/s would be downgraded to 2.5 GT/s during enumeration and for
> increasing speed would be needed another round of "enumeration" to set a
> new TLS and retrain link again. As TLS affects link only after link goes
> into Recovery state.
> 
> So this would just complicate card enumeration and settings.

The current quirk complicates the ASPM code.  I'm hoping that if we
set the bridge's Target Link Speed during enumeration, the link
retrain will "just work" without complicating the ASPM code.

An enumeration quirk wouldn't have to set the bridge's TLS to 2.5
GT/s; the quirk would be attached to specific endpoint devices and
could set the bridge's TLS to whatever the endpoint supports.

> Moreover here we are dealing with specific OTP/EEPROM bug in Atheros
> chips, which was confirmed that exists. As I wrote in previous email, I
> was told that semi-official workaround is do Warm Reset or Cold Reset
> with turning power off from card. Which on most platforms / boards is
> not possible.

If there's a specific bug with a real root-cause analysis, please cite
it.  The threads mentioned in the current commit log are basically
informed speculation.

Bjorn