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

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Date:   Sun, 30 Apr 2023 17:24:26 -0400
From:   Jim Quinlan <james.quinlan@...adcom.com>
To:     Bjorn Helgaas <helgaas@...nel.org>
Cc:     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 Wilczyński <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 3:13 PM Bjorn Helgaas <helgaas@...nel.org> wrote:
>
> On Fri, Apr 28, 2023 at 06:34:57PM -0400, Jim Quinlan wrote:
> > Since the STB PCIe HW will cause a CPU abort on a PCIe transaction
> > completion timeout abort, we might as well extend the default timeout
> > limit.  Further, different devices and systems may requires a larger or
> > smaller amount commensurate with their L1SS exit time, so the property
> > "brcm,completion-timeout-us" may be used to set a custom timeout value.
>
> s/requires/require/
>
> AFAIK, other platforms do not tweak Configuration Timeout values based
> on L1SS exit time.  Why is brcm different?

Keep in mind that our Brcm PCIe HW signals a CPU abort on a PCIe
completion timeout.  Other PCIe HW just returns 0xffffffff.

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.

But I'm certainly willing to do that -- would that  be acceptable?

Regards,
Jim

>
> Bjorn

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