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

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Date:	Tue, 01 Oct 2013 20:35:56 -0600
From:	Alex Williamson <alex.williamson@...hat.com>
To:	Christoffer Dall <christoffer.dall@...aro.org>
Cc:	Kim Phillips <kim.phillips@...aro.org>, gregkh@...uxfoundation.org,
	linux-kernel@...r.kernel.org, a.motakis@...tualopensystems.com,
	agraf@...e.de, stuart.yoder@...escale.com, B07421@...escale.com,
	B16395@...escale.com, R65777@...escale.com,
	peter.maydell@...aro.org, santosh.shukla@...aro.org,
	kvm@...r.kernel.org
Subject: Re: RFC: (re-)binding the VFIO platform driver to a platform device

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@...uxfoundation.org> 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,

Alex

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