linux-kernel - Re: [PATCH v3] PCI: Introduce new device binding path using pci_dev.driver

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Message-ID: <20140528033019.GA11547@kroah.com>
Date:	Tue, 27 May 2014 20:30:19 -0700
From:	Greg KH <gregkh@...uxfoundation.org>
To:	Bjorn Helgaas <bhelgaas@...gle.com>
Cc:	Alex Williamson <alex.williamson@...hat.com>,
	linux-pci@...r.kernel.org, kvm@...r.kernel.org,
	konrad.wilk@...cle.com, kim.phillips@...aro.org,
	stuart.yoder@...escale.com, agraf@...e.de, libvir-list@...hat.com,
	iommu@...ts.linux-foundation.org, christoffer.dall@...aro.org,
	tech@...tualopensystems.com, kvmarm@...ts.cs.columbia.edu,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] PCI: Introduce new device binding path using
 pci_dev.driver_override

On Tue, May 27, 2014 at 09:07:42PM -0600, Bjorn Helgaas wrote:
> On Tue, May 20, 2014 at 08:53:21AM -0600, Alex Williamson wrote:
> > The driver_override field allows us to specify the driver for a device
> > rather than relying on the driver to provide a positive match of the
> > device.  This shortcuts the existing process of looking up the vendor
> > and device ID, adding them to the driver new_id, binding the device,
> > then removing the ID, but it also provides a couple advantages.
> > 
> > First, the above existing process allows the driver to bind to any
> > device matching the new_id for the window where it's enabled.  This is
> > often not desired, such as the case of trying to bind a single device
> > to a meta driver like pci-stub or vfio-pci.  Using driver_override we
> > can do this deterministically using:
> > 
> > echo pci-stub > /sys/bus/pci/devices/0000:03:00.0/driver_override
> > echo 0000:03:00.0 > /sys/bus/pci/devices/0000:03:00.0/driver/unbind
> > echo 0000:03:00.0 > /sys/bus/pci/drivers_probe
> > 
> > Previously we could not invoke drivers_probe after adding a device
> > to new_id for a driver as we get non-deterministic behavior whether
> > the driver we intend or the standard driver will claim the device.
> > Now it becomes a deterministic process, only the driver matching
> > driver_override will probe the device.
> > 
> > To return the device to the standard driver, we simply clear the
> > driver_override and reprobe the device:
> > 
> > echo > /sys/bus/pci/devices/0000:03:00.0/driver_override
> > echo 0000:03:00.0 > /sys/bus/pci/devices/0000:03:00.0/driver/unbind
> > echo 0000:03:00.0 > /sys/bus/pci/drivers_probe
> > 
> > Another advantage to this approach is that we can specify a driver
> > override to force a specific binding or prevent any binding.  For
> > instance when an IOMMU group is exposed to userspace through VFIO
> > we require that all devices within that group are owned by VFIO.
> > However, devices can be hot-added into an IOMMU group, in which case
> > we want to prevent the device from binding to any driver (override
> > driver = "none") or perhaps have it automatically bind to vfio-pci.
> > With driver_override it's a simple matter for this field to be set
> > internally when the device is first discovered to prevent driver
> > matches.
> > 
> > Signed-off-by: Alex Williamson <alex.williamson@...hat.com>
> > Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> 
> Greg, are you going to weigh in on this?  It does seem to solve some real
> problems.  ISTR you had an opinion once, but I don't know your current
> thoughts.

Give me a few more days, still digging through my patch backlog...

thanks,

greg k-h
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