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

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Date:   Mon, 22 Aug 2022 15:29:35 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     "Gupta, Nipun" <Nipun.Gupta@....com>
Cc:     "robh+dt@...nel.org" <robh+dt@...nel.org>,
        "krzysztof.kozlowski+dt@...aro.org" 
        <krzysztof.kozlowski+dt@...aro.org>,
        "rafael@...nel.org" <rafael@...nel.org>,
        "eric.auger@...hat.com" <eric.auger@...hat.com>,
        "alex.williamson@...hat.com" <alex.williamson@...hat.com>,
        "cohuck@...hat.com" <cohuck@...hat.com>,
        "Gupta, Puneet (DCG-ENG)" <puneet.gupta@....com>,
        "song.bao.hua@...ilicon.com" <song.bao.hua@...ilicon.com>,
        "mchehab+huawei@...nel.org" <mchehab+huawei@...nel.org>,
        "maz@...nel.org" <maz@...nel.org>,
        "f.fainelli@...il.com" <f.fainelli@...il.com>,
        "jeffrey.l.hugo@...il.com" <jeffrey.l.hugo@...il.com>,
        "saravanak@...gle.com" <saravanak@...gle.com>,
        "Michael.Srba@...nam.cz" <Michael.Srba@...nam.cz>,
        "mani@...nel.org" <mani@...nel.org>,
        "yishaih@...dia.com" <yishaih@...dia.com>,
        "jgg@...pe.ca" <jgg@...pe.ca>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "okaya@...nel.org" <okaya@...nel.org>,
        "Anand, Harpreet" <harpreet.anand@....com>,
        "Agarwal, Nikhil" <nikhil.agarwal@....com>,
        "Simek, Michal" <michal.simek@....com>,
        "git (AMD-Xilinx)" <git@....com>,
        "jgg@...dia.com" <jgg@...dia.com>,
        Robin Murphy <robin.murphy@....com>
Subject: Re: [RFC PATCH v2 2/6] bus/cdx: add the cdx bus driver

On Mon, Aug 22, 2022 at 01:21:47PM +0000, Gupta, Nipun wrote:
> [AMD Official Use Only - General]
> 
> 
> 
> > -----Original Message-----
> > From: Greg KH <gregkh@...uxfoundation.org>
> > Sent: Wednesday, August 17, 2022 9:03 PM
> > To: Gupta, Nipun <Nipun.Gupta@....com>
> > Cc: robh+dt@...nel.org; krzysztof.kozlowski+dt@...aro.org; rafael@...nel.org;
> > eric.auger@...hat.com; alex.williamson@...hat.com; cohuck@...hat.com;
> > Gupta, Puneet (DCG-ENG) <puneet.gupta@....com>;
> > song.bao.hua@...ilicon.com; mchehab+huawei@...nel.org; maz@...nel.org;
> > f.fainelli@...il.com; jeffrey.l.hugo@...il.com; saravanak@...gle.com;
> > Michael.Srba@...nam.cz; mani@...nel.org; yishaih@...dia.com;
> > jgg@...pe.ca; linux-kernel@...r.kernel.org; devicetree@...r.kernel.org;
> > kvm@...r.kernel.org; okaya@...nel.org; Anand, Harpreet
> > <harpreet.anand@....com>; Agarwal, Nikhil <nikhil.agarwal@....com>;
> > Simek, Michal <michal.simek@....com>; git (AMD-Xilinx) <git@....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/ ?

thanks,

greg k-h