Re: RFC: (re-)binding the VFIO platform driver to a platform device

[Scott Wood <scottwood@freescale.com>]

On Wed, 2013-10-02 at 16:40 -0700, gregkh@linuxfoundation.org wrote:
> On Wed, Oct 02, 2013 at 04:35:15PM -0500, Scott Wood wrote:
> > On Wed, 2013-10-02 at 14:16 -0700, gregkh@linuxfoundation.org wrote:
> > > On Wed, Oct 02, 2013 at 04:08:41PM -0500, Scott Wood wrote:
> > > > I don't see any equivalent functionality to PCI_ANY_ID for platform
> > > > devices.
> > > 
> > > Nor should it.  If you are wanting to bind platform devices to different
> > > things based on "ids" or "strings" or something else, then you had
> > > better not be using a platform device because that is not what you have
> > > anymore.
> > 
> > I don't see how anything could be considered a platform device under
> > your definition.
> 
> Devices that you just "know" are at a specific memory location ahead of
> time.
>
> > Even before all the device tree stuff came along,
> > platform devices were still bound based on strings.
> 
> Not all of them, there are lots that are not, look at ISA devices on a
> PC platform for one example (your pc speaker, keyboard controller,
> etc.)

Using platform devices to let board files provide this information to
driver files was a big improvement over hacking up the drivers directly
to know about all sorts of different boards/SoCs.  Once you split that
knowledge into a different place you need a way of matching the two.

BTW, this is done with the pc speaker as far as I can tell --
arch/x86/kernel/pcspeaker.c registers a device using the string
"pcspkr" (as do some non-PC platforms).

> > And even if we did still have a separate OF platform bus, my point about
> > there not being a wildcard match applies to of_device_id as well.  It
> > certainly is not the case that "this is already there, and has been for
> > years with no problems".
> 
> That's an OF problem then, feel free to fix it there, but not in the
> driver core with a crazy "ignore this bus type string" hack :)

Even if we did this for OF and ACPI, you'd still have a problem if you
want to use VFIO with a platform device that only has plain old
platform data; thus, the platform bus (not driver core) seems to be the
appropriate place for a wildcard match if we end up needing it.  It may
be moot though, since for platform devices we may want explicit kernel
support for a device even with vfio, in order to reset/quiesce it
between users.

What it looks like we do still want from the driver core is the ability
for a driver to say that it should not be bound to a device except via
explicit sysfs bind, and the ability for a user to say that a device
should not be bound to a driver except via explicit sysfs bind.  This is
a separate issue from making driver_match_device() happy (in some
earlier e-mails in the thread these two issues were not properly
separated).

-Scott