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

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Date:   Sun, 30 Apr 2023 17:38:49 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Jim Quinlan <james.quinlan@...adcom.com>
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 05:24:26PM -0400, Jim Quinlan wrote:
> 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.

Most does, but I'm pretty sure there are other controllers used on
arm64 that signal CPU aborts, e.g., imx6q_pcie_abort_handler() seems
similar.

> 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?

That would be fine with me.

Bjorn