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Date:   Fri, 18 Dec 2020 19:09:11 +0000
From:   Lee Jones <lee.jones@...aro.org>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     Mark Brown <broonie@...nel.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Pierre-Louis Bossart <pierre-louis.bossart@...ux.intel.com>,
        alsa-devel@...a-project.org, Kiran Patil <kiran.patil@...el.com>,
        linux-rdma <linux-rdma@...r.kernel.org>,
        Shiraz Saleem <shiraz.saleem@...el.com>,
        Martin Habets <mhabets@...arflare.com>,
        Liam Girdwood <lgirdwood@...il.com>,
        Ranjani Sridharan <ranjani.sridharan@...ux.intel.com>,
        Fred Oh <fred.oh@...ux.intel.com>,
        Dave Ertman <david.m.ertman@...el.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Netdev <netdev@...r.kernel.org>,
        Leon Romanovsky <leonro@...dia.com>,
        David Miller <davem@...emloft.net>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Parav Pandit <parav@...lanox.com>
Subject: Re: [resend/standalone PATCH v4] Add auxiliary bus support

On Fri, 18 Dec 2020, Jason Gunthorpe wrote:

> On Fri, Dec 18, 2020 at 06:03:10PM +0000, Mark Brown wrote:
> > On Fri, Dec 18, 2020 at 12:28:17PM -0400, Jason Gunthorpe wrote:
> > > On Fri, Dec 18, 2020 at 03:52:04PM +0000, Mark Brown wrote:
> > > > On Fri, Dec 18, 2020 at 10:08:54AM -0400, Jason Gunthorpe wrote:
> > 
> > > > > I thought the recent LWN article summed it up nicely, auxillary bus is
> > > > > for gluing to subsystems together using a driver specific software API
> > > > > to connect to the HW, MFD is for splitting a physical HW into disjoint
> > > > > regions of HW.
> > 
> > > > This conflicts with the statements from Greg about not using the
> > > > platform bus for things that aren't memory mapped or "direct firmware",
> > > > a large proportion of MFD subfunctions are neither at least in so far as
> > > > I can understand what direct firmware means.
> > 
> > > I assume MFD will keep existing and it will somehow stop using
> > > platform device for the children it builds.
> > 
> > If it's not supposed to use platform devices so I'm assuming that the
> > intention is that it should use aux devices, otherwise presumably it'd
> > be making some new clone of the platform bus but I've not seen anyone
> > suggesting this.
> 
> I wouldn't assume that, I certainly don't want to see all the HW
> related items in platform_device cloned roughly into aux device.
> 
> I've understood the bus type should be basically related to the thing
> that is creating the device. In a clean view platform code creates
> platform devices. DT should create DT devices, ACPI creates ACPI
> devices, PNP does pnp devices, etc
> 
> So, I strongly suspect, MFD should create mfd devices on a MFD bus
> type.
> 
> Alexandre's point is completely valid, and I think is the main
> challenge here, somehow avoiding duplication.
> 
> If we were to look at it with some OOP viewpoint I'd say the generic
> HW resource related parts should be some shared superclass between
> 'struct device' and 'struct platform/pnp/pci/acpi/mfd/etc_device'.

You're confusing things here.

ACPI, DT and MFD are not busses.  They are just methods to
describe/register devices which will operate on buses.

Busses include things like; I2C, SPI, PCI, USB and Platform (MMIO).

-- 
Lee Jones [李琼斯]
Senior Technical Lead - Developer Services
Linaro.org │ Open source software for Arm SoCs
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