linux-kernel - Re: [PATCH v4 2/2] PCI: Enable NO_BUS

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Date:   Wed, 5 May 2021 15:40:16 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Alex Williamson <alex.williamson@...hat.com>
Cc:     Amey Narkhede <ameynarkhede03@...il.com>,
        Oliver O'Halloran <oohall@...il.com>,
        Shanker R Donthineni <sdonthineni@...dia.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        linux-pci <linux-pci@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Sinan Kaya <okaya@...nel.org>, Vikram Sethi <vsethi@...dia.com>
Subject: Re: [PATCH v4 2/2] PCI: Enable NO_BUS_RESET quirk for Nvidia GPUs

On Wed, May 05, 2021 at 01:13:57PM -0600, Alex Williamson wrote:
> On Wed, 5 May 2021 23:10:32 +0530
> Amey Narkhede <ameynarkhede03@...il.com> wrote:
> > On 21/05/05 01:56PM, Oliver O'Halloran wrote:
> > > On Wed, May 5, 2021 at 12:50 PM Bjorn Helgaas <helgaas@...nel.org> wrote:  
> > > > On Mon, May 03, 2021 at 09:07:11PM -0500, Shanker R Donthineni wrote:  
> > > > > On 5/3/21 5:42 PM, Bjorn Helgaas wrote:  
> > > > > > Obviously _RST only works for built-in devices, since there's no AML
> > > > > > for plug-in devices, right?  So if there's a plug-in card with this
> > > > > > GPU, neither SBR nor _RST will work?  
> > > > > These are not plug-in PCIe GPU cards, will exist on upcoming server
> > > > > baseboards. ACPI-reset should wok for plug-in devices as well as long
> > > > > as firmware has _RST method defined in ACPI-device associated with
> > > > > the PCIe hot-plug slot.  
> > > >
> > > > Maybe I'm missing something, but I don't see how _RST can work for
> > > > plug-in devices.  _RST is part of the system firmware, and that
> > > > firmware knows nothing about what will be plugged into the slot.  So
> > > > if system firmware supplies _RST that knows how to reset the Nvidia
> > > > GPU, it's not going to do the right thing if you plug in an NVMe
> > > > device instead.
> > > >
> > > > Can you elaborate on how _RST would work for plug-in devices?  
> 
> I'm not sure I really understand these concerns about plug-in devices.

I'm not really concerned about plug-in devices.  Shanker said above
that _RST would work for them:

  ACPI-reset should work for plug-in devices as well as long as
  firmware has _RST method defined in ACPI-device associated with the
  PCIe hot-plug slot.

and I disagreed.  _RST *cannot* work for plug-in devices because
firmware doesn't know what device will be plugged in and therefore
cannot provide the required device-specific _RST.

That's all I wanted to clarify.

Bjorn