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

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Message-ID: <DM6PR12MB3082D966CFC0FA1C2148D8FAE8719@DM6PR12MB3082.namprd12.prod.outlook.com>
Date:   Mon, 22 Aug 2022 13:21:47 +0000
From:   "Gupta, Nipun" <Nipun.Gupta@....com>
To:     Greg KH <gregkh@...uxfoundation.org>
CC:     "robh+dt@...nel.org" <robh+dt@...nel.org>,
        "krzysztof.kozlowski+dt@...aro.org" 
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Subject: RE: [RFC PATCH v2 2/6] bus/cdx: add the cdx bus driver

[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.

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.

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?

Thanks,
Nipun

> 
> > CDX bus is capable of scanning devices dynamically,
> > supporting rescanning of dynamically added, removed or
> > updated devices.
> 
> Wonderful, that's a real bus, so be a real bus please.
> 
> thanks,
> 
> greg k-h