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Date:   Fri, 18 Dec 2020 13:46:03 +0000
From:   Lee Jones <>
To:     Mark Brown <>
Cc:     Greg KH <>,
        Alexandre Belloni <>,
        Dan Williams <>,
        Pierre-Louis Bossart <>,, Kiran Patil <>,
        linux-rdma <>,
        Shiraz Saleem <>,
        Martin Habets <>,
        Liam Girdwood <>,
        Ranjani Sridharan <>,
        Fred Oh <>,
        Jason Gunthorpe <>,
        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 Fri, 18 Dec 2020, Mark Brown wrote:

> On Fri, Dec 18, 2020 at 08:10:51AM +0100, Greg KH wrote:
> > On Thu, Dec 17, 2020 at 10:19:37PM +0100, Alexandre Belloni wrote:
> > > There is something I don't get from the documentation and it is what is
> > > this introducing that couldn't already be done using platform drivers
> > > and platform devices?
> > Because platform drivers and devices should ONLY be for actual platform
> > devices.  Do NOT use that interface to fake up a non-platform device
> > (i.e. something that is NOT connected to a cpu through a memory-mapped
> > or direct-firmware interface).
> > Do not abuse the platform code anymore than it currently is, it's bad
> > enough what has been done to it over time, let's not make it any worse.
> I am not clear on why you're giving direct-firmware devices (which I
> assume means things like ARM SCMI where we're talking directly to some
> firmware?) a pass here but not for example a GPIO controlled devices.
> If this is mainly about improving abstractions it seems like the
> boundary here isn't great.  Or perhaps I'm just missing what
> direct-firmware is supposed to mean?
> In any case, to be clear part of what you're saying here is that all
> I2C and SPI MFDs should be rewritten to use this new bus - I've just
> copied Lee in again since he keeps getting missed from these threads.
> As previously discussed this will need the auxilliary bus extending to
> support at least interrupts and possibly also general resources.

Thanks Mark.

Not entirely sure why this needed an entirely new subsystem to handle
non-MMIO Multi-Functional Devices (MFD).  Or why I was not approached
by any of the developers during the process.

Having 2 entirely separate subsystems where MFDs can now be registered
sounds confusing and convoluted at best.  Why not simply extend actual
MFD to be capable of registering non-pure platform devices via other
means?  By doing so you keep things bound to a central location
resulting in less chance of misuse.

I turn away MFD implementation abuses all the time.  Seeing as the 2
subsystems are totally disjoint, this just unwittingly opened up
another back-channel opportunity for those abuses to make it into the
mainline kernel.

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