linux-kernel - Re: [PATCH] mfd: Add Simple PCI MFD driver

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Date:   Wed, 25 Jan 2023 11:15:38 +0100
From:   Vincent Whitchurch <vincent.whitchurch@...s.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
CC:     Lee Jones <lee@...nel.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        kernel <kernel@...s.com>, <robh@...nel.org>
Subject: Re: [PATCH] mfd: Add Simple PCI MFD driver

On Mon, Jan 23, 2023 at 05:31:21PM +0100, Greg Kroah-Hartman wrote:
> On Mon, Jan 23, 2023 at 03:32:55PM +0000, Lee Jones wrote:
> > On Mon, 23 Jan 2023, Vincent Whitchurch wrote:
> > 
> > > Add a PCI driver which registers all child nodes specified in the
> > > devicetree.  It will allow platform devices to be used on virtual
> > > systems which already support PCI and devicetree, such as UML with
> > > virt-pci.
> > > 
> > > The driver has no id_table by default; user space needs to provide one
> > > using the new_id mechanism in sysfs.
> > 
> > This feels wrong for several reasons.
> > 
> > Firstly, I think Greg (Cc:ed) will have something to say about this.
> 
> Yes, this isn't ok.  Please write a real driver for the hardware under
> control here, and that would NOT be a MFD driver (hint, if you want to
> split up a PCI device into different drivers, use the aux bus code, that
> is what it is there for.)

I hope it's clear from my other replies in this thread that the entire
purpose of this driver is to allow arbitrary platform devices to be used
via a PCI device in virtual environments like User Mode Linux in order
to test existing platform drivers using mocked hardware.

Given this "hardware", it's not clear what a "real driver" would do
differently.  The auxiliary bus cannot be used since it naturally does
not support platform devices.  A hard coded list of sub-devices cannot
be used since arbitrary platform devices with arbitrary devicetree
properties need to be supported.

I could move this driver to drivers/bus/ and pitch it as a
"PCI<->platform bridge for testing in virtual environments", if that
makes more sense.