Re: [PATCH v2] fpga: dfl: afu: support Rev 2 of DFL Port feature

[matthew . gerlach]

[snip]

> > Hi Yilun,
> >
> > I think this conversation has gotten a little off track. This patch only
> > changes the port reset behavior at driver initialization for revision 2 of
> > the port IP. The behavior and the requirements of port reset during run time
> > have not changed. The existing implementation requires the user performing
> > the port reset to ensure appropriate SW was quiesced. This patch does not
> > change this requirement.
>
> You are actually adding support to the new devices behavior defined by
> revision 2.  Previously the user requires the management PF driver to do port
> reset, and this only affects some logic in the PF itself and the impact could
> be handled inside the driver.  The current PF driver doesn't actually do anything
> because it (or any kernel component) never touches the disabled logic, and the
> user accessing of the mmapped but disabled logic won't cause system issues.

I'm wondering about the use case when Virtual Functions (VFs) 
are enabled with the current driver. If partial reconfiguration of the port 
occurs, which includes a reset, what happens to the user software attached 
to the VFs when the hardware has changed out from under it? While the 
current port implementation won't cause PCIe errors when the logic is 
disable, something has to notify the user software that the hardware 
changed. I think the user space software would be typically notified of 
the change by the orchestration software performing the partial 
reconfiguration. I think the use case of VFs with the original port 
implementation is logically equivalent to the new port implementation with 
multiple PFs/VFs.

> But the new management PF does port reset and affect other PFs, and may
> cause disorder in other drivers. so you need extra code to support the
> behavior.

I think the real problem is that the new port is not doing as good of a 
job preventing PCIe errors during port reset as the previous version.

Matthew
> This patch does make some progress, as you said, "With revision 2,the
> initial state of the logic inside the port is guaranteed to be valid, and
> a port reset is not required during driver initialization".  But it
> should not be the only patch to enable the new port reset behavior.
>
> > > > > > image. I don't think the driver performing the port_reset() can know all the
> > > > >
> > > > > Other PF drivers should know their own components. They should be aware
> > > > > that their devices are being reset.
> > > >
> > > > The other PF drivers depend on the actual FPGA image being run.
> > > >
> > > > > > components to be able to provide any meaningful management.
> > > > >
> > > > > If the reset provider and reset consumer are not in the same driver,
> > > > > they should interact with each other. IIRC, some reset controller class
> > > > > works for this purpose.
> > > >
> > > > The other PFs in many cases can present as standard devices with existing
> > > > drivers like virtio-net or virtio-blk. It does not seem desireable to have
> > > > to change existing drivers for a particular FPGA implementation
> > >
> > > If you have to use a specific method for reset, it is not a standard virtio
> > > pci device, and you have to make some change.
> >
> > From the perspective of the PCI PF/VF implementing the virtio or other
> > standard device, the pci endpoint is completely compliant to the standard,
> > and no driver changes should be required. As mentioned above, this patch
> > does nothing to change any of this behavior. Consider a port reset that is
> > part of a partial configuration flow. The virtio endpoint can become
> > something completely different with a completely different driver. This
>
> Then how could the virtio driver stop and remove itself to avoid crashing the
> kernel, and how could the new driver be probed.  If the answer is still
> userspace responsible for everything, that's not good to me.
>
> Thanks,
> Yilun
>
> > patch does not affect this flow either.
> >
> > Thanks,
> > Matthew
> >
> > > Thanks,
> > > Yilun
> > >
> > > > Thanks,
> > > > Matthew
> > > > > Thanks,
> > > > > Yilun
> > > > >
> > > > > > Thanks,
> > > > > > Matthew
> > > > > >
> > > > > > > Thanks,
> > > > > > > Yilun
> > > > > > >
> > > > > > > > Do you want me to update the commit message with this information?
> > > > > > > >
> > > > > > > > Thanks,
> > > > > > > > Matthew