Re: [RFC PATCH v2 2/6] bus/cdx: add the cdx bus driver

[Greg KH <gregkh@linuxfoundation.org>]

On Wed, Aug 24, 2022 at 08:50:19AM +0000, Gupta, Nipun wrote:
> [AMD Official Use Only - General]
> 
> 
> 
> > -----Original Message-----
> > From: Greg KH <gregkh@linuxfoundation.org>
> > Sent: Monday, August 22, 2022 7:00 PM
> > To: Gupta, Nipun <Nipun.Gupta@amd.com>
> > Cc: robh+dt@kernel.org; krzysztof.kozlowski+dt@linaro.org;
> > rafael@kernel.org; eric.auger@redhat.com; alex.williamson@redhat.com;
> > cohuck@redhat.com; Gupta, Puneet (DCG-ENG)
> > <puneet.gupta@amd.com>; song.bao.hua@hisilicon.com;
> > mchehab+huawei@kernel.org; maz@kernel.org; f.fainelli@gmail.com;
> > jeffrey.l.hugo@gmail.com; saravanak@google.com;
> > Michael.Srba@seznam.cz; mani@kernel.org; yishaih@nvidia.com;
> > jgg@ziepe.ca; linux-kernel@vger.kernel.org; devicetree@vger.kernel.org;
> > kvm@vger.kernel.org; okaya@kernel.org; Anand, Harpreet
> > <harpreet.anand@amd.com>; Agarwal, Nikhil <nikhil.agarwal@amd.com>;
> > Simek, Michal <michal.simek@amd.com>; git (AMD-Xilinx) <git@amd.com>;
> > jgg@nvidia.com; Robin Murphy <robin.murphy@arm.com>
> > Subject: Re: [RFC PATCH v2 2/6] bus/cdx: add the cdx bus driver
> > 
> > [CAUTION: External Email]
> > 
> > On Mon, Aug 22, 2022 at 01:21:47PM +0000, Gupta, Nipun wrote:
> > > [AMD Official Use Only - General]
> > >
> > >
> > >
> > > > -----Original Message-----
> > > > From: Greg KH <gregkh@linuxfoundation.org>
> > > > Sent: Wednesday, August 17, 2022 9:03 PM
> > > > To: Gupta, Nipun <Nipun.Gupta@amd.com>
> > > > Cc: robh+dt@kernel.org; krzysztof.kozlowski+dt@linaro.org;
> > rafael@kernel.org;
> > > > eric.auger@redhat.com; alex.williamson@redhat.com;
> > cohuck@redhat.com;
> > > > Gupta, Puneet (DCG-ENG) <puneet.gupta@amd.com>;
> > > > song.bao.hua@hisilicon.com; mchehab+huawei@kernel.org;
> > maz@kernel.org;
> > > > f.fainelli@gmail.com; jeffrey.l.hugo@gmail.com; saravanak@google.com;
> > > > Michael.Srba@seznam.cz; mani@kernel.org; yishaih@nvidia.com;
> > > > jgg@ziepe.ca; linux-kernel@vger.kernel.org;
> > devicetree@vger.kernel.org;
> > > > kvm@vger.kernel.org; okaya@kernel.org; Anand, Harpreet
> > > > <harpreet.anand@amd.com>; Agarwal, Nikhil
> > <nikhil.agarwal@amd.com>;
> > > > Simek, Michal <michal.simek@amd.com>; git (AMD-Xilinx)
> > <git@amd.com>
> > > > Subject: Re: [RFC PATCH v2 2/6] bus/cdx: add the cdx bus driver
> > > >
> > > > [CAUTION: External Email]
> > > >
> > > > On Wed, Aug 17, 2022 at 08:35:38PM +0530, Nipun Gupta wrote:
> > > > > CDX bus driver manages the scanning and populating FPGA
> > > > > based devices present on the CDX bus.
> > > > >
> > > > > The bus driver sets up the basic infrastructure and fetches
> > > > > the device related information from the firmware. These
> > > > > devices are registered as platform devices.
> > > >
> > > > Ick, why?  These aren't platform devices, they are CDX devices.  Make
> > > > them real devices here, don't abuse the platform device interface for
> > > > things that are not actually on the platform bus.
> > >
> > > CDX is a virtual bus (FW based) which discovers FPGA based platform
> > > devices based on communication with FW.
> > 
> > virtual busses are fine to have as a real bus in the kernel, no problem
> > there.
> > 
> > > These devices are essentially platform devices as these are memory
> > mapped
> > > on system bus, but having a property that they are dynamically discovered
> > > via FW and are rescannable.
> > 
> > If they are dynamically discoverable and rescannable, then great, it's a
> > bus in the kernel and NOT a platform device.
> > 
> > > I think your point is correct in the sense that CDX bus is not an actual bus,
> > > but a FW based mechanism to discover FPGA based platform devices.
> > >
> > > Can you kindly suggest us if we should have the CDX platform device
> > scanning
> > > code as a CDX bus in "drivers/bus/" folder OR have it in "drivers/fpga/" or
> > > "drivers/platform/" or which other suitable location?
> > 
> > drivers/cdx/ ?
> 
> I agree that the approach, which is correct should be used, just wanted
> to reconfirm as adding a new bus would lead to change in other areas
> like SMMU, MSI and VFIO too and we will need vfio-cdx interface for CDX
> bus, similar to vfio-platform.
> 
> On another mail Robin and Jason have suggested to use OF_DYNAMIC.
> Can you please also let us know in case that is a suited option where we
> use OF_DYNAMIC and have our code as part of "drivers/fpga" instead of
> using the bus. (something like pseries CPU hotplug is using to add new
> CPU platform devices on runtime:
> https://elixir.bootlin.com/linux/v5.19.3/source/arch/powerpc/platforms/pseries/hotplug-cpu.c#L534).
> We can share the RFC in case you are interested in looking at code flow
> using the of_dynamic approach.

Please no more abuse of the platform device.

If your device can be discovered by scanning a bus, it is not a platform
device.

greg k-h