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

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Date:	Wed, 21 May 2014 10:25:27 +0200
From:	Alexander Graf <agraf@...e.de>
To:	Alex Williamson <alex.williamson@...hat.com>, bhelgaas@...gle.com,
	linux-pci@...r.kernel.org
CC:	kvm@...r.kernel.org, konrad.wilk@...cle.com,
	kim.phillips@...aro.org, gregkh@...uxfoundation.org,
	stuart.yoder@...escale.com, 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 20.05.14 16:53, 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>

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

I suppose Konrad's RB stays as well?


Alex

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