Date:	Tue, 06 May 2014 11:22:03 -0600
From:	Alex Williamson <alex.williamson@...hat.com>
To:	Greg KH <gregkh@...uxfoundation.org>
Cc:	linux-pci@...r.kernel.org, agraf@...e.de, kvm@...r.kernel.org,
	konrad.wilk@...cle.com, kim.phillips@...aro.org,
	gregkh@...uxfoundation.org, stuart.yoder@...escale.com,
	linux-kernel@...r.kernel.org, libvir-list@...hat.com,
	iommu@...ts.linux-foundation.org, tech@...tualopensystems.com,
	kvmarm@...ts.cs.columbia.edu, christoffer.dall@...aro.org,
	Bjorn Helgaas <bhelgaas@...gle.com>
Subject: Re: [PATCH] PCI: Introduce new device binding path using
 pci_dev.driver_override

Hi Greg,

I think Bjorn is waiting for an ack from you on this.  Do you approve of
this approach from a driver-core perspective?  Thanks,

Alex

On Fri, 2014-04-04 at 14:19 -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/preferred_driver
> 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 (preferred
> 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>
> ---
> 
> Changes since RFC:
>  - Add ABI documentation (gregkh)
>  - Documentation wording clarification (Christoffer)
> 
> Nobody puked on the RFC and platform folks have posted a working
> version of this for platform devices, so I guess the only thing left
> to do is formally propose this as a new driver binding mechanism.  I
> don't see much incentive to push this into the driver core since the
> match ultimately needs to be done by the bus driver.  I think this is
> therefore like new_id/remove_id where PCI and USB implement separate,
> but consistent interfaces.
> 
> I've pruned the CC list a bit from the RFC, but I've added libvirt
> folks since I expect they would be the first userspace tool that would
> adopt this.  Thanks,
> 
> Alex
> 
>  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@...hat.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 25f0bc6..f780eb8 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 276ef9c..70cb39d 100644
> --- a/drivers/pci/pci-sysfs.c
> +++ b/drivers/pci/pci-sysfs.c
> @@ -510,6 +510,45 @@ 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;
> +
> +	if (count > PATH_MAX)
> +		return -EINVAL;
> +
> +	driver_override = kstrndup(buf, count, GFP_KERNEL);
> +	if (!driver_override)
> +		return -ENOMEM;
> +
> +	while (strlen(driver_override) &&
> +	       driver_override[strlen(driver_override) - 1] == '\n')
> +		driver_override[strlen(driver_override) - 1] = '\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,
> @@ -532,6 +571,7 @@ 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/include/linux/pci.h b/include/linux/pci.h
> index 33aa2ca..6b666af 100644
> --- a/include/linux/pci.h
> +++ b/include/linux/pci.h
> @@ -364,6 +364,7 @@ struct pci_dev {
>  #endif
>  	phys_addr_t rom; /* Physical address of ROM if it's not from the BAR */
>  	size_t romlen; /* Length of ROM if it's not from the BAR */
> +	char *driver_override; /* Driver name to force a match */
>  };
>  
>  static inline struct pci_dev *pci_physfn(struct pci_dev *dev)
> 



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