From: Vincent Whitchurch <vincent.whitchurch@axis.com>
To: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: 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>, "robh@kernel.org" <robh@kernel.org>
Subject: Re: [PATCH] mfd: Add Simple PCI MFD driver
Date: Wed, 25 Jan 2023 14:06:26 +0100 [thread overview]
Message-ID: <Y9EpUnRDmWPobcrL@axis.com> (raw)
In-Reply-To: <Y9EgrKT3hDyx+ULy@kroah.com>
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.
next prev parent reply other threads:[~2023-01-25 13:06 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-01-23 14:32 [PATCH] mfd: Add Simple PCI MFD driver Vincent Whitchurch
2023-01-23 15:32 ` Lee Jones
2023-01-23 16:02 ` Vincent Whitchurch
2023-01-23 16:36 ` Rob Herring
2023-01-24 2:30 ` Lizhi Hou
2023-01-24 13:15 ` Vincent Whitchurch
2023-01-23 16:31 ` Greg Kroah-Hartman
2023-01-25 10:15 ` Vincent Whitchurch
2023-01-25 12:29 ` Greg Kroah-Hartman
2023-01-25 13:06 ` Vincent Whitchurch [this message]
2023-01-25 13:34 ` Greg Kroah-Hartman
2023-01-25 14:54 ` Rob Herring
2023-01-25 15:00 ` Greg Kroah-Hartman
2023-01-25 15:34 ` Rob Herring
2023-01-31 15:07 ` Greg Kroah-Hartman
2023-01-23 16:13 ` Rob Herring
2023-01-24 12:54 ` Vincent Whitchurch
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