PCI: Introduce new device binding path using pci_dev.driver_override

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@redhat.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Reviewed-by: Alexander Graf <agraf@suse.de>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 21 insertions, 0 deletions

diff --git a/Documentation/ABI/testing/sysfs-bus-pci b/Documentation/ABI/testing/sysfs-bus-pci
index a3c5a6685036..898ddc4440e6 100644
--- a/Documentation/ABI/testing/sysfs-bus-pci
+++ b/Documentation/ABI/testing/sysfs-bus-pci
@@ -250,3 +250,24 @@ Description:
                 valid.  For example, writing a 2 to this file when sriov_numvfs
                 is not 0 and not 2 already will return an error. Writing a 10
                 when the value of sriov_totalvfs is 8 will return an error.
+
+What:           /sys/bus/pci/devices/.../driver_override
+Date:           April 2014
+Contact:        Alex Williamson <alex.williamson@redhat.com>
+Description:
+                This file allows the driver for a device to be specified which
+                will override standard static and dynamic ID matching.  When
+                specified, only a driver with a name matching the value written
+                to driver_override will have an opportunity to bind to the
+                device.  The override is specified by writing a string to the
+                driver_override file (echo pci-stub > driver_override) and
+                may be cleared with an empty string (echo > driver_override).
+                This returns the device to standard matching rules binding.
+                Writing to driver_override does not automatically unbind the
+                device from its current driver or make any attempt to
+                automatically load the specified driver.  If no driver with a
+                matching name is currently loaded in the kernel, the device
+                will not bind to any driver.  This also allows devices to
+                opt-out of driver binding using a driver_override name such as
+                "none".  Only a single driver may be specified in the override,
+                there is no support for parsing delimiters.