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

[Christoffer Dall <christoffer.dall@linaro.org>]

On Wed, Oct 02, 2013 at 01:32:38PM -0500, Scott Wood wrote:
> On Wed, 2013-10-02 at 13:25 -0500, Yoder Stuart-B08248 wrote:
> > 
> > > -----Original Message-----
> > > From: Christoffer Dall [mailto:christoffer.dall@linaro.org]
> > > Sent: Wednesday, October 02, 2013 10:14 AM
> > > To: Alex Williamson
> > > Cc: Kim Phillips; gregkh@linuxfoundation.org; linux-
> > > kernel@vger.kernel.org; a.motakis@virtualopensystems.com; agraf@suse.de;
> > > Yoder Stuart-B08248; Wood Scott-B07421; Sethi Varun-B16395; Bhushan
> > > Bharat-R65777; peter.maydell@linaro.org; santosh.shukla@linaro.org;
> > > kvm@vger.kernel.org
> > > Subject: Re: RFC: (re-)binding the VFIO platform driver to a platform
> > > device
> > > 
> > > 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.
> 
> Needing to specify the compatible is hacky (we already know what device
> we want to bind; why do we need to scrounge up more information than
> that, and add a new sysfs interface for extending compatible matches,
> and a more flexible data structure to back that up?), and is racy on
> buses that can hotplug (which driver gets the new device?).

Why hacky?  It seems quite reasonable to me that the user has to tell a
subsystem that from a certain point it should be capable of handling
some device.

As for the data structure, isn't this a simple linked list?

The problem with the race seems to be a common problem that hasn't even
been solved for PCI yet, so I'm wondering if this is not an orthogonal
issue with a separate solution, such as a priority or something like
that.

Yes, once you've added the new_compatible to the vfio-platform driver,
it's up for grabs from both the new and the old driver, but that could
be solved by always making sure that the vfio-platform driver is checked
first.

(I'm not familiar with these data structures, but I would imagine
something like re-inserting the vfio-platform driver in the
list/tree/... whenever adding a new_compatible value might possibly be
one solution).

> 
> What's wrong with a non-vfio-specific flag that a driver can set, that
> indicates that the driver is willing to try to bind to any device on the
> bus if explicitly requested via the existing sysfs bind mechanism?
> 
It sounds more hackish to me to invent some 'generic' flag to solve a
very specific case.  What you're suggesting would let users specify that
a serial driver should handle a NIC hardware, no?  That sounds much much
worse to me.

-Christoffer