Date:   Tue, 31 Jan 2023 16:07:50 +0100
From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     Rob Herring <robh@kernel.org>
Cc:     Vincent Whitchurch <vincent.whitchurch@axis.com>,
        Lee Jones <lee@kernel.org>,
        "devicetree@vger.kernel.org" <devicetree@vger.kernel.org>,
        "linux-pci@vger.kernel.org" <linux-pci@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        kernel <kernel@axis.com>
Subject: Re: [PATCH] mfd: Add Simple PCI MFD driver
Message-ID: <Y9kuxrL3XaCG+blk@kroah.com>
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On Wed, Jan 25, 2023 at 09:34:01AM -0600, Rob Herring wrote:
> On Wed, Jan 25, 2023 at 9:00 AM Greg Kroah-Hartman
> <gregkh@linuxfoundation.org> wrote:
> >
> > On Wed, Jan 25, 2023 at 08:54:21AM -0600, Rob Herring wrote:
> > > On Wed, Jan 25, 2023 at 6:29 AM Greg Kroah-Hartman
> > > <gregkh@linuxfoundation.org> wrote:
> > > >
> > > > On Wed, Jan 25, 2023 at 11:15:38AM +0100, Vincent Whitchurch wrote:
> > > > > 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.
> > > >
> > > > 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.
> > >
> > > My question as well. However, that's only for Vincent's usecase. The
> > > other ones I'm aware of are definitely non-discoverable MMIO devices
> > > behind a PCI device.
> > >
> > > It is perfectly valid in DT to have the same device either directly on
> > > an MMIO bus or behind some other MMIO capable bus. So what bus type
> > > should they all be?
> >
> > If the mmio space is behind a PCI device, then why isn't that a special
> > bus type for that "pci-mmio" or something, right?  Otherwise what
> > happens when you yank out that PCI device?  Does that work today for
> > these platform devices?
> 
> Well, yes, I'm sure there's lots of issues with hot-unplug and DT.
> That's pretty much anything using DT, not just platform devices. Those
> will only get fixed when folks try to do that, but so far we've mostly
> prevented doing that. For example, we don't support a generic
> mechanism to add and remove DT overlays because most drivers aren't
> ready for their DT node to disappear.
> 
> Is there some restriction that says platform_bus can't do hotplug? I
> thought everything is hotpluggable (in theory).
> 
> > > > > 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.
> > > >
> > > > > 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).
> > > >
> > > > > 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 about of_platform_bus[1]?
> >
> > Fair enough :)
> >
> > > At this point, it would be easier to create a new bus type for
> > > whatever it is you think *should* be a platform device and move those
> > > to the new bus leaving platform_bus as the DT/ACPI devices bus.
> >
> > platfom bus should be for DT/ACPI devices like that, but that's not what
> > a "hang a DT off a PCI device" should be, right?  Why is mmio space
> > somehow special here?
> 
> Only because platform_bus is the bus type in the kernel that supports
> MMIO devices and that the DT code uses to instantiate them. The DT
> code doesn't care if those are at the root level or behind some other
> bus type.
> 
> > Perhaps we just add support for that to the aux
> > bus?
> 
> Yes, we could add IOMEM resources, DT ID table and matching, etc., but
> we'd just end up back at of_platform_bus with a new name. Every driver
> doing both would have 2 driver structs and register calls. What do we
> gain from that?

As you know, nothing :)

Ok, I'll stop arguing now, maybe this is a valid use of a platform
device, but it feels really wrong that such a thing could live below a
PCI device that can be removed from the system at any point in time.

thanks,

greg k-h