Re: [PATCH v2] PCI: Introduce new device binding path using pci_dev.driver_override

[Alexander Graf <agraf@suse.de>]

On 09.05.14 18:50, 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@redhat.com>
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> ---
>
> v2: Use strchr() as suggested by Guenter Roeck and adopted by the
>      platform driver version of this same interface.
>
>   Documentation/ABI/testing/sysfs-bus-pci |   21 ++++++++++++++++
>   drivers/pci/pci-driver.c                |   25 +++++++++++++++++--
>   drivers/pci/pci-sysfs.c                 |   40 +++++++++++++++++++++++++++++++
>   include/linux/pci.h                     |    1 +
>   4 files changed, 84 insertions(+), 3 deletions(-)
>
> diff --git a/Documentation/ABI/testing/sysfs-bus-pci b/Documentation/ABI/testing/sysfs-bus-pci
> index a3c5a66..898ddc4 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.
> diff --git a/drivers/pci/pci-driver.c b/drivers/pci/pci-driver.c
> index d911e0c..4393c12 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 4e0acef..faa4ab5 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);

Is there anything we have to do on hot remove of this device to free 
that variable?

Otherwise,

   Reviewed-by: Alexander Graf <agraf@suse.de>


Alex