linux-kernel - Re: [PATCH v4 3/5] PCI: brcmstb: Set PCIe transaction completion timeout

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Message-ID: <20230501205529.GA3626@wunner.de>
Date:   Mon, 1 May 2023 22:55:29 +0200
From:   Lukas Wunner <lukas@...ner.de>
To:     Jim Quinlan <james.quinlan@...adcom.com>
Cc:     Bjorn Helgaas <helgaas@...nel.org>,
        Jim Quinlan <jim2101024@...il.com>, linux-pci@...r.kernel.org,
        Nicolas Saenz Julienne <nsaenz@...nel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Cyril Brulebois <kibi@...ian.org>,
        Phil Elwell <phil@...pberrypi.com>,
        bcm-kernel-feedback-list@...adcom.com,
        Florian Fainelli <f.fainelli@...il.com>,
        Lorenzo Pieralisi <lpieralisi@...nel.org>,
        Krzysztof Wilczy??ski <kw@...ux.com>,
        Rob Herring <robh@...nel.org>,
        "moderated list:BROADCOM BCM2711/BCM2835 ARM ARCHITECTURE" 
        <linux-rpi-kernel@...ts.infradead.org>,
        "moderated list:BROADCOM BCM2711/BCM2835 ARM ARCHITECTURE" 
        <linux-arm-kernel@...ts.infradead.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 3/5] PCI: brcmstb: Set PCIe transaction completion
 timeout

On Sun, Apr 30, 2023 at 05:24:26PM -0400, Jim Quinlan wrote:
> I've been maintaining this driver for over eight years or so and we've
> done fine with the HW default completion timeout value.
> Only recently has a major customer requested that this timeout value
> be changed, and their reason was so they could
> avoid a CPU abort when using L1SS.
> 
> Now we could set this value to a big number for all cases and not
> require "brcm,completion-timeout-us".  I cannot see any
> downsides, other than another customer coming along asking us to
> double the default or lessen it.

The Completion Timeout is configurable in the Device Control 2 Register
(PCIe r2.1 sec 7.8.16).  Your controller does expose that register:

  DevCtl2: Completion Timeout: 50us to 50ms, TimeoutDis- LTR+ OBFF Disabled, ARIFwd-
           AtomicOpsCtl: ReqEn- EgressBlck-

Why does your controller allow configuration of the timeout in a
proprietary register in addition to DevCtl2?

If you make the Completion Timeout configurable, please do so in
a spec-compliant way, i.e. via DevCtl2, so that it works for
other products as well.

If the proprietary register has advantages over DevCtl2 (e.g.
finer granularity), perhaps you can divert accesses to the
Completion Timeout Value in DevCtl2 to your proprietary register.

Thanks,

Lukas