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Date:   Fri, 18 Dec 2020 22:16:58 +0100
From:   Alexandre Belloni <>
To:     Jason Gunthorpe <>
Cc:     Mark Brown <>,
        Greg KH <>,
        Dan Williams <>,
        Pierre-Louis Bossart <>,, Kiran Patil <>,
        linux-rdma <>,
        Shiraz Saleem <>,
        Martin Habets <>,
        Liam Girdwood <>,
        Ranjani Sridharan <>,
        Fred Oh <>,
        Dave Ertman <>,
        Jakub Kicinski <>,
        Netdev <>,
        Leon Romanovsky <>,
        David Miller <>,
        Linux Kernel Mailing List <>,
        Parav Pandit <>,
Subject: Re: [resend/standalone PATCH v4] Add auxiliary bus support

On 18/12/2020 16:58:56-0400, Jason Gunthorpe wrote:
> On Fri, Dec 18, 2020 at 08:32:11PM +0000, Mark Brown wrote:
> > > So, I strongly suspect, MFD should create mfd devices on a MFD bus
> > > type.
> > 
> > Historically people did try to create custom bus types, as I have
> > pointed out before there was then pushback that these were duplicating
> > the platform bus so everything uses platform bus.
> Yes, I vaugely remember..
> I don't know what to say, it seems Greg doesn't share this view of
> platform devices as a universal device.
> Reading between the lines, I suppose things would have been happier
> with some kind of inheritance scheme where platform device remained as
> only instantiated directly in board files, while drivers could bind to
> OF/DT/ACPI/FPGA/etc device instantiations with minimal duplication &
> boilerplate.
> And maybe that is exactly what we have today with platform devices,
> though the name is now unfortunate.
> > I can't tell the difference between what it's doing and what SOF is
> > doing, the code I've seen is just looking at the system it's running
> > on and registering a fixed set of client devices.  It looks slightly
> > different because it's registering a device at a time with some wrapper
> > functions involved but that's what the code actually does.
> SOF's aux bus usage in general seems weird to me, but if you think
> it fits the mfd scheme of primarily describing HW to partition vs
> describing a SW API then maybe it should use mfd.
> The only problem with mfd as far as SOF is concerned was Greg was not
> happy when he saw PCI stuff in the MFD subsystem.

But then again, what about non-enumerable devices on the PCI device? I
feel this would exactly fit MFD. This is a collection of IPs that exist
as standalone but in this case are grouped in a single device.

Note that I then have another issue because the kernel doesn't support
irq controllers on PCI and this is exactly what my SoC has. But for now,
I can just duplicate the irqchip driver in the MFD driver.

> This whole thing started when Intel first proposed to directly create
> platform_device's in their ethernet driver and Greg had a quite strong
> NAK to that.

Let me point to drivers/net/ethernet/cadence/macb_pci.c which is a
fairly recent example. It does exactly that and I'm not sure you could
do it otherwise while still not having to duplicate most of macb_probe.

Alexandre Belloni, Bootlin
Embedded Linux and Kernel engineering

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