Date:   Wed, 25 Jan 2023 14:06:26 +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" <robh@...nel.org>
Subject: Re: [PATCH] mfd: Add Simple PCI MFD driver

On Wed, Jan 25, 2023 at 01:29:32PM +0100, Greg Kroah-Hartman wrote:
> On Wed, Jan 25, 2023 at 11:15:38AM +0100, Vincent Whitchurch wrote:
> > 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.
> 
> That still feels wrong, why is PCI involved here at all?
>
> Don't abuse platform devices like this please, mock up a platform device
> framework instead if you want to test them that way, don't think that
> adding a platform device "below" a PCI device is somehow allowed at all.

As you know, PCI allows exposing an MMIO region to the host, so the host
can use ioremap() and readl()/writel() on it.  This allows reusing
platform drivers even though the device is on the other side of a PCI
bus.

There is hardware already supported by the kernel since a long time ago
which is handled by putting platform devices below PCI devices.  See
add_bus_probe() in arch/x86/kernel/devicetree.c.

And this hardware also wants to do the same thing:

 https://lore.kernel.org/lkml/1674183732-5157-1-git-send-email-lizhi.hou@amd.com/

Also, UML already supports out-of-process PCI, and there is ongoing work
in QEMU to add support for out-of-process PCI emulation.  So using PCI
will allow this to work on different kinds of virtual environments
without having to invent a new method specifically for platform devices.

> > Given this "hardware", it's not clear what a "real driver" would do
> > differently.
> 
> Again, you can not have a platform device below a PCI device, that's not
> what a platform device is for at all.

See above.

> > The auxiliary bus cannot be used since it naturally does
> > not support platform devices.
> 
> The aux bus can support any type of bus (it's there to be used as you
> want, it's just that people are currently using it for PCI devices right
> now).

I assume we're talking about drivers/base/auxiliary.c?  The kernel doc
says:

 * A key requirement for utilizing the auxiliary bus is that there is no
 * dependency on a physical bus, device, register accesses or regmap support.
 * These individual devices split from the core cannot live on the platform bus
 * as they are not physical devices that are controlled by DT/ACPI.

But this case the sub-devices do need standard register access with
readl()/writel() and _are_ controlled by devicetree.

> > A hard coded list of sub-devices cannot be used since arbitrary
> > platform devices with arbitrary devicetree properties need to be
> > supported.
> 
> Then make a new bus type and again, do not abuse platform devices.

How can existing platform drivers be re-used if you invent a new bus
type and don't create platform devices?

> > 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.
> 
> Again, nope, a platform device is NOT ever a child of a PCI device.
> That's just not how PCI works at all.
> 
> Would you do the attempt to do this for USB?  (hint, no.)  So why is PCI
> somehow special here?

PCI is special because it allows exposing an MMIO region to the host and
allowing the host to access it like any other I/O memory.  USB doesn't
allow that.

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