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

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Date:   Wed, 5 May 2021 23:10:32 +0530
From:   Amey Narkhede <ameynarkhede03@...il.com>
To:     Oliver O'Halloran <oohall@...il.com>
Cc:     Bjorn Helgaas <helgaas@...nel.org>,
        Shanker R Donthineni <sdonthineni@...dia.com>,
        Alex Williamson <alex.williamson@...hat.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 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?
>
> Power cycling the slot or just re-asserting #PERST probably. IBM has
> been doing that on Power boxes since forever and it mostly works.
> Mostly.
According to ACPI spec v6.3 section 7.3.25, _RST just performs normal
FLR in most cases but if the device supports _PRR(Power Resource for Reset)
then reset operation causes the device to be reported as missing from the bus
that indicates that it affects all the devices on the bus.

Thanks,
Amey