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>

3 files changed, 63 insertions, 3 deletions

diff --git a/drivers/pci/pci-driver.c b/drivers/pci/pci-driver.c
index d911e0c1f359..4393c12e9135 100644
--- a/drivers/pci/pci-driver.c
+++ b/drivers/pci/pci-driver.c
@@ -216,6 +216,13 @@ const struct pci_device_id *pci_match_id(const struct pci_device_id *ids,
 	return NULL;
 }
 
+static const struct pci_device_id pci_device_id_any = {
+	.vendor = PCI_ANY_ID,
+	.device = PCI_ANY_ID,
+	.subvendor = PCI_ANY_ID,
+	.subdevice = PCI_ANY_ID,
+};
+
 /**
  * pci_match_device - Tell if a PCI device structure has a matching PCI device id structure
  * @drv: the PCI driver to match against
@@ -229,18 +236,30 @@ static const struct pci_device_id *pci_match_device(struct pci_driver *drv,
 						    struct pci_dev *dev)
 {
 	struct pci_dynid *dynid;
+	const struct pci_device_id *found_id = NULL;
+
+	/* When driver_override is set, only bind to the matching driver */
+	if (dev->driver_override && strcmp(dev->driver_override, drv->name))
+		return NULL;
 
 	/* Look at the dynamic ids first, before the static ones */
 	spin_lock(&drv->dynids.lock);
 	list_for_each_entry(dynid, &drv->dynids.list, node) {
 		if (pci_match_one_device(&dynid->id, dev)) {
-			spin_unlock(&drv->dynids.lock);
-			return &dynid->id;
+			found_id = &dynid->id;
+			break;
 		}
 	}
 	spin_unlock(&drv->dynids.lock);
 
-	return pci_match_id(drv->id_table, dev);
+	if (!found_id)
+		found_id = pci_match_id(drv->id_table, dev);
+
+	/* driver_override will always match, send a dummy id */
+	if (!found_id && dev->driver_override)
+		found_id = &pci_device_id_any;
+
+	return found_id;
 }
 
 struct drv_dev_and_id {
diff --git a/drivers/pci/pci-sysfs.c b/drivers/pci/pci-sysfs.c
index 4e0acefb7565..faa4ab554d68 100644
--- a/drivers/pci/pci-sysfs.c
+++ b/drivers/pci/pci-sysfs.c
@@ -499,6 +499,45 @@ static struct device_attribute sriov_numvfs_attr =
 		       sriov_numvfs_show, sriov_numvfs_store);
 #endif /* CONFIG_PCI_IOV */
 
+static ssize_t driver_override_store(struct device *dev,
+				     struct device_attribute *attr,
+				     const char *buf, size_t count)
+{
+	struct pci_dev *pdev = to_pci_dev(dev);
+	char *driver_override, *old = pdev->driver_override, *cp;
+
+	if (count > PATH_MAX)
+		return -EINVAL;
+
+	driver_override = kstrndup(buf, count, GFP_KERNEL);
+	if (!driver_override)
+		return -ENOMEM;
+
+	cp = strchr(driver_override, '\n');
+	if (cp)
+		*cp = '\0';
+
+	if (strlen(driver_override)) {
+		pdev->driver_override = driver_override;
+	} else {
+		kfree(driver_override);
+		pdev->driver_override = NULL;
+	}
+
+	kfree(old);
+
+	return count;
+}
+
+static ssize_t driver_override_show(struct device *dev,
+				    struct device_attribute *attr, char *buf)
+{
+	struct pci_dev *pdev = to_pci_dev(dev);
+
+	return sprintf(buf, "%s\n", pdev->driver_override);
+}
+static DEVICE_ATTR_RW(driver_override);
+
 static struct attribute *pci_dev_attrs[] = {
 	&dev_attr_resource.attr,
 	&dev_attr_vendor.attr,
@@ -521,6 +560,7 @@ static struct attribute *pci_dev_attrs[] = {
 #if defined(CONFIG_PM_RUNTIME) && defined(CONFIG_ACPI)
 	&dev_attr_d3cold_allowed.attr,
 #endif
+	&dev_attr_driver_override.attr,
 	NULL,
 };
 
diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
index ef09f5f2fe6c..54268de45f59 100644
--- a/drivers/pci/probe.c
+++ b/drivers/pci/probe.c
@@ -1215,6 +1215,7 @@ static void pci_release_dev(struct device *dev)
 	pci_release_of_node(pci_dev);
 	pcibios_release_device(pci_dev);
 	pci_bus_put(pci_dev->bus);
+	kfree(pci_dev->driver_override);
 	kfree(pci_dev);
 }