linux-kernel - Re: [RFC PATCH] PCI: avoid SBR for NVIDIA T4

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Date:   Thu, 30 Mar 2023 10:49:26 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Wu Zongyong <wuzongyong@...ux.alibaba.com>
Cc:     sdonthineni@...dia.com, bhelgaas@...gle.com,
        linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
        wllenyj@...ux.alibaba.com, wutu.xq2@...ux.alibaba.com,
        gerry@...ux.alibaba.com
Subject: Re: [RFC PATCH] PCI: avoid SBR for NVIDIA T4

On Thu, Mar 30, 2023 at 10:10:16AM +0800, Wu Zongyong wrote:
> On Wed, Mar 29, 2023 at 12:05:15PM -0500, Bjorn Helgaas wrote:
> > On Wed, Mar 29, 2023 at 07:58:45PM +0800, Wu Zongyong wrote:
> > > Secondary bus reset will fail if NVIDIA T4 card is direct attached to a
> > > root port.
> > 
> > Is this only a problem when direct attached to a Root Port?  Why would
> > that be?  If it's *not* related to being directly under a Root Port,
> > don't mention that at all.
>
> Yes, this problem occurs only when the T4 card is direct attached to a
> Root Port.
> I have test it with a T4 card attached to a PCIe Switch or a PCI Bridge,
> and it works well.

>From an electrical and protocol point of view, the device should not
be able to tell the difference, so Lukas' suggestion that it may be
related to reset delays seems very pertinent.

Bjorn