Re: [PATCH v4 2/2] PCI: Enable NO_BUS_RESET quirk for Nvidia GPUs

[Bjorn Helgaas <helgaas@kernel.org>]

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