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

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Date:   Mon, 3 May 2021 17:42:20 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Shanker R Donthineni <sdonthineni@...dia.com>
Cc:     Alex Williamson <alex.williamson@...hat.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>, linux-pci@...r.kernel.org,
        linux-kernel@...r.kernel.org, Sinan Kaya <okaya@...nel.org>,
        Vikram Sethi <vsethi@...dia.com>,
        Amey Narkhede <ameynarkhede03@...il.com>
Subject: Re: [PATCH v4 2/2] PCI: Enable NO_BUS_RESET quirk for Nvidia GPUs

On Fri, Apr 30, 2021 at 05:11:23PM -0500, Shanker R Donthineni wrote:
> Thanks Bjorn for reviewing patch.
> 
> On 4/30/21 12:01 PM, Bjorn Helgaas wrote:
> > External email: Use caution opening links or attachments
> >
> >
> > On Wed, Apr 28, 2021 at 07:49:07PM -0500, Shanker Donthineni wrote:
> >> On select platforms, some Nvidia GPU devices do not work with SBR.
> >> Triggering SBR would leave the device inoperable for the current
> >> system boot. It requires a system hard-reboot to get the GPU device
> >> back to normal operating condition post-SBR. For the affected
> >> devices, enable NO_BUS_RESET quirk to fix the issue.
> > Since 1/2 adds _RST support, should I infer that _RST works on these
> > Nvidia GPUs even though SBR does not?  If so, how does _RST do the
> > reset?
> Yes, _RST method works but not SBR. The _RST method in DSDT-AML uses
> platform-specific initialization steps outside of the GPU BARs for resetting
> the GPU device.

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?

> > Do you have a root cause for why SBR doesn't work?  
> It is a hardware implementation specific issue. GPU end-point device
> is inoperative after receiving SBR from the RP/SwitchPort. This quirk is
> to prevent SBR.

That's not a root cause; it's basically still "it doesn't work."  But
OK.

I'm wondering if we should log something to dmesg in
quirk_no_bus_reset(), quirk_no_pm_reset(), quirk_no_flr(), etc., just
so we have a hint about the fact that resets won't work quite as
expected on these devices.

Bjorn