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

[Yoder Stuart-B08248]

> -----Original Message-----
> From: Christoffer Dall [mailto:christoffer.dall@xxxxxxxxxx]
> Sent: Wednesday, October 02, 2013 10:14 AM
> To: Alex Williamson
> Cc: Kim Phillips; gregkh@xxxxxxxxxxxxxxxxxxx; linux-
> kernel@xxxxxxxxxxxxxxx; a.motakis@xxxxxxxxxxxxxxxxxxxxxx; agraf@xxxxxxx;
> Yoder Stuart-B08248; Wood Scott-B07421; Sethi Varun-B16395; Bhushan
> Bharat-R65777; peter.maydell@xxxxxxxxxx; santosh.shukla@xxxxxxxxxx;
> kvm@xxxxxxxxxxxxxxx
> Subject: Re: RFC: (re-)binding the VFIO platform driver to a platform
> device
> 
> On Tue, Oct 01, 2013 at 08:35:56PM -0600, Alex Williamson wrote:
> > On Wed, 2013-10-02 at 02:53 +0100, Christoffer Dall wrote:
> > > On Tue, Oct 01, 2013 at 05:02:44PM -0500, Kim Phillips wrote:
> > > > On Tue, 1 Oct 2013 13:00:54 -0700
> > > > Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
> > > >
> > > > > On Tue, Oct 01, 2013 at 01:38:31PM -0500, Kim Phillips wrote:
> > > > > > Hi,
> > > > > >
> > > > > > Santosh and I are having a problem figuring out how to enable
> binding
> > > > > > (and re-binding) platform devices to a platform VFIO driver
> (see
> > > > > > Antonis' WIP: [1]) in an upstream-acceptable manner.
> > > > > >
> > > > > > Binding platform drivers currently depends on a string match in
> the
> > > > > > device node's compatible entry.  On an arndale, one can
> currently
> > > > > > rebind the same device to the same driver like so:
> > > > > >
> > > > > > echo 12ce0000.i2c > /sys/bus/platform/drivers/s3c-
> i2c/12ce0000.i2c/driver/unbind
> > > > > > echo 12ce0000.i2c > /sys/bus/platform/drivers/s3c-i2c/bind
> > > > > >
> > > > > > And one can bind it to the vfio-dt driver, as Antonis
> instructs, by
> > > > > > appending a 'vfio-dt' string to the device tree compatible
> entry for
> > > > > > the device.  Then this would work:
> > > > > >
> > > > > > echo 12ce0000.i2c > /sys/bus/platform/drivers/s3c-
> i2c/12ce0000.i2c/driver/unbind
> > > > > > echo 12ce0000.i2c > /sys/bus/platform/drivers/vfio-dt/bind
> > > > > >
> > > > > > Consequently, the hack patch below [2] allows any platform
> device to be
> > > > > > bound to the vfio-dt driver, without making changes to the
> device
> > > > > > tree.  It's a hack because I don't see having any driver name
> specific
> > > > > > code in drivers/base/bus.c being upstream acceptable.
> > > > >
> > > > > You are correct.
> > > > >
> > > > > What is wrong with just doing the above unbind/bind things
> through
> > > > > sysfs, that is what it is there for, right?
> > > >
> > > > The bind fails because the compatible string in the device tree
> doesn't
> > > > match that of the VFIO platform driver, so driver_match_device
> always
> > > > returns false.
> > > >
> > > It sounds like this is not going to be pretty almost no matter what
> > > we'll end up doing: Inherently VFIO is going to bind to a device
> without
> > > the device tree entry for that device ever saying anything about
> VFIO.
> > >
> > > How is this solved for PCI?  Can we use some analogy from that work
> to
> > > construct the missing piece?
> >
> > PCI supports a dynamic ID table for driver/device matching, see
> > pci_add_dynid().  The problem is that this gets a little sloppy for the
> > period where you have multiple drivers that can claim the same device,
> > especially in the presence of hotplug.  Thus the desire to improve the
> > situation with some kind of direct binding interface.  Thanks,
> >
> So that's called on the vfio pci driver?
> 
> Wouldn't a sysfs file to add compatibility strings to the vfio-platform
> driver make driver_match_device return true and make everyone happy?

I had a similar thought.  Why can't we do something like:

  echo "fsl,i2c" > /sys/bus/platform/drivers/vfio-platform/new_compatible
  echo 12ce0000.i2c > /sys/bus/platform/drivers/vfio-platform/bind

The first steps tell vfio-platform to register itself to handle
"fsl,i2c" compatible devices.  The second step does the bind.

Stuart

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